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{{short description|Mixture of organic matter, minerals, gases, liquids, and organisms that together support life}}
{{For|other uses| Soil (disambiguation)}}
{{other uses}}
{{pp-move-indef|small= yes}}
{{pp-move|small= yes}}
{{short description|mixture of organic matter, minerals, gases, liquids, and organisms that together support life}}


[[File:Stagnogley.JPG|thumb|upright=1.5|Surface-water-[[Gley soil|gley]] developed in [[glacial till]] in [[Northern Ireland]]]]
{{wikt | soil}}
[[File:Estructura-suelo.jpg|thumb|right|alt= This is a diagram and related photograph of soil layers from bedrock to soil.|A, B, and C represent the [[soil profile]], a notation firstly coined by [[Vasily Dokuchaev]] (1846-1903), the father of [[pedology]]; A is the [[topsoil]]; B is a [[regolith]]; C is a [[saprolite]] (a less-weathered regolith); the bottom-most layer represents the [[bedrock]].]]
[[File:Stagnogley.JPG|thumb|Surface-water-[[Gley soil|gley]] developed in [[glacial till]], [[Northern Ireland]].]]


'''Soil''' is a [[mixture]] of [[organic matter]], [[minerals]], [[gas]]es, [[liquid]]s, and [[organism]]s that together support [[life]]. [[Earth]]'s body of soil, called the [[pedosphere]], has four important [[soil functions|functions]]:
'''Soil''', also commonly referred to as '''earth''', is a [[mixture]] of [[organic matter]], [[minerals]], [[gas]]es, [[liquid]]s, and [[organism]]s that together support the [[life]] of [[plant]]s and [[Soil biology|soil organisms]]. Some scientific definitions distinguish [[dirt]] from ''soil'' by restricting the former term specifically to displaced soil.
[[File:EAgronom 4okt2023 L-1120.jpg|thumb|Soil measuring and surveying device]]
Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a [[Porosity|porous]] phase that holds [[Soil gas|gases]] (the soil atmosphere) and [[water]] (the soil solution).<ref>{{cite book |last1=Voroney |first1=R. Paul |title=Soil microbiology, ecology and biochemistry |last2=Heck |first2=Richard J. |date=2015 |publisher=[[Elsevier]] |isbn=978-0-12-415955-6 |editor-last=Paul |editor-first=Eldor A. |edition=4th |location=Amsterdam, the Netherlands |pages=15–39 |chapter=The soil habitat |doi=10.1016/B978-0-12-415955-6.00002-5 |access-date=22 December 2024 |chapter-url=https://fr.1lib.sk/book/67708166/606823/soil-microbiology-ecology-and-biochemistry-the-soil-habitat.html }}</ref><ref>{{cite book |last1=Taylor |first1=Sterling A. |url=https://archive.org/details/physicaledapholo0000tayl |title=Physical edaphology: the physics of irrigated and nonirrigated soils |last2=Ashcroft |first2=Gaylen L. |date=1972 |publisher=[[W. H. Freeman and Company|W.H. Freeman]] |isbn=978-0-7167-0818-6 |location=San Francisco, California |access-date=22 December 2024 }}</ref> Accordingly, soil is a three-[[state of matter|state]] system of solids, liquids, and gases.<ref>{{cite book |last=McCarthy |first=David F. |url=https://fr.1lib.sk/book/3555343/8f031e/essentials-of-soil-mechanics-and-foundations-basic-geotechnics.html |title=Essentials of soil mechanics and foundations: basic geotechnics |date=2014 |publisher=[[Pearson Education|Pearson]] |isbn=9781292039398 |edition=7th |location=London, United Kingdom |access-date=22 December 2024 |archive-date=16 October 2022 |archive-url=https://web.archive.org/web/20221016144604/https://fr.b-ok.cc/book/3555343/0f8f97 |url-status=live }}</ref> Soil is a product of several factors: the influence of [[climate]], [[terrain|relief]] (elevation, orientation, and slope of terrain), organisms, and the soil's [[parent material]]s (original minerals) interacting over time.<ref name="Gilluly1975">{{cite book |last1=Gilluly |first1=James |url=https://archive.org/details/principlesofgeol0000gill |title=Principles of geology |last2=Waters |first2=Aaron Clement |last3=Woodford |first3=Alfred Oswald |date=1975 |publisher=[[W. H. Freeman and Company|W.H. Freeman]] |isbn=978-0-7167-0269-6 |edition=4th |location=San Francisco, California |author-link1=James Gilluly |access-date=22 December 2024 }}</ref> It continually undergoes development by way of numerous physical, chemical and biological processes, which include [[weathering]] with associated [[erosion]].<ref>{{cite book |first=Richard John |last=Huggett |chapter=What is geomorphology? |title=Fundamentals of geomorphology |edition=4th |series=Routledge Fundamentals of Physical Geography |publisher=[[Routledge]] |location=London, United Kingdom |date=2017 |pages=3–30 |isbn=9781138940659 |url=https://fr.1lib.sk/book/19205504/2af594/fundamentals-of-geomorphology.html |access-date=22 December 2024 }}</ref> Given its complexity and strong internal [[connectedness]], [[Soil ecology|soil ecologists]] regard soil as an [[ecosystem]].<ref>{{cite journal |last=Ponge |first=Jean-François |year=2015 |title=The soil as an ecosystem |url=https://www.researchgate.net/publication/276090499 |journal=Biology and Fertility of Soils |volume=51 |issue=6 |pages=645–648 |doi=10.1007/s00374-015-1016-1 |bibcode=2015BioFS..51..645P |access-date=22 December 2024 |s2cid=18251180}}</ref>


Most soils have a dry [[bulk density]] (density of soil taking into account voids when dry) between 1.1 and 1.6&nbsp;g/cm<sup>3</sup>, though the soil [[Particle density (packed density)|particle density]] is much higher, in the range of 2.6 to 2.7&nbsp;g/cm<sup>3</sup>.<ref name="Yu2015">{{cite web |last1=Yu |first1=Charley |last2=Kamboj |first2=Sunita |last3=Wang |first3=Cheng |last4=Cheng |first4=Jing-Jy |year=2015 |title=Data collection handbook to support modeling impacts of radioactive material in soil and building structures |url=https://resrad.evs.anl.gov/docs/data_collection.pdf |url-status=live |archive-url=https://web.archive.org/web/20180804105951/http://resrad.evs.anl.gov/docs/data_collection.pdf |archive-date=4 August 2018 |access-date=3 April 2022 |website=[[Argonne National Laboratory]] |pages=13–21}}</ref> Little of the soil of [[planet Earth]] is older than the [[Pleistocene]] and none is older than the [[Cenozoic]],<ref name="Buol">{{cite book |last1=Buol |first1=Stanley W. |url=https://fr1lib.org/book/2156097/707d35 |title=Soil genesis and classification |last2=Southard |first2=Randal J. |last3=Graham |first3=Robert C. |last4=McDaniel |first4=Paul A. |date=2011 |publisher=[[Wiley-Blackwell]] |isbn=978-0-470-96060-8 |edition=6th |location=Ames, Iowa |access-date=3 April 2022 |archive-date=22 April 2023 |archive-url=https://web.archive.org/web/20230422182641/https://fr1lib.org/book/2156097/707d35 |url-status=dead }}</ref> although [[Paleopedological record|fossilized soils]] are preserved from as far back as the [[Archean]].<ref>{{cite journal |last1=Retallack |first1=Gregory J. |last2=Krinsley |first2=David H. |last3=Fischer |first3=Robert |last4=Razink |first4=Joshua J. |last5=Langworthy |first5=Kurt A. |year=2016 |title=Archean coastal-plain paleosols and life on land |url=https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/d/3735/files/2013/07/Retallack-et-al.-2016-Farrel-1gt7uft.pdf |url-status=live |journal=[[Gondwana Research]] |volume=40 |pages=1–20 |bibcode=2016GondR..40....1R |doi=10.1016/j.gr.2016.08.003 |archive-url=https://web.archive.org/web/20181113075710/https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/d/3735/files/2013/07/Retallack-et-al.-2016-Farrel-1gt7uft.pdf |archive-date=13 November 2018 |access-date=3 April 2022 |doi-access=free}}</ref>
* as a medium for plant growth pj

* as a means of [[water storage]], supply and purification
Collectively the Earth's body of soil is called the [[pedosphere]]. The pedosphere interfaces with the [[lithosphere]], the [[hydrosphere]], the [[atmosphere]], and the [[biosphere]].<ref name="ches">{{cite book |url=https://fr1lib.org/book/563235/8e916e |title=Encyclopedia of soil science |date=2008 |publisher=[[Springer Science+Business Media|Springer]] |isbn=978-1-4020-3994-2 |editor-last=Chesworth |editor-first=Ward |edition=1st |location=Dordrecht, The Netherlands |access-date=27 March 2022 |archive-url=https://web.archive.org/web/20180905002957/http://www.encyclopedias.biz/dw/Encyclopedia%20of%20Soil%20Science.pdf |archive-date=5 September 2018 |url-status=live}}</ref> Soil has four important [[soil functions|functions]]:

* as a medium for plant growth
* as a means of [[water storage]], supply, and purification
* as a modifier of [[Atmosphere of Earth|Earth's atmosphere]]
* as a modifier of [[Atmosphere of Earth|Earth's atmosphere]]
* as a habitat for organisms
* as a habitat for organisms


All of these functions, in their turn, modify the soil.
All of these functions, in their turn, modify the soil and its properties.

The pedosphere interfaces with the [[lithosphere]], the [[hydrosphere]], the [[atmosphere]], and the [[biosphere]].<ref name="ches">{{cite book |editor-last= Chesworth |editor-first= Ward |date= 2008 |title= Encyclopedia of soil science |isbn= 978-1-4020-3994-2 |publisher= [[Springer Science+Business Media|Springer]] |location= Dordrecht, The Netherlands |url= http://www.encyclopedias.biz/dw/Encyclopedia%20of%20Soil%20Science.pdf |archive-url= https://web.archive.org/web/20180905002957/http://www.encyclopedias.biz/dw/Encyclopedia%20of%20Soil%20Science.pdf |archive-date= 2018-09-05 |dead-url= no |access-date= 2019-01-14}}</ref> The term ''[[:wikt:pedolith|pedolith]]'', used commonly to refer to the soil, translates to ''[[wikt:ground|ground]] stone'' in the sense "fundamental stone".<ref>{{Cite OED|pedo-}}, from the ancient Greek πέδον "ground", "earth".</ref> Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a [[Porosity|porous]] phase that holds gases (the soil atmosphere) and water (the soil solution).<ref>{{cite book |last1= Voroney |first1= R. Paul |last2= Heck |first2= Richard J. |lastauthoramp= yes |date= 2007 |chapter= The soil habitat |doi= 10.1016/B978-0-08-047514-1.50006-8 |title= Soil microbiology, ecology and biochemistry |edition= 3rd |editor-first= Eldor A. |editor-last= Paul |publisher= [[Elsevier]] |location= Amsterdam |pages= 25–49 |isbn= 978-0-12-546807-7 |url= http://csmi.issas.ac.cn/uploadfiles/Soil%20Microbiology,%20Ecology%20&%20Biochemistry.pdf |archive-url= https://web.archive.org/web/20180710102532/http://csmi.issas.ac.cn/uploadfiles/Soil%20Microbiology%2C%20Ecology%20%26%20Biochemistry.pdf |archive-date= 10 July 2018 |dead-url= no |access-date= 2019-01-15 |df= dmy-all }}</ref><ref>{{cite web |last= Danoff-Burg |first= James A. |publisher= [[Columbia University Press]] |location= New York |url= http://ccnmtl.columbia.edu/projects/seeu/dr/restrict/modules/module10.html |title= The terrestrial influence: geology and soils |website= [[Earth Institute Center for Environmental Sustainability]] |access-date= 17 December 2017}}</ref><ref>{{cite book |last1= Taylor |first1= Sterling A. |last2= Ashcroft |first2= Gaylen L. |lastauthoramp= yes |date= 1972 |title= Physical edaphology: the physics of irrigated and nonirrigated soils |publisher= W.H. Freeman |location= San Francisco |isbn= 978-0-7167-0818-6}}</ref> Accordingly, soil scientists can envisage soils as a three-[[state of matter|state]] system of solids, liquids, and gases.<ref>{{cite book |last= McCarthy |first= David F. |date= 2006 |title= Essentials of soil mechanics and foundations: basic geotechnics |edition= 7th |publisher= Prentice Hall |location= Upper Saddle River, New Jersey |isbn= 978-0-13-114560-3}}</ref>

Soil is a product of several factors: the influence of [[climate]], [[terrain|relief]] (elevation, orientation, and slope of terrain), organisms, and the soil's [[parent material]]s (original minerals) interacting over time.<ref name="Gilluly1975">{{cite book|authorlink1= James Gilluly |last1= Gilluly |first1= James |last2= Waters |first2= Aaron Clement |last3= Woodford |first3= Alfred Oswald |lastauthoramp= yes |title= Principles of geology |date= 1975 |edition= 4th |publisher= W.H. Freeman |location= San Francisco |isbn= 978-0-7167-0269-6}}</ref> It continually undergoes development by way of numerous physical, chemical and biological processes, which include [[weathering]] with associated [[erosion]]. Given its complexity and strong internal [[connectedness]], [[Soil ecology|soil ecologists]] regard soil as an [[ecosystem]].<ref>{{cite journal |last= Ponge |first= Jean-François |journal= Biology and Fertility of Soils |title= The soil as an ecosystem |year= 2015 |volume= 51 |issue= 6 |pages= 645–48 |doi= 10.1007/s00374-015-1016-1 |url= https://www.researchgate.net/publication/276090499 |accessdate= 17 December 2017 |format= [[Portable Document Format|PDF]]}}</ref>

Most soils have a dry [[bulk density]] (density of soil taking into account voids when dry) between 1.1 and 1.6&nbsp;g/cm<sup>3</sup>, while the soil [[Particle density (packed density)|particle density]] is much higher, in the range of 2.6 to 2.7&nbsp;g/cm<sup>3</sup>.<ref name="Yu2015">{{cite web |url= http://resrad.evs.anl.gov/docs/data_collection.pdf |last1= Yu |first1= Charley |last2= Kamboj |first2= Sunita |last3= Wang |first3= Cheng |last4= Cheng |first4= Jing-Jy |lastauthoramp= yes |title= Data collection handbook to support modeling impacts of radioactive material in soil and building structures |pages= 13–21 |website= [[Argonne National Laboratory]] |year= 2015 |archive-url= https://web.archive.org/web/20180804105951/http://resrad.evs.anl.gov/docs/data_collection.pdf |archive-date= 2018-08-04 |dead-url= no |access-date= 17 December 2017}}</ref> Little of the soil of planet Earth is older than the [[Pleistocene]] and none is older than the [[Cenozoic]],<ref name="Buol">{{cite book |last1= Buol |first1= Stanley W. |last2= Southard |first2= Randal J. |last3= Graham |first3= Robert C. |last4= McDaniel |first4= Paul A. |lastauthoramp= yes |title= Soil genesis and classification |edition= 7th |date= 2011 |publisher= Wiley-Blackwell |location= Ames, Iowa |isbn= 978-0-470-96060-8}}</ref> although [[Paleopedological record|fossilized soils]] are preserved from as far back as the [[Archean]].<ref>{{cite journal |last1= Retallack |first1= Gregory J. |last2= Krinsley |first2= David H |last3= Fischer |first3= Robert | last4= Razink |first4= Joshua J. |last5= Langworthy |first5= Kurt A. |lastauthoramp= yes |journal= [[Gondwana Research]] |volume= 40 |title= Archean coastal-plain paleosols and life on land |url= https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/d/3735/files/2013/07/Retallack-et-al.-2016-Farrel-1gt7uft.pdf |year= 2016 |pages= 1–20 |doi= 10.1016/j.gr.2016.08.003 |archive-url= https://web.archive.org/web/20181113075710/https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/d/3735/files/2013/07/Retallack-et-al.-2016-Farrel-1gt7uft.pdf |archive-date= 2018-11-13 |dead-url= no |access-date= 2019-01-15 }}</ref>

[[Soil science]] has two basic branches of study: [[edaphology]] and [[pedology]]. Edaphology studies the influence of soils on living things.<ref>{{cite web |url= http://sis.agr.gc.ca/cansis/glossary/e/index.html |title= Glossary of Terms in Soil Science |website= [[Agriculture and Agri-Food Canada]] |archive-url= https://web.archive.org/web/20181027045042/http://sis.agr.gc.ca/cansis/glossary/e/index.html |archive-date= 2018-10-27 |access-date= 2019-01-15}}</ref> Pedology focuses on the formation, description (morphology), and classification of soils in their natural environment.<ref>{{cite web |url= http://natres.psu.ac.th/Link/SoilCongress/bdd/symp45/75-t.pdf |title= Soil preservation and the future of pedology |first= Ronald |last= Amundson |website= Faculty of Natural Resources |publisher= Prince of Songkla University |location= Songkhla, Thailand|archive-url= https://web.archive.org/web/20180612140029/http://natres.psu.ac.th/Link/SoilCongress/bdd/symp45/75-t.pdf |archive-date= 2018-06-12 |dead-url= no |access-date= 2019-01-15}}</ref> In engineering terms, soil is included in the broader concept of [[regolith]], which also includes other loose material that lies above the bedrock, as can be found on the Moon and on other celestial objects as well.<ref>{{cite web |url= https://www.mps.mpg.de/phd/planetary-interiors-and-surfaces-2011-part-05 |title= Impacts and formation of regolith |last1= Küppers |first1= Michael |last2= Vincent |first2= Jean-Baptiste |website= [[Max Planck Institute for Solar System Research]] |archive-url= https://web.archive.org/web/20180804200824/https://www.mps.mpg.de/phd/planetary-interiors-and-surfaces-2011-part-05 |archive-date= 2018-08-04 |dead-url= no |access-date= 2019-01-15}}</ref> Soil is also commonly referred to as '''earth''' or '''[[dirt]]'''; some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former term specifically to displaced soil.<ref>
{{cite web | url= https://www.soils.org/files/about-soils/soils-overview.pdf |title= Soils overview provided by The Soil Science Society of America | access-date= 2019-02-24}}</ref>


[[Soil science]] has two basic branches of study: [[edaphology]] and [[pedology]]. ''Edaphology'' studies the influence of soils on living things.<ref>{{cite web |url=https://sis.agr.gc.ca/cansis/glossary/e/index.html |title=Glossary of terms in soil science |website=[[Agriculture and Agri-Food Canada]] |date=13 December 2013 |archive-url=https://web.archive.org/web/20181027045042/http://sis.agr.gc.ca/cansis/glossary/e/index.html |archive-date=27 October 2018 |url-status=live |access-date=3 April 2022}}</ref> ''Pedology'' focuses on the formation, description (morphology), and classification of soils in their natural environment.<ref>{{cite web |title=Soil preservation and the future of pedology |first=Ronald |last=Amundson |citeseerx=10.1.1.552.237 |url=http://natres.psu.ac.th/Link/SoilCongress/bdd/symp45/75-t.pdf |archive-url=https://web.archive.org/web/20180612140029/http://natres.psu.ac.th/Link/SoilCongress/bdd/symp45/75-t.pdf |archive-date=12 June 2018 |url-status=dead }}</ref> In engineering terms, soil is included in the broader concept of [[regolith]], which also includes other loose material that lies above the bedrock, as can be found on the [[Moon]] and other [[Astronomical object|celestial objects]].<ref>{{cite web |url=https://www.mps.mpg.de/phd/planetary-interiors-and-surfaces-2011-part-05 |title=Impacts and formation of regolith |last1=Küppers |first1=Michael |last2=Vincent |first2=Jean-Baptiste |website=[[Max Planck Institute for Solar System Research]] |archive-url=https://web.archive.org/web/20180804200824/https://www.mps.mpg.de/phd/planetary-interiors-and-surfaces-2011-part-05 |archive-date=4 August 2018 |url-status=live |access-date=3 April 2022}}</ref>
==Overview==
[[File:Soil profile.png|thumb|Soil Profile: Darkened topsoil and reddish subsoil [[soil horizons|layers]] are typical in [[humid subtropical climate|some regions.]]]]


===Functions===
== Processes ==
Soil is a major component of the [[Earth]]'s [[ecosystem]]. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, from [[ozone depletion]] and [[global warming]] to [[rainforest destruction]] and [[water pollution]]. With respect to Earth's [[carbon cycle]], soil is an important [[carbon sink|carbon reservoir]], and it is potentially one of the most reactive to human disturbance<ref>{{cite journal |last1=Pouyat |first1=Richard |last2=Groffman |first2=Peter |last3=Yesilonis |first3=Ian |last4=Hernandez |first4=Luis |lastauthoramp=yes |journal=[[Environmental Pollution (journal)|Environmental Pollution]] |volume=116 |issue=Supplement 1 |title=Soil carbon pools and fluxes in urban ecosystems |url=https://www.ncrs.fs.fed.us/pubs/jrnl/2002/ne_2002_Pouyat_001.pdf |year=2002 |pages=S107–S118 |doi=10.1016/S0269-7491(01)00263-9 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> and climate change.<ref name="Davidson">{{cite journal |last1=Davidson |first1=Eric A. |last2=Janssens |first2=Ivan A. |lastauthoramp=yes |journal=[[Nature (journal)|Nature]] |volume=440 |title=Temperature sensitivity of soil carbon decomposition and feedbacks to climate change |url=https://www.researchgate.net/publication/7253750 |year=2006 |issue=9 March 2006 |pages=165‒73 |doi=10.1038/nature04514 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]] |pmid=16525463|bibcode=2006Natur.440..165D }}</ref> As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased [[Soil biology|biological]] activity at higher temperatures, a [[positive feedback]] (amplification).<ref>{{cite journal |last=Powlson |first=David |journal=[[Nature (journal)|Nature]] |volume=433 |title=Climatology: will soil amplify climate change? |url=https://www.nature.com/articles/433204a.pdf |year=2005 |issue=20 January 2005 |pages=204‒05 |doi=10.1038/433204a |pmid=15662396 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2005Natur.433..204P }}</ref> This prediction has, however, been questioned on consideration of more recent knowledge on soil carbon turnover.<ref>{{cite journal |last1=Bradford |first1=Mark A. |last2=Wieder |first2=William R. |last3=Bonan |first3=Gordon B. |last4=Fierer |first4=Noah |last5=Raymond |first5=Peter A. |last6=Crowther |first6=Thomas W. |lastauthoramp=yes |journal=[[Nature Climate Change]] |volume=6 |title=Managing uncertainty in soil carbon feedbacks to climate change |url=http://fiererlab.org/wp-content/uploads/2014/09/Bradford_etal_2016_NCC.pdf |year=2016 |issue=27 July 2016 |pages=751–58 |doi=10.1038/nclimate3071 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2016NatCC...6..751B }}</ref>
Soil is a major component of the [[Earth]]'s [[ecosystem]]. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, with effects ranging from [[ozone depletion]] and [[global warming]] to [[rainforest destruction]] and [[water pollution]]. With respect to Earth's [[carbon cycle]], soil acts as an important [[carbon sink|carbon reservoir]],<ref>{{Cite journal |last1=Amelung |first1=Wulf |last2=Bossio |first2=Deborah |last3=De Vries |first3=Wim |last4=Kögel-Knabner |first4=Ingrid |last5=Lehmann |first5=Johannes |last6=Amundson |first6=Ronald |last7=Bol |first7=Roland |last8=Collins |first8=Chris |last9=Lal |first9=Rattan |last10=Leifeld |first10=Jens |last11=Minasny |first11=Buniman |last12=Pan |first12=Gen-Xing |last13=Paustian |first13=Keith |last14=Rumpel |first14=Cornelia |last15=Sanderman |first15=Jonathan |last16=Van Groeningen |first16=Jan Willem |last17=Mooney |first17=Siân |last18=Van Wesemael |first18=Bas |last19=Wander |first19=Michelle |last20=Chabbi |first20=Abad |date=27 October 2020 |title=Towards a global-scale soil climate mitigation strategy |journal=[[Nature Communications]] |language=en |volume=11 |issue=1 |pages=5427 |doi=10.1038/s41467-020-18887-7 |pmid=33110065 |pmc=7591914 |bibcode=2020NatCo..11.5427A |issn=2041-1723 |url=https://www.nature.com/articles/s41467-020-18887-7.pdf |access-date=3 April 2022 |doi-access=free}}</ref> and it is potentially one of the most reactive to human disturbance<ref>{{cite journal |last1=Pouyat |first1=Richard |last2=Groffman |first2=Peter |last3=Yesilonis |first3=Ian |last4= Hernandez |first4=Luis |journal=[[Environmental Pollution (journal)|Environmental Pollution]] |volume=116 |issue=Supplement 1 |title=Soil carbon pools and fluxes in urban ecosystems |url=https://www.researchgate.net/publication/11526697 |year=2002 |pages=S107–S118 |doi=10.1016/S0269-7491(01)00263-9 |pmid=11833898 |access-date=3 April 2022 |quote=Our analysis of pedon data from several disturbed soil profiles suggests that physical disturbances and anthropogenic inputs of various materials (direct effects) can greatly alter the amount of C stored in these human "made" soils.}}</ref> and [[climate change]].<ref name="Davidson">{{cite journal |last1=Davidson |first1=Eric A. |last2=Janssens |first2=Ivan A. |journal=[[Nature (journal)|Nature]] |volume=440 |title=Temperature sensitivity of soil carbon decomposition and feedbacks to climate change |year=2006 |issue=9 March 2006 |pages=165‒73 |url=https://www.nature.com/articles/nature04514.pdf |doi=10.1038/nature04514 |pmid=16525463 |bibcode=2006Natur.440..165D |s2cid=4404915 |access-date=3 April 2022 |doi-access=free}}</ref> As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased [[Soil biology|biological]] activity at higher temperatures, a [[positive feedback]] (amplification).<ref>{{cite journal |last=Powlson |first=David |journal=[[Nature (journal)|Nature]] |volume=433 |title=Will soil amplify climate change? |year=2005 |issue=20 January 2005 |pages=204‒05 |url=https://fr.art1lib.org/book/10543301/528a68 |doi=10.1038/433204a |pmid=15662396 |bibcode=2005Natur.433..204P |s2cid=35007042 |access-date=3 April 2022 |archive-date=22 September 2022 |archive-url=https://web.archive.org/web/20220922110017/https://fr.art1lib.org/book/10543301/528a68 |url-status=dead }}</ref> This prediction has, however, been questioned on consideration of more recent knowledge on [[soil carbon]] turnover.<ref>{{cite journal |last1=Bradford |first1=Mark A. |last2=Wieder |first2=William R. |last3=Bonan |first3=Gordon B. |last4=Fierer |first4=Noah |last5=Raymond |first5=Peter A. |last6=Crowther |first6=Thomas W. |journal=[[Nature Climate Change]] |volume=6 |title=Managing uncertainty in soil carbon feedbacks to climate change |url=http://fiererlab.org/wp-content/uploads/2014/09/Bradford_etal_2016_NCC.pdf |year=2016 |issue=27 July 2016 |pages=751–758 |doi=10.1038/nclimate3071 |access-date=3 April 2022 |bibcode=2016NatCC...6..751B |hdl=20.500.11755/c1792dbf-ce96-4dc7-8851-1ca50a35e5e0 |s2cid=43955196 |hdl-access=free |archive-date=10 April 2017 |archive-url=https://web.archive.org/web/20170410025316/http://fiererlab.org/wp-content/uploads/2014/09/Bradford_etal_2016_NCC.pdf |url-status=dead }}</ref>


Soil acts as an engineering medium, a habitat for [[soil organisms]], a recycling system for [[nutrients]] and [[organic waste]]s, a regulator of [[water quality]], a modifier of [[Atmospheric chemistry|atmospheric composition]], and a medium for [[plant growth]], making it a critically important provider of [[ecosystem services]].<ref>{{cite journal |last1=Dominati |first1=Estelle |last2=Patterson |first2=Murray |last3=Mackay |first3=Alec |lastauthoramp=yes |journal=[[Ecological Economics (journal)|Ecological Economics]] |volume=69 |issue=9 |title=A framework for classifying and quantifying the natural capital and ecosystem services of soils |year=2010 |url=http://esanalysis.colmex.mx/Sorted%20Papers/2010/2010%20NZL%20-3F%20Phys.pdf |pages=1858‒68 |doi=10.1016/j.ecolecon.2010.05.002 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Since soil has a tremendous range of available niches and habitats, it contains most of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and in the main still unexplored.<ref>{{cite journal |last=Dykhuizen |first=Daniel E. |journal=Antonie van Leeuwenhoek |volume=73 |issue=1 |title=Santa Rosalia revisited: why are there so many species of bacteria? |year=1998 |url=https://www.researchgate.net/publication/13682480 |pages=25‒33 |doi=10.1023/A:1000665216662 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Torsvik |first1=Vigdis |last2=Øvreås |first2=Lise |lastauthoramp=yes |journal=[[Current Opinion in Microbiology]] |volume=5 |issue=3 |title=Microbial diversity and function in soil: from genes to ecosystems |year=2002 |url=https://s3.amazonaws.com/academia.edu.documents/41679616/Torsvik_2002_CurrOp-Microbial_diversity_and_function_in_soil.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1513504055&Signature=ErdiJ%2Bb55S31YyoGDCHzocXl2AY%3D&response-content-disposition=inline%3B%20filename%3DMicrobial_diversity_and_function_in_soil.pdf |pages=240‒45 |doi=10.1016/S1369-5274(02)00324-7 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Soil has a [[mean]] [[Prokaryote|prokaryotic]] density of roughly 10<sup>8</sup> organisms per gram,<ref>{{cite journal |last1= Raynaud |first1=Xavier |last2=Nunan |first2=Naoise |lastauthoramp=yes |journal=[[PLOS ONE]] |volume=9 |issue=1 |title=Spatial ecology of bacteria at the microscale in soil |year=2014 |page=e87217 |doi=10.1371/journal.pone.0087217 |pmid=24489873 |pmc=3905020 |bibcode=2014PLoSO...987217R }}</ref> whereas the ocean has no more than 10<sup>7</sup> procaryotic organisms per milliliter (gram) of seawater.<ref>{{cite journal |last1=Whitman |first1=William B. |last2=Coleman |first2=David C. |last3=Wiebe |first3=William J. |lastauthoramp=yes |journal=[[Proceedings of the National Academy of Sciences of the USA]] |volume=95 |issue=12 |title=Prokaryotes: the unseen majority |year=1998 |url=http://www.pnas.org/content/95/12/6578.full.pdf |pages=6578‒83 |doi=10.1073/pnas.95.12.6578 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]] |pmid=9618454 |pmc=33863|bibcode=1998PNAS...95.6578W }}</ref> [[Soil organic matter|Organic carbon]] held in soil is eventually returned to the atmosphere through the process of [[cellular respiration|respiration]] carried out by [[heterotrophic]] organisms, but a substantial part is retained in the soil in the form of [[soil organic matter]]; [[tillage]] usually increases the rate of soil respiration, leading to the depletion of soil organic matter.<ref>{{cite journal |last1=Schlesinger |first1=William H. |last2=Andrews |first2=Jeffrey A. |lastauthoramp=yes |journal=Biogeochemistry |volume=48 |issue=1 |title=Soil respiration and the global carbon cycle |year=2000 |url=https://www.researchgate.net/publication/51997678 |pages=7‒20 |doi=10.1023/A:1006247623877 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Since plant roots need oxygen, ventilation is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected [[Pore space in soil|soil pores]], which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the [[Soil water (retention)|water-holding capacity]] of soils is vital for plant survival.<ref>{{cite journal |last1=Denmead |first1=Owen Thomas |last2=Shaw |first2=Robert Harold |lastauthoramp=yes |journal=Agronomy Journal |volume=54 |issue=5 |title=Availability of soil water to plants as affected by soil moisture content and meteorological conditions |year=1962 |url=https://www.researchgate.net/publication/250098028 |pages=385‒90 |doi=10.2134/agronj1962.00021962005400050005x |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>
Soil acts as an engineering medium, a habitat for [[soil organisms]], a recycling system for [[nutrients]] and [[organic waste]]s, a regulator of [[water quality]], a modifier of [[Atmospheric chemistry|atmospheric composition]], and a medium for [[plant growth]], making it a critically important provider of [[ecosystem services]].<ref>{{cite journal |last1=Dominati |first1=Estelle |last2=Patterson |first2=Murray |last3=Mackay |first3=Alec |journal=[[Ecological Economics (journal)|Ecological Economics]] |volume=69 |issue=9 |title=A framework for classifying and quantifying the natural capital and ecosystem services of soils |year=2010 |url=https://www.researchgate.net/publication/223852147 |pages=1858‒68 |doi=10.1016/j.ecolecon.2010.05.002 |bibcode=2010EcoEc..69.1858D |access-date=10 April 2022 |archive-url=https://web.archive.org/web/20170808082847/http://esanalysis.colmex.mx/Sorted%20Papers/2010/2010%20NZL%20-3F%20Phys.pdf |archive-date=8 August 2017 |url-status=live}}</ref> Since soil has a tremendous range of available [[Ecological niche|niches]] and [[habitat]]s, it contains a prominent part of the Earth's [[genetic diversity]]. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored.<ref>{{cite journal |last=Dykhuizen |first=Daniel E. |journal=Antonie van Leeuwenhoek |volume=73 |issue=1 |title=Santa Rosalia revisited: why are there so many species of bacteria? |year=1998 |url=https://www.researchgate.net/publication/13682480 |pages=25‒33 |doi=10.1023/A:1000665216662 |pmid=9602276 |s2cid=17779069 |access-date=10 April 2022}}</ref><ref>{{cite journal |last1=Torsvik |first1=Vigdis |last2=Øvreås |first2=Lise |journal=[[Current Opinion in Microbiology]] |volume=5 |issue=3 |title=Microbial diversity and function in soil: from genes to ecosystems |year=2002 |pages=240‒45 |url=https://www.academia.edu/13038690 |doi=10.1016/S1369-5274(02)00324-7 |pmid=12057676 |access-date=10 April 2022}}</ref> Soil has a [[mean]] [[Prokaryote|prokaryotic]] density of roughly 10<sup>8</sup> organisms per gram,<ref>{{cite journal |last1=Raynaud |first1=Xavier |last2=Nunan |first2=Naoise |journal=[[PLOS ONE]] |volume=9 |issue=1 |title=Spatial ecology of bacteria at the microscale in soil |year=2014 |page=e87217 |doi=10.1371/journal.pone.0087217 |pmid=24489873 |pmc=3905020 |bibcode=2014PLoSO...987217R |doi-access=free}}</ref> whereas the ocean has no more than 10<sup>7</sup> prokaryotic organisms per milliliter (gram) of seawater.<ref>{{cite journal |last1=Whitman |first1=William B. |last2=Coleman |first2=David C. |last3=Wiebe |first3=William J. |journal=[[Proceedings of the National Academy of Sciences of the USA]] |volume=95 |issue=12 |title=Prokaryotes: the unseen majority |year=1998 |pages=6578‒83 |doi=10.1073/pnas.95.12.6578 |pmid=9618454 |pmc=33863 |bibcode=1998PNAS...95.6578W |doi-access=free}}</ref> [[Soil organic matter|Organic carbon]] held in soil is eventually returned to the atmosphere through the process of [[cellular respiration|respiration]] carried out by [[heterotrophic]] organisms, but a substantial part is retained in the soil in the form of soil organic matter; [[tillage]] usually increases the rate of [[soil respiration]], leading to the depletion of soil organic matter.<ref>{{cite journal |last1=Schlesinger |first1=William H. |last2=Andrews |first2=Jeffrey A. |journal=Biogeochemistry |volume=48 |issue=1 |title=Soil respiration and the global carbon cycle |year=2000 |url=https://www.researchgate.net/publication/51997678 |pages=7‒20 |doi=10.1023/A:1006247623877 |s2cid=94252768 |access-date=10 April 2022}}</ref> Since plant roots need oxygen, [[aeration]] is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected [[Pore space in soil|soil pores]], which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the [[Soil water (retention)|water-holding capacity]] of soils is vital for plant survival.<ref>{{cite journal |last1=Denmead |first1=Owen Thomas |last2=Shaw |first2=Robert Harold |journal=[[Agronomy Journal]] |volume=54 |issue=5 |title=Availability of soil water to plants as affected by soil moisture content and meteorological conditions |year=1962 |url=https://www.researchgate.net/publication/250098028 |pages=385‒90 |doi=10.2134/agronj1962.00021962005400050005x |bibcode=1962AgrJ...54..385D |access-date=10 April 2022}}</ref>


Soils can effectively remove impurities,<ref>{{cite journal |last1=House |first1=Christopher H. |last2=Bergmann |first2=Ben A. |last3=Stomp |first3=Anne-Marie |last4=Frederick |first4=Douglas J. |lastauthoramp=yes |journal=Ecological Engineering |volume=12 |issue=1–2 |title=Combining constructed wetlands and aquatic and soil filters for reclamation and reuse of water |year=1999 |url=https://www.researchgate.net/publication/222464331 |pages=27–38 |doi=10.1016/S0925-8574(98)00052-4 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> kill disease agents,<ref>{{cite journal |last1=Van Bruggen |first1=Ariena H.C. |last2=Semenov |first2=Alexander M. |lastauthoramp=yes |journal=Applied Soil Ecology |volume=15 |issue=1 |title=In search of biological indicators for soil health and disease suppression |year=2000 |url=https://www.researchgate.net/publication/222520930 |pages=13–24 |doi=10.1016/S0929-1393(00)00068-8 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> and degrade contaminants, this latter property being called natural attenuation.<ref>{{cite web |url=https://clu-in.org/download/Citizens/a_citizens_guide_to_monitored_natural_attenuation.pdf |title=A citizen's guide to monitored natural attenuation |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Typically, soils maintain a net absorption of [[oxygen]] and [[methane]] and undergo a net release of [[carbon dioxide]] and [[nitrous oxide]].<ref>{{cite journal |last1=Linn |first1=Daniel Myron |last2=Doran |first2=John W. |journal=[[Soil Science Society of America Journal]] |volume=48 |issue=6 |title=Effect of water-filled pore space on carbon dioxide and nitrous oxide production in tilled and nontilled soils |year=1984 |url=https://naldc.nal.usda.gov/download/16745/PDF |pages=1267–72 |doi=10.2136/sssaj1984.03615995004800060013x |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1984SSASJ..48.1267L }}</ref> Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins.<ref>{{cite book |last1=Miller |first1=Raymond W. |last2=Donahue |first2= Roy Luther |date=1990 |title=Soils: an introduction to soils and plant growth |isbn=978-0-13-820226-2 |publisher=[[Prentice Hall]] |location=Upper Saddle River, New Jersey}}</ref> Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.<ref>{{cite book |last1=Bot |first1=Alexandra |last2=Benites |first2=José |date=2005 |title=The importance of soil organic matter: key to drought-resistant soil and sustained food and production |isbn=978-92-5-105366-9 |publisher=[[Food and Agriculture Organization of the United Nations]] |location=Rome |url=http://www.fao.org/3/a-a0100e.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>
Soils can effectively remove impurities,<ref>{{cite journal |last1=House |first1=Christopher H. |last2=Bergmann |first2=Ben A. |last3=Stomp |first3=Anne-Marie |last4=Frederick |first4=Douglas J. |journal=Ecological Engineering |volume=12 |issue=1–2 |title=Combining constructed wetlands and aquatic and soil filters for reclamation and reuse of water |year=1999 |url=https://www.researchgate.net/publication/222464331 |pages=27–38 |doi=10.1016/S0925-8574(98)00052-4 |bibcode=1999EcEng..12...27H |access-date=10 April 2022}}</ref> kill disease agents,<ref>{{cite journal |last1=Van Bruggen |first1=Ariena H.C. |last2=Semenov |first2=Alexander M. |journal=Applied Soil Ecology |volume=15 |issue=1 |title=In search of biological indicators for soil health and disease suppression |year=2000 |url=https://www.researchgate.net/publication/222520930 |pages=13–24 |doi=10.1016/S0929-1393(00)00068-8 |bibcode=2000AppSE..15...13V |access-date=10 April 2022}}</ref> and degrade [[contaminants]], this latter property being called [[natural attenuation]].<ref>{{cite web |url=https://semspub.epa.gov/work/HQ/401611.pdf |title=Community guide to monitored natural attenuation |access-date=10 April 2022}}</ref> Typically, soils maintain a net absorption of [[oxygen]] and [[methane]] and undergo a net release of [[carbon dioxide]] and [[nitrous oxide]].<ref>{{cite journal |last1=Linn |first1=Daniel Myron |last2=Doran |first2=John W. |journal=[[Soil Science Society of America Journal]] |volume=48 |issue=6 |title=Effect of water-filled pore space on carbon dioxide and nitrous oxide production in tilled and nontilled soils |year=1984 |url=https://fr.art1lib.org/book/23108771/821c3f |pages=1267–1272 |doi=10.2136/sssaj1984.03615995004800060013x |access-date=10 April 2022 |bibcode=1984SSASJ..48.1267L |archive-date=18 March 2023 |archive-url=https://web.archive.org/web/20230318043457/https://fr.art1lib.org/book/23108771/821c3f |url-status=dead }}</ref> Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins.<ref>{{cite book |last1=Gregory |first1=Peter J. |last2=Nortcliff |first2=Stephen |date=2013 |title=Soil conditions and plant growth |isbn=9781405197700 |publisher=[[Wiley-Blackwell]] |location=Hoboken, New Jersey |url=https://fr.book4you.org/book/2156095/fd863f |access-date=10 April 2022 |archive-date=22 April 2023 |archive-url=https://web.archive.org/web/20230422182643/https://fr.book4you.org/book/2156095/fd863f |url-status=dead }}</ref> Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.<ref>{{cite book |last1=Bot |first1=Alexandra |last2=Benites |first2=José |date=2005 |title=The importance of soil organic matter: key to drought-resistant soil and sustained food and production |isbn=978-92-5-105366-9 |publisher=[[Food and Agriculture Organization of the United Nations]] |location=Rome |url=http://www.fao.org/3/a-a0100e.pdf |access-date=10 April 2022}}</ref>


===Description===
== Composition ==
[[File:Estructura-suelo.jpg|thumb|right|alt= This is a diagram and related photograph of soil layers from bedrock to soil.|A, B, and C represent the [[soil horizon|soil profile]], a notation firstly coined by [[Vasily Dokuchaev]] (1846–1903), the father of pedology. Here, A is the [[topsoil]]; B is a [[regolith]]; C is a [[saprolite]] (a less-weathered regolith); the bottom-most layer represents the [[bedrock]].]]
{{Pie chart
{{Pie chart
|caption = Components of a loam soil by percent volume
|caption = Components of a silt loam soil by percent volume
|value1 = 25
|value1 = 25
|label1 = Water
|label1 = Water
|color1= blue
|color1 = blue
|value2 = 25
|value2 = 25
|label2 = Gases
|label2 = Gases
Line 58: Line 53:
}}
}}


A typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas.<ref name="McClellan2017">{{cite web |last=McClellan |first=Tai |title=Soil composition |url=https://www.ctahr.hawaii.edu/mauisoil/a_comp.aspx |publisher=University of Hawai‘i College of Tropical Agriculture and Human Resources |accessdate=29 April 2018}}</ref> The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other.<ref>{{cite web |title=Arizona Master Gardener Manual |url=http://ag.arizona.edu/pubs/garden/mg/soils/soils.html |publisher=Cooperative Extension, College of Agriculture, University of Arizona |accessdate=17 December 2017 |deadurl=no |archiveurl=https://web.archive.org/web/20160529015259/http://ag.arizona.edu/pubs/garden/mg/soils/soils.html |archivedate=29 May 2016 |df=dmy-all }}</ref> The pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil.<ref name="Vannier1987">{{cite journal |last=Vannier |first=Guy |journal=Biology and Fertility of Soils |volume=3 |issue=1 |title=The porosphere as an ecological medium emphasized in Professor Ghilarov's work on soil animal adaptations |year=1987 |url=https://eurekamag.com/pdf/001/001713676.pdf |pages=39–44 |doi=10.1007/BF00260577 |accessdate=29 July 2018}}</ref> Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.<ref>{{cite journal |last1=Torbert |first1=H. Allen |last2=Wood |first2=Wes |lastauthoramp=yes |journal=Communications in Soil Science and Plant Analysis |volume=23 |issue=11 |title=Effect of soil compaction and water-filled pore space on soil microbial activity and N losses |year=1992 |url=https://www.researchgate.net/publication/240546132 |pages=1321‒31 |doi=10.1080/00103629209368668 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>
A typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas.<ref name="McClellan2017">{{cite web |last=McClellan |first=Tai |title=Soil composition |url=https://www.ctahr.hawaii.edu/mauisoil/a_comp.aspx |publisher=[[University of Hawaiʻi]] at Mānoa, College of Tropical Agriculture and Human Resources |access-date=18 April 2022}}</ref> The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other.<ref>{{cite web |title=Arizona Master Gardener Manual |url=http://ag.arizona.edu/pubs/garden/mg/soils/soils.html |publisher=Cooperative Extension, College of Agriculture, [[University of Arizona]] |access-date=17 December 2017 |url-status=dead |archive-url=https://web.archive.org/web/20160529015259/http://ag.arizona.edu/pubs/garden/mg/soils/soils.html |archive-date=29 May 2016 |date=9 November 2017}}</ref> The [[pore space]] allows for the infiltration and movement of air and water, both of which are critical for life existing in soil.<ref name="Vannier1987">{{cite journal |last=Vannier |first=Guy |journal=Biology and Fertility of Soils |volume=3 |issue=1 |title=The porosphere as an ecological medium emphasized in Professor Ghilarov's work on soil animal adaptations |year=1987 |url=https://link.springer.com/content/pdf/10.1007/BF00260577.pdf |pages=39–44 |doi=10.1007/BF00260577 |s2cid=297400 |access-date=18 April 2022}}</ref> [[Soil compaction|Compaction]], a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.<ref>{{cite journal |last1=Torbert |first1=H. Allen |last2=Wood |first2=Wes |journal=Communications in Soil Science and Plant Analysis |volume=23 |issue=11 |title=Effect of soil compaction and water-filled pore space on soil microbial activity and N losses |year=1992 |url=https://www.researchgate.net/publication/240546132 |pages=1321‒31 |doi=10.1080/00103629209368668 |bibcode=1992CSSPA..23.1321T |access-date=18 April 2022}}</ref>


Given sufficient time, an undifferentiated soil will evolve a [[soil horizon|soil profile]] which consists of two or more layers, referred to as [[soil horizon]]s, that differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity.<ref name="Buol"/> The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type of [[parent material]], the processes that modify those parent materials, and the [[Soil#soil-forming factors|soil-forming factors]] that influence those processes. The biological influences on soil properties are strongest near the surface, while the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The [[solum]] normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon.{{sfn|Simonson|1957|p=17}}
Given sufficient time, an undifferentiated soil will evolve a [[soil horizon|soil profile]] that consists of two or more layers, referred to as soil horizons. These differ in one or more properties such as in their [[Soil texture|texture]], [[structure]], [[density]], porosity, consistency, temperature, color, and [[Reactivity (chemistry)|reactivity]].<ref name="Buol"/> The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type of [[parent material]], the processes that modify those parent materials, and the [[#soil-forming factors|soil-forming factors]] that influence those processes. The biological influences on soil properties are strongest near the surface, though the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The [[solum]] normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon.{{sfn|Simonson|1957|p=17}} It has been suggested that the ''pedon'', a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the ''humipedon'' (the living part, where most soil organisms are dwelling, corresponding to the ''humus form''), the ''copedon'' (in intermediary position, where most [[weathering]] of minerals takes place) and the ''lithopedon'' (in contact with the subsoil).<ref>{{cite journal |last1=Zanella |first1=Augusto |last2=Katzensteiner |first2=Klaus |last3=Ponge |first3=Jean-François |last4=Jabiol |first4=Bernard |last5=Sartori |first5=Giacomo |last6=Kolb |first6=Eckart |last7=Le Bayon |first7=Renée-Claire |last8=Aubert |first8=Michaël |last9=Ascher-Jenull |first9=Judith |last10=Englisch |first10=Michael |last11=Hager |first11=Herbert |title=TerrHum: an iOS App for classifying terrestrial humipedons and some considerations about soil classification |journal=[[Soil Science Society of America Journal]] |date=June 2019 |volume=83 |issue=S1 |pages=S42–S48 |doi=10.2136/sssaj2018.07.0279 |hdl=11577/3315165 |s2cid=197555747 |url=https://www.researchgate.net/publication/332080061 |access-date=18 April 2022|hdl-access=free }}</ref>


The [[soil texture]] is determined by the relative proportions of the individual particles of sand, silt, and clay that make up the soil. The interaction of the individual mineral particles with organic matter, water, gases via [[Biotic component|biotic]] and [[abiotic]] processes causes those particles to [[flocculate]] (stick together) to form [[soil structure|aggregates]] or [[ped]]s.<ref name="Bronick2005">{{cite journal |last1=Bronick |first1=Carol J. |last2=Lal |first2=Ratan |lastauthoramp=yes |title=Soil structure and management: a review |journal=Geoderma |date=January 2005 |volume=124 |issue=1/2 |pages=3–22 |doi=10.1016/j.geoderma.2004.03.005 |url=http://tinread.usarb.md:8888/tinread/fulltext/lal/soil_structure.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2005Geode.124....3B }}</ref> Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, [[porosity]], [[consistency]], reaction ([[acidity]]), etc.
The soil texture is determined by the relative proportions of the individual particles of [[sand]], [[silt]], and [[clay]] that make up the soil. [[File:SoilTextureTriangle.svg|thumb|A [[Soil triangle|soil texture triangle]] plot is a visual representation of the proportions of sand, silt, and clay in a soil sample.]] The interaction of the individual mineral particles with organic matter, water, gases via [[Biotic component|biotic]] and [[abiotic]] processes causes those particles to [[flocculate]] (stick together) to form [[soil structure|aggregates]] or [[ped]]s.<ref name="Bronick2005">{{cite journal |last1=Bronick |first1=Carol J. |last2=Lal |first2=Ratan |title=Soil structure and management: a review |journal=Geoderma |date=January 2005 |volume=124 |issue=1–2 |pages=3–22 |doi=10.1016/j.geoderma.2004.03.005 |url=http://tinread.usarb.md:8888/tinread/fulltext/lal/soil_structure.pdf |access-date=18 April 2022 |bibcode=2005Geode.124....3B}}</ref> Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction ([[acidity]]), etc.


Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed.<ref>{{cite web |url=http://www.fao.org/docrep/r4082e/r4082e03.htm |title=Soil and water |website=[[Food and Agriculture Organization of the United Nations]] |accessdate=17 December 2017}}</ref> The mixture of water and dissolved or suspended materials that occupy the soil [[pore space]] is called the soil solution. Since [[#Water|soil water]] is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called the soil solution. Water is central to the [[Dissolution (chemistry)|dissolution]], [[Precipitation (chemistry)|precipitation]] and [[Leaching (agriculture)|leaching]] of minerals from the [[soil profile]]. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.<ref>{{cite journal |last1=Valentin |first1=Christian |last2=d'Herbès |first2=Jean-Marc |last3=Poesen |first3=Jean |lastauthoramp=yes |journal=Catena |volume=37 |issue=1 |title=Soil and water components of banded vegetation patterns |year=1999 |url=https://www.researchgate.net/publication/248379223 |pages=1‒24 |doi=10.1016/S0341-8162(99)00053-3 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>
Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed.<ref>{{cite web |url=https://www.fao.org/3/r4082e/r4082e03.htm |title=Soil and water |website=[[Food and Agriculture Organization of the United Nations]] |access-date=18 April 2022}}</ref> The mixture of water and dissolved or suspended materials that occupy the soil [[pore space]] is called the soil solution. Since soil water is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called the soil solution. Water is central to the [[Dissolution (chemistry)|dissolution]], [[Precipitation (chemistry)|precipitation]] and [[Leaching (agriculture)|leaching]] of minerals from the [[soil profile]]. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.<ref>{{cite journal |last1=Valentin |first1=Christian |last2=d'Herbès |first2=Jean-Marc |last3=Poesen |first3=Jean |journal=Catena |volume=37 |issue=1 |title=Soil and water components of banded vegetation patterns |year=1999 |url=https://www.academia.edu/35300713 |pages=1‒24 |doi=10.1016/S0341-8162(99)00053-3 |bibcode=1999Caten..37....1V |access-date=18 April 2022}}</ref>


Soils supply plants with nutrients, most of which are held in place by particles of [[Soil texture#Soil separates|clay]] and [[Soil organic matter|organic matter]] ([[colloid]]s)<ref>{{cite book |last=Barber |first=Stanley A. |date=1995 |chapter=Chemistry of soil-nutrient associations |title=Soil nutrient bioavailability: a mechanistic approach |pages=9–48 |edition=2nd |editor-last=Barber |editor-first=Stanley A. |publisher=John Wiley & Sons |location=New York |isbn=978-0-471-58747-7}}</ref> The nutrients may be [[Adsorption|adsorbed]] on clay mineral surfaces, bound within clay minerals ([[Absorption (chemistry)|absorbed]]), or bound within organic compounds as part of the living [[Soil organism|organisms]] or dead [[soil organic matter]]. These bound nutrients interact with soil water to buffer the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.<ref>{{cite web |url=http://eagri.org/eagri50/SSAC121/lec14.pdf |title=Soil colloids: properties, nature, types and significance |website=[[Tamil Nadu Agricultural University]] |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref><ref name="CEC">{{cite web |url=http://www.soilminerals.com/Cation_Exchange_Simplified.htm |title=Cation exchange capacity in soils, simplified |accessdate=17 December 2017}}</ref>
Soils supply [[plant]]s with [[nutrient]]s, most of which are held in place by particles of [[Soil texture#Soil separates|clay]] and organic matter ([[colloid]]s)<ref>{{cite book |last1=Brady |first1=Nyle C. |last2=Weil |first2=Ray R. |date=2007 |chapter=The colloidal fraction: seat of soil chemical and physical activity |title=The nature and properties of soils |pages=310–357 |edition=14th |editor-last1=Brady |editor-first1=Nyle C. |editor-last2=Weil |editor-first2=Ray R. |publisher=[[Pearson Education|Pearson]] |location=London, United Kingdom |isbn=978-0132279383 |chapter-url=https://www.researchgate.net/publication/309630422 |access-date=18 April 2022}}</ref> The nutrients may be [[Adsorption|adsorbed]] on clay mineral surfaces, bound within clay minerals ([[Absorption (chemistry)|absorbed]]), or bound within organic compounds as part of the living [[Soil organism|organisms]] or dead soil organic matter. These bound nutrients interact with soil water to [[Buffer solution|buffer]] the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.<ref>{{cite web |url=http://eagri.org/eagri50/SSAC121/lec14.pdf |title=Soil colloids: properties, nature, types and significance |website=[[Tamil Nadu Agricultural University]] |access-date=18 April 2022}}</ref>


Plant nutrient availability is affected by [[soil pH]], which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acid) where weathering is more advanced.<ref>{{cite web |url=http://drum.lib.umd.edu/bitstream/handle/1903/18519/FS-1054%20Soil%20pH%20and%20Nutrient%20Availbility.pdf |last=Miller |first=Jarrod O. |title=Soil pH affects nutrient availability |website=[[University of Maryland]] |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>
Plant nutrient availability is affected by [[soil pH]], which is a measure of the [[hydrogen]] [[Thermodynamic activity|ion activity]] in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acidic) where weathering is more advanced.<ref>{{cite web |url=https://www.researchgate.net/publication/305775103 |last=Miller |first=Jarrod O. |title=Soil pH affects nutrient availability |access-date=18 April 2022}}</ref>


Most plant nutrients, with the exception of nitrogen, originate from the [[mineral]]s that make up the soil parent material. Some nitrogen originates from rain as dilute [[nitric acid]] and [[ammonia]],<ref>{{cite journal |last1=Goulding |first1=Keith W.T. |last2=Bailey |first2=Neal J. |last3=Bradbury |first3=Nicola J. |last4=Hargreaves |first4=Patrick |last5=Howe |first5=MT |last6=Murphy |first6=Daniel V. |last7=Poulton |first7=Paul R. |last8=Willison |first8=Toby W. |lastauthoramp=yes |journal=[[New Phytologist]] |volume=139 |issue=1 |title=Nitrogen deposition and its contribution to nitrogen cycling and associated soil processes |year=1998 |pages=49‒58 |doi=10.1046/j.1469-8137.1998.00182.x }}</ref> but most of the nitrogen is available in soils as a result of [[nitrogen fixation]] by bacteria. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues ([[soil organic matter]]), mineral-bound forms, and the soil solution. Both living [[Soil microbe|microorganisms]] and [[soil organic matter]] are of critical importance to this recycling, and thereby to soil formation and soil fertility.<ref>{{cite book |last=Kononova |first=M.M. |date=2013 |title=Soil organic matter: its nature, its role in soil formation and in soil fertility |edition=2nd |publisher=[[Elsevier]] |location=Amsterdam|isbn=978-1-4831-8568-2}}</ref> Microbial activity in soils may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil by [[volatilisation]] (loss to the atmosphere as gases) or [[Leaching (agriculture)|leaching]].
Most plant nutrients, with the exception of [[nitrogen]], originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute [[nitric acid]] and [[ammonia]],<ref>{{cite journal |last1=Goulding |first1=Keith W.T. |last2=Bailey |first2=Neal J. |last3=Bradbury |first3=Nicola J. |last4=Hargreaves |first4=Patrick |last5=Howe |first5=M.T. |last6=Murphy |first6=Daniel V. |last7=Poulton |first7=Paul R. |last8=Willison |first8=Toby W. |journal=[[New Phytologist]] |volume=139 |issue=1 |title=Nitrogen deposition and its contribution to nitrogen cycling and associated soil processes |year=1998 |pages=49‒58 |doi=10.1046/j.1469-8137.1998.00182.x |doi-access=free}}</ref> but most of the nitrogen is available in soils as a result of [[nitrogen fixation]] by [[bacteria]]. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and the soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby to [[soil formation]] and [[soil fertility]].<ref>{{cite book |last=Kononova |first=M.M. |date=2013 |title=Soil organic matter: its nature, its role in soil formation and in soil fertility |edition=2nd |publisher=[[Elsevier]] |location=Amsterdam, the Netherlands |isbn=978-1-4831-8568-2 |url=https://fr1lib.org/book/2275488/ea4395 |access-date=24 April 2022 |archive-date=22 March 2023 |archive-url=https://web.archive.org/web/20230322091500/https://fr1lib.org/book/2275488/ea4395 |url-status=dead }}</ref> Microbial [[soil enzyme]]s may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil by [[volatilisation]] (loss to the atmosphere as gases) or leaching.<ref>{{cite journal |last1=Burns |first1=Richards G. |last2=DeForest |first2=Jared L. |last3=Marxsen |first3=Jürgen |last4=Sinsabaugh |first4=Robert L. |last5=Stromberger |first5=Mary E. |last6=Wallenstein |first6=Matthew D. |last7=Weintraub |first7=Michael N. |last8=Zoppini |first8=Annamaria |journal=[[Soil Biology and Biochemistry]] |volume=58 |title=Soil enzymes in a changing environment: current knowledge and future directions |year=2013 |pages=216‒34 |doi=10.1016/j.soilbio.2012.11.009 |bibcode=2013SBiBi..58..216B |url=https://www.academia.edu/25235991 |access-date=24 April 2022}}</ref>


==History of studies==
== Formation ==
{{main|Soil formation}}
{{Further|Soil mechanics#Genesis}}
Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, [[humus]], [[iron oxide]], [[carbonate]], and [[gypsum]], producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time.<ref>{{cite journal |last1=Sengupta |first1=Aditi |last2=Kushwaha |first2=Priyanka |last3=Jim |first3=Antonia |last4=Troch |first4=Peter A. |last5=Maier |first5=Raina |date=2020 |title=New soil, old plants, and ubiquitous microbes: evaluating the potential of incipient basaltic soil to support native plant growth and influence belowground soil microbial community composition |journal=[[Sustainability (journal)|Sustainability]] |volume=12 |issue=10 |pages=4209 |doi=10.3390/su12104209 |doi-access=free}}</ref> These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive [[soil horizons]]. However, more recent definitions of soil embrace soils without any organic matter, such as those [[regolith]]s that formed on Mars<ref>{{cite journal |last1=Bishop |first1=Janice L. |last2=Murchie |first2=Scott L. |last3=Pieters |first3=Carlé L. |last4=Zent |first4=Aaron P. |date=2002 |title=A model for formation of dust, soil, and rock coatings on Mars: physical and chemical processes on the Martian surface |journal=[[Journal of Geophysical Research]] |volume=107 |issue=E11 |pages=7-1–7-17 |doi=10.1029/2001JE001581 |bibcode=2002JGRE..107.5097B |doi-access=free}}</ref> and analogous conditions in planet Earth deserts.<ref>{{cite journal |last1=Navarro-González |first1=Rafael |last2=Rainey |first2=Fred A. |last3=Molina |first3=Paola |last4=Bagaley |first4=Danielle R. |last5=Hollen |first5=Becky J. |last6=de la Rosa |first6=José |last7=Small |first7=Alanna M. |last8=Quinn |first8=Richard C. |last9=Grunthaner |first9=Frank J. |last10=Cáceres |first10=Luis |last11=Gomez-Silva |first11=Benito |last12=McKay |first12=Christopher P. |date=2003 |title=Mars-like soils in the Atacama desert, Chile, and the dry limit of microbial life |journal=[[Science (journal)|Science]] |volume=302 |issue=5647 |pages=1018–1021 |doi=10.1126/science.1089143 |pmid=14605363 |url=https://www.researchgate.net/publication/9020258 |access-date=24 April 2022 |bibcode=2003Sci...302.1018N |s2cid=18220447}}</ref>


An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage [[nitrogen-fixing]] [[lichen]]s and [[cyanobacteria]] then [[epilithic]] [[higher plants]]) become established very quickly on [[basalt]]ic lava, even though there is very little organic material.<ref>{{cite journal |last1=Guo |first1=Yong |last2=Fujimura |first2=Reiko |last3=Sato |first3=Yoshinori |last4=Suda |first4=Wataru |last5=Kim |first5=Seok-won |last6=Oshima |first6=Kenshiro |last7=Hattori |first7=Masahira |last8=Kamijo |first8=Takashi |last9=Narisawa |first9=Kazuhiko |last10=Ohta |first10=Hiroyuki |date=2014 |title=Characterization of early microbial communities on volcanic deposits along a vegetation gradient on the island of Miyake, Japan |journal=Microbes and Environments |volume=29 |issue=1 |pages=38–49 |doi=10.1264/jsme2.ME13142 |pmid=24463576 |pmc=4041228 |doi-access=free}}</ref> Basaltic minerals commonly weather relatively quickly, according to the [[Goldich dissolution series]].<ref>{{cite journal |last=Goldich |first=Samuel S. |date=1938 |title=A study in rock-weathering |url=https://fr.art1lib.org/book/60175497/a54b2b |journal=[[The Journal of Geology]] |volume=46 |issue=1 |pages=17–58 |bibcode=1938JG.....46...17G |doi=10.1086/624619 |issn=0022-1376 |access-date=24 April 2022 |s2cid=128498195 |archive-date=27 March 2022 |archive-url=https://web.archive.org/web/20220327065200/https://fr.art1lib.org/book/60175497/a54b2b |url-status=dead }}</ref> The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering [[Mycorrhiza|mycorrhizal fungi]]<ref name="Van Schöll2006">{{cite journal |last1=Van Schöll |first1=Laura |last2=Smits |first2=Mark M. |last3=Hoffland |first3=Ellis |date=2006 |title=Ectomycorrhizal weathering of the soil minerals muscovite and hornblende |journal=[[New Phytologist]] |volume=171 |issue=4 |pages=805–814 |doi=10.1111/j.1469-8137.2006.01790.x |pmid=16918551 |doi-access=free}}</ref> that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes,<ref>{{cite journal |last1=Stretch |first1=Rachelle C. |last2=Viles |first2=Heather A. |year=2002 |title=The nature and rate of weathering by lichens on lava flows on Lanzarote |journal=[[Geomorphology (journal)|Geomorphology]] |volume=47 |issue=1 |pages=87–94 |doi=10.1016/S0169-555X(02)00143-5 |bibcode=2002Geomo..47...87S |url=https://fr.art1lib.org/book/17831662/8253cd |access-date=24 April 2022 |archive-date=22 April 2023 |archive-url=https://web.archive.org/web/20230422182644/https://fr.art1lib.org/book/17831662/8253cd |url-status=dead }}</ref> inselbergs,<ref>{{cite journal |last1=Dojani |first1=Stephanie |last2=Lakatos |first2=Michael |last3=Rascher |first3=Uwe |last4=Waneck |first4=Wolfgang |last5=Luettge |first5=Ulrich |last6=Büdel |first6=Burkhard |year=2007 |title=Nitrogen input by cyanobacterial biofilms of an inselberg into a tropical rainforest in French Guiana |journal=Flora |volume=202 |issue=7 |pages=521–529 |doi=10.1016/j.flora.2006.12.001 |bibcode=2007FMDFE.202..521D |url=https://www.researchgate.net/publication/224026482 |access-date=21 March 2021}}</ref> and glacial moraines.<ref>{{cite journal |last1=Kabala |first1=Cesary |last2=Kubicz |first2=Justyna |year=2012 |title=Initial soil development and carbon accumulation on moraines of the rapidly retreating Werenskiold Glacier, SW Spitsbergen, Svalbard archipelago |journal=Geoderma |volume=175–176 |pages=9–20 |url=https://www.academia.edu/31221217 |doi=10.1016/j.geoderma.2012.01.025 |access-date=24 April 2022 |bibcode=2012Geode.175....9K}}</ref>
===Fertility===
The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for our animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.<ref>{{cite book |last=Hillel |first=Daniel |date=1993 |title=Out of the Earth: civilization and the life of the soil |publisher=[[University of California Press]] |location=Berkeley|isbn=978-0-520-08080-5}}</ref>


How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil: parent material, climate, topography (relief), organisms, and time.<ref name="Jenny1941">{{cite book |last=Jenny |first=Hans |title=Factors of soil formation: a system of qunatitative pedology |year=1941 |publisher=[[McGraw-Hill]] |location=New York |url=http://netedu.xauat.edu.cn/sykc/hjx/content/ckzl/6/2.pdf |access-date=24 April 2022 |archive-url=https://web.archive.org/web/20170808104008/http://netedu.xauat.edu.cn/sykc/hjx/content/ckzl/6/2.pdf |archive-date=8 August 2017 |url-status=live}}</ref> When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.<ref>{{cite web |url=http://www.tsu-excel4ed.org/reviews/Geography%20Template_The%20Physical%20Environment_Cunha.pdf |title=The physical environment: an introduction to physical geography |first=Michael E. |last=Ritter |access-date=24 April 2022}}</ref>
The Greek historian [[Xenophon]] (450–355 BCE) is credited with being the first to expound upon the merits of green-manuring crops: "But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much as dung."{{sfn|Donahue|Miller|Shickluna|1977|p=4}}


== Physical properties ==
[[Columella]]'s "Husbandry," circa 60 CE, advocated the use of lime and that [[clover]] and [[alfalfa]] ([[green manure]]) should be turned under, and was used by 15 generations (450 years) under the [[Roman Empire]] until its collapse.{{sfn|Donahue|Miller|Shickluna|1977|p=4}}{{sfn|Kellogg|1957|p=1}} From the [[fall of Rome]] to the [[French Revolution]], knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European [[Middle Ages]], [[Ibn al-'Awwam|Yahya Ibn al-'Awwam]]'s handbook,<ref>{{cite book |language=fr |last=[[Ibn al-'Awwam]] |date=1864 |title=Le livre de l'agriculture, traduit de l'arabe par Jean Jacques Clément-Mullet |publisher=Librairie A. Franck |location=Paris|url=https://catalog.hathitrust.org/Record/009953450 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> with its emphasis on irrigation, guided the people of North Africa, Spain and the Middle East; a translation of this work was finally carried to the southwest of the United States when under Spanish influence.<ref>{{cite book |last=Jelinek |first=Lawrence J. |date=1982 |title=Harvest empire: a history of California agriculture |publisher=Boyd and Fraser |location=San Francisco |isbn=978-0-87835-131-2}}</ref> [[Olivier de Serres]], considered as the father of French [[agronomy]], was the first to suggest the abandonment of [[fallowing]] and its replacement by hay [[meadows]] within [[crop rotation]]s, and he highlighted the importance of soil (the French [[terroir]]) in the management of [[vineyard]]s. His famous book ''Le Théâtre d’Agriculture et mesnage des champs''<ref>{{cite book |language=fr |last=de Serres |first=Olivier |date=1600 |title=Le Théâtre d'Agriculture et mesnage des champs |publisher=Jamet Métayer |location=Paris |url=http://gallica.bnf.fr/ark:/12148/bpt6k738381/f1.image |accessdate=17 December 2017}}</ref> contributed to the rise of modern, [[sustainable agriculture]] and to the collapse of old [[agricultural practices]] such as the lifting of [[forest litter]] for the [[amendment]] of crops (the French ''soutrage'') and [[assarting]], which ruined the soils of western Europe during [[Middle Ages]] and even later on according to regions.<ref>{{cite journal |last1=Virto |first1=Iñigo |last2=Imaz |first2=María José |last3=Fernández-Ugalde |first3=Oihane |last4=Gartzia-Bengoetxea |first4=Nahia |last5=Enrique |first5=Alberto |last6=Bescansa |first6=Paloma |lastauthoramp=yes |journal=Sustainability |volume=7 |issue=1 |title=Soil degradation and soil quality in western Europe: current situation and future perspectives |url=http://www.mdpi.com/2071-1050/7/1/313/htm |year=2015 |pages=313–65 |doi=10.3390/su7010313 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>
{{main|Physical properties of soil}}
{{for|the academic discipline|Soil physics}}


The physical properties of soils, in order of decreasing importance for ecosystem services such as [[crop production]], are [[Soil texture|texture]], [[Soil structure|structure]], [[bulk density]], [[Pore space in soil|porosity]], consistency, [[temperature]], [[Soil color|colour]] and [[Soil resistivity|resistivity]].<ref>{{cite book |last1=Gardner |first1=Catriona M.K. |last2=Laryea |first2=Kofi Buna |last3=Unger |first3=Paul W. |date=1999 |title=Soil physical constraints to plant growth and crop production |edition=first |location=Rome, Italy |publisher=[[Food and Agriculture Organization of the United Nations]] |url=http://www.plantstress.com/Files/Soil_Physical_Constraints.pdf |archive-url=https://web.archive.org/web/20170808175354/http://www.plantstress.com/Files/Soil_Physical_Constraints.pdf |archive-date=8 August 2017 |url-status=dead }}</ref> Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: [[sand]], [[silt]], and [[clay]]. At the next larger scale, soil structures called [[ped]]s or more commonly ''soil aggregates'' are created from the soil separates when [[iron oxide]]s, [[carbonate]]s, clay, [[silica]] and [[humus]], coat particles and cause them to adhere into larger, relatively [[Soil aggregate stability|stable]] secondary structures.<ref>{{cite journal |last1=Six |first1=Johan |last2=Paustian |first2=Keith |last3=Elliott |first3=Edward T. |last4=Combrink |first4=Clay |journal=[[Soil Science Society of America Journal]] |volume=64 |issue=2 |title=Soil structure and organic matter. I. Distribution of aggregate-size classes and aggregate-associated carbon |url=https://www.researchgate.net/publication/280798601 |year=2000 |pages=681–689 |doi=10.2136/sssaj2000.642681x |access-date=7 August 2022 |bibcode=2000SSASJ..64..681S}}</ref> Soil [[bulk density]], when determined at standardized moisture conditions, is an estimate of [[Soil compaction (agriculture)|soil compaction]].<ref>{{cite journal |last1=Håkansson |first1=Inge |last2=Lipiec |first2=Jerzy |journal=Soil and Tillage Research |volume=53 |issue=2 |title=A review of the usefulness of relative bulk density values in studies of soil structure and compaction |url=https://www.researchgate.net/publication/222541793 |year=2000 |pages=71–85 |doi=10.1016/S0167-1987(99)00095-1 |bibcode=2000STilR..53...71H |s2cid=30045538 |access-date=26 October 2023 |archive-date=16 May 2022 |archive-url=https://web.archive.org/web/20220516120555/http://directory.umm.ac.id/Data%20Elmu/jurnal/S/Soil%20%26%20Tillage%20Research/Vol53.Issue2.Jan2000/1452.pdf |url-status=live }}</ref> Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil.<ref>{{cite journal |last=Schwerdtfeger |first=William J. |journal=[[Journal of Research of the National Bureau of Standards]] |volume=69C |issue=1 |title=Soil resistivity as related to underground corrosion and cathodic protection |year=1965 |pages=71–77 |doi=10.6028/jres.069c.012 |url=https://nvlpubs.nist.gov/nistpubs/jres/69C/jresv69Cn1p71_A1b.pdf |access-date=7 August 2022}}</ref> These properties vary through the depth of a soil profile, i.e. through [[soil horizon]]s. Most of these properties determine the [[Permeability of soils|aeration]] of the soil and the ability of water to [[Infiltration (hydrology)|infiltrate]] and to be [[Soil water (retention)|held]] within the soil.<ref>{{cite book |last=Tamboli |first=Prabhakar Mahadeo |date=1961 |title=The influence of bulk density and aggregate size on soil moisture retention |location=Ames, Iowa |publisher=[[Iowa State University]] |url=https://dr.lib.iastate.edu/bitstreams/85621186-4b03-4140-ad1c-b18c3ab3b4a8/download |access-date=7 August 2022}}</ref>
Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.<ref>{{cite journal |last1=Van der Ploeg |first1=Rienk R. |last2=Schweigert |first2=Peter |last3=Bachmann |first3=Joerg |lastauthoramp=yes |journal=[[Scientific World Journal]] |volume=1 |issue=S2 |title=Use and misuse of nitrogen in agriculture: the German story |url=https://www.hindawi.com/journals/tswj/2001/683180/abs/ |year=2001 |pages=737–44 |doi=10.1100/tsw.2001.263 |pmid=12805882 |pmc=6084271 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> In about 1635, the Flemish chemist [[Jan Baptist van Helmont]] thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight.<ref name="Brady"/>{{sfn|Kellogg|1957|p=3}} [[John Woodward (naturalist)|John Woodward]] (d. 1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, [[Jethro Tull (agriculturist)|Jethro Tull]] demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.<ref name="Brady">{{cite book |last=Brady |first=Nyle C. |title=The nature and properties of soils |edition=9th |year=1984 |publisher=[[Collier Macmillan]] |location=New York |isbn=978-0-02-313340-4}}</ref>{{sfn|Kellogg|1957|p=2}}


== Soil moisture ==
As chemistry developed, it was applied to the investigation of [[soil fertility]]. The French chemist [[Antoine Lavoisier]] showed in about 1778 that plants and animals must [combust] oxygen internally to live and was able to deduce that most of the 165-pound weight of [[Jan Baptist van Helmont|van Helmont]]'s willow tree derived from air.<ref>{{cite journal |language=fr |last=de Lavoisier |first=Antoine-Laurent |journal=Mémoires de l'Académie Royale des Sciences |title=Mémoire sur la combustion en général |year=1777 |url=http://www.academie-sciences.fr/pdf/dossiers/Franklin/Franklin_pdf/Mem1777_p592.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> It was the French agriculturalist [[Jean-Baptiste Boussingault]] who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil.<ref>{{cite book |language=fr |last=Boussingault |first=Jean-Baptiste |title=Agronomie, chimie agricole et physiologie, volumes 1-5 |year=1860–1874 |publisher=Mallet-Bachelier |location=Paris|url=https://archive.org/details/8TSUP364_1 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> [[Justus von Liebig]] in his book ''Organic chemistry in its applications to agriculture and physiology'' (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced.<ref>{{cite book |last=von Liebig |first=Justus |title=Organic chemistry in its applications to agriculture and physiology |year=1840 |publisher=Taylor and Walton |location=London |url=https://archive.org/details/organicchemistry00liebrich |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by [[Alexander von Humboldt]]. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.<ref>{{cite journal |last=Way |first=J. Thomas |journal=Journal of the Royal Agricultural Society of England |title=On the composition and money value of the different varieties of guano |year=1849 |volume=10 |pages=196–230 |url=https://www.biodiversitylibrary.org/item/37078#page/220/mode/1up |accessdate=17 December 2017}}</ref>
{{Main|Soil moisture}}
Soil [[water content]] can be measured as volume or [[Specific weight#Soil mechanics|weight]]. Soil moisture levels, in order of decreasing water content, are saturation, [[field capacity]], [[wilting point]], air dry, and oven dry. Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity. Irrigating soil above field capacity risks percolation losses. Wilting point describes the dry limit for growing plants. During growing season, soil moisture is unaffected by functional groups or specie richness.<ref name="auto1">{{Cite journal |last1=Spehn |first1=Eva M. |last2=Joshi |first2=Jasmin |last3=Schmid |first3=Bernhard |last4=Alphei |first4=Jörn |last5=Körner |first5=Christian |date=2000 |title=Plant diversity effects on soil heterotrophic activity in experimental grassland ecosystems |url=http://link.springer.com/10.1023/A:1004891807664 |journal=Plant and Soil |volume=224 |issue=2 |pages=217–230 |doi=10.1023/A:1004891807664|s2cid=25639544 }}</ref>


[[Available water capacity]] is the amount of water held in a soil profile available to plants. As water content drops, plants have to work against increasing forces of [[adhesion]] and [[sorptivity]] to withdraw water. [[Irrigation scheduling]] avoids [[moisture stress]] by replenishing depleted water before stress is induced.<ref>{{cite web |title=Water holding capacity |work=[[Oregon State University]] |date=24 June 2016 |url=https://forages.oregonstate.edu/ssis/soils/characteristics/water-holding-capacity |quote=Irrigators must have knowledge of the readily available moisture capacity so that water can be applied before plants have to expend excessive energy to extract moisture |access-date=9 October 2022}}</ref><ref>{{cite web |title=Basics of irrigation scheduling |work=[[University of Minnesota Extension]] |url=https://extension.umn.edu/irrigation/basics-irrigation-scheduling |quote=Only a portion of the available water holding capacity is easily used by the crop before crop water stress develop |access-date=9 October 2022}}</ref>
The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England [[John Bennet Lawes]] and [[Joseph Henry Gilbert]] worked in the [[Rothamsted Research|Rothamsted Experimental Station]], founded by the former, and (re)discovered that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the "[[superphosphate]]", consisting in the acid treatment of phosphate rock.{{sfn|Kellogg|1957|p=4}} This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of [[coke (fuel)|coke]] was recovered and used as fertiliser.<ref>{{cite web |last=Tandon |first=Hari L.S. |url=http://www.tandontech.net/fertilisers.html |title=A short history of fertilisers |website=Fertiliser Development and Consultation Organisation |accessdate=17 December 2017}}</ref> Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms still awaited discovery.


[[Capillary action]] is responsible for moving [[groundwater]] from wet regions of the soil to dry areas. [[Subirrigation]] designs (e.g., [[wicking bed]]s, [[sub-irrigated planter]]s) rely on [[Capillary action|capillarity]] to supply water to plant roots. Capillary action can result in an evaporative concentration of salts, causing land degradation through [[Soil salinity#Dry land salinity|salination]].
In 1856 J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates,<ref>{{cite journal |last=Way |first=J. Thomas |journal=Journal of the Royal Agricultural Society of England |title=On the power of soils to absorb manure |year=1852 |volume=13 |pages=123–43 |url=https://biodiversitylibrary.org/page/45583402 |accessdate=17 December 2017}}</ref> and twenty years later [[Robert Warington]] proved that this transformation was done by living organisms.<ref>{{cite book |last=Warington |first=Robert |title=Note on the appearance of nitrous acid during the evaporation of water: a report of experiments made in the Rothamsted laboratory |year=1878 |publisher=[[Harrison and Sons]] |location=London}}</ref> In 1890 [[Sergei Winogradsky]] announced he had found the bacteria responsible for this transformation.<ref>{{cite journal |language=fr |last=Winogradsky |first=Sergei |journal=[[Comptes Rendus de l'Académie des Sciences|Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences]] |title=Sur les organismes de la nitrification |year=1890 |volume=110 |issue=1 |pages=1013–16 |url=https://gallica.bnf.fr/ark:/12148/bpt6k30663/f1087 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>


[[Soil moisture measurement]]—measuring the water content of the soil, as can be expressed in terms of volume or weight—can be based on ''in situ'' probes (e.g., [[capacitance probe]]s, [[neutron probe]]s), or [[remote sensing]] methods. Soil moisture measurement is an important factor in determining changes in soil activity.<ref name="auto1"/>
It was known that certain [[legume]]s could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in [[nitrogen fixation]] by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist [[Hermann Hellriegel]] and the Dutch microbiologist [[Martinus Beijerinck]].{{sfn|Kellogg|1957|p=4}}


== Soil gas ==
[[Crop rotation]], mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.{{sfn|Kellogg|1957|pp=1–4}}
{{main|Soil gas}}
The atmosphere of soil, or [[soil gas]], is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO<sub>2</sub> concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.<ref>{{cite journal |last1=Qi |first1=Jingen |last2=Marshall |first2=John D. |last3=Mattson |first3=Kim G. |journal=[[New Phytologist]] |volume=128 |issue=3 |title=High soil carbon dioxide concentrations inhibit root respiration of Douglas fir |year=1994 |pages=435–442 |doi=10.1111/j.1469-8137.1994.tb02989.x |pmid=33874575 |doi-access=free}}</ref> Calcareous soils regulate CO<sub>2</sub> concentration by [[carbonate]] [[Buffering agent|buffering]], contrary to acid soils in which all CO<sub>2</sub> respired accumulates in the soil pore system.<ref>{{cite journal |last1=Karberg |first1=Noah J. |last2=Pregitzer |first2=Kurt S. |last3=King |first3=John S. |last4=Friend |first4=Aaron L. |last5=Wood |first5=James R. |journal=[[Oecologia]] |volume=142 |issue=2 |title=Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone |url=https://www.researchgate.net/publication/8337234 |year=2005 |pages=296–306 |doi=10.1007/s00442-004-1665-5 |pmid=15378342 |access-date=13 November 2022 |bibcode=2005Oecol.142..296K |s2cid=6161016}}</ref> At extreme levels, CO<sub>2</sub> is toxic.<ref>{{cite journal |last1=Chang |first1=H.T. |last2=Loomis |first2=Walter E. |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=20 |issue=2 |title=Effect of carbon dioxide on absorption of water and nutrients by roots |year=1945 |pages=221–232 |doi=10.1104/pp.20.2.221 |pmid=16653979 |pmc=437214 }}</ref> This suggests a possible [[negative feedback]] control of soil CO<sub>2</sub> concentration through its inhibitory effects on root and microbial respiration (also called [[soil respiration]]).<ref>{{cite journal |last1=McDowell |first1=Nate J. |last2=Marshall |first2=John D. |last3=Qi |first3=Jingen |last4=Mattson |first4=Kim |journal=Tree Physiology |volume=19 |issue=9 |title=Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations |year=1999 |pages=599–605 |doi=10.1093/treephys/19.9.599 |pmid=12651534 |doi-access=free}}</ref> In addition, the soil voids are saturated with water vapour, at least until the point of maximal [[hygroscopic]]ity, beyond which a [[vapour-pressure deficit]] occurs in the soil pore space.<ref name="Vannier1987"/> Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by [[diffusion]] from high concentrations to lower, the [[diffusion coefficient]] decreasing with [[Soil compaction (agriculture)|soil compaction]].<ref>{{cite journal |last1=Xu |first1=Xia |last2=Nieber |first2=John L. |last3=Gupta |first3=Satish C. |journal=[[Soil Science Society of America Journal]] |volume=56 |issue=6 |title=Compaction effect on the gas diffusion coefficient in soils |url=https://www.academia.edu/6547475 |year=1992 |pages=1743–1750 |doi=10.2136/sssaj1992.03615995005600060014x |access-date=13 November 2022 |bibcode=1992SSASJ..56.1743X}}</ref> Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including [[greenhouse gases]]) as well as water.<ref name="Smith2003">{{cite journal |last1=Smith |first1=Keith A. |last2=Ball |first2=Tom |last3=Conen |first3=Franz |last4=Dobbie |first4=Karen E. |last5=Massheder |first5=Jonathan |last6=Rey |first6=Ana |journal=European Journal of Soil Science |volume=54 |issue=4 |title=Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes |url=https://www.academia.edu/14433607 |year=2003 |pages=779–791 |doi=10.1046/j.1351-0754.2003.0567.x |bibcode=2003EuJSS..54..779S |s2cid=18442559 |access-date=13 November 2022}}</ref> [[Soil texture]] and [[soil structure|structure]] strongly affect soil porosity and gas diffusion. It is the total pore space ([[porosity]]) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air [[turbulence]] and temperature, that determine the rate of diffusion of gases into and out of soil.{{sfn|Russell|1957|pp=35–36}}<ref name="Smith2003"/> [[Ped#Platy|Platy]] soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO<sub>3</sub> to the gases N<sub>2</sub>, N<sub>2</sub>O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called [[denitrification]].<ref>{{cite journal |last1=Ruser |first1=Reiner |last2=Flessa |first2=Heiner |last3=Russow |first3=Rolf |last4=Schmidt |first4=G. |last5=Buegger |first5=Franz |last6=Munch |first6=J.C. |journal=[[Soil Biology and Biochemistry]] |volume=38 |issue=2 |title=Emission of N<sub>2</sub>O, N<sub>2</sub> and CO<sub>2</sub> from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting |url=https://www.sciencedirect.com/science/article/abs/pii/S0038071705001975 |year=2006 |pages=263–274 |doi=10.1016/j.soilbio.2005.05.005}}</ref> Aerated soil is also a net sink of methane (CH<sub>4</sub>)<ref>{{cite journal |last1=Hartmann |first1=Adrian A. |last2=Buchmann |first2=Nina |last3=Niklaus |first3=Pascal A. |journal=[[Plant and Soil]] |volume=342 |issue=1–2 |title=A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization |year=2011 |pages=265–275 |doi=10.1007/s11104-010-0690-x |bibcode=2011PlSoi.342..265H |hdl=20.500.11850/34759 |s2cid=25691034 |url=https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/34759/2/11104_2010_Article_690.pdf |access-date=13 November 2022}}</ref> but a net producer of methane (a strong heat-absorbing [[greenhouse gas]]) when soils are depleted of oxygen and subject to elevated temperatures.<ref>{{cite journal |last1=Moore |first1=Tim R. |last2=Dalva |first2=Moshe |journal=Journal of Soil Science |volume=44 |issue=4 |title=The influence of temperature and water table position on carbon dioxide and methane emissions from laboratory columns of peatland soils |url=https://www.researchgate.net/publication/229878721 |year=1993 |pages=651–664 |doi=10.1111/j.1365-2389.1993.tb02330.x |access-date=13 November 2022}}</ref>


Soil atmosphere is also the seat of emissions of [[Volatile (astrogeology)|volatiles]] other than carbon and nitrogen oxides from various soil organisms, e.g. roots,<ref>{{cite journal |last1=Hiltpold |first1=Ivan |last2=Toepfer |first2=Stefan |last3=Kuhlmann |first3=Ulrich |last4=Turlings |first4=Ted C.J. |journal=Chemoecology |volume=20 |issue=2 |title=How maize root volatiles affect the efficacy of entomopathogenic nematodes in controlling the western corn rootworm? |url=https://www.researchgate.net/publication/215470509 |year=2010 |pages=155–162 |doi=10.1007/s00049-009-0034-6 |bibcode=2010Checo..20..155H |s2cid=30214059 |access-date=13 November 2022}}</ref> bacteria,<ref>{{cite journal |last1=Ryu |first1=Choong-Min |last2=Farag |first2=Mohamed A. |last3=Hu |first3=Chia-Hui |last4=Reddy |first4=Munagala S. |last5= Wei |first5= Han-Xun |last6= Paré |first6=Paul W. |last7= Kloepper |first7= Joseph W. |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=100 |issue=8 |title=Bacterial volatiles promote growth in Arabidopsis |year=2003 |pages=4927–4932 |doi=10.1073/pnas.0730845100 |pmid=12684534 |pmc=153657 |bibcode=2003PNAS..100.4927R |doi-access=free}}</ref> fungi,<ref>{{cite journal |last1=Hung |first1=Richard |last2=Lee |first2=Samantha |last3=Bennett |first3=Joan W. |journal=[[Applied Microbiology and Biotechnology]] |volume=99 |issue=8 |title=Fungal volatile organic compounds and their role in ecosystems |url=https://www.researchgate.net/publication/273638498 |year=2015 |pages=3395–3405 |doi=10.1007/s00253-015-6494-4 |pmid=25773975 |s2cid=14509047 |access-date=13 November 2022}}</ref> animals.<ref>{{cite journal |last1=Purrington |first1=Foster Forbes |last2=Kendall |first2=Paricia A. |last3=Bater |first3=John E. |last4=Stinner |first4=Benjamin R. |journal=Great Lakes Entomologist |volume=24 |issue=2 |title=Alarm pheromone in a gregarious poduromorph collembolan (Collembola: Hypogastruridae) |url=https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1732&context=tgle |year=1991 |pages=75–78 |access-date=13 November 2022}}</ref> These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks<ref>{{cite journal |last1=Badri |first1=Dayakar V. |last2=Weir |first2=Tiffany L. |last3=Van der Lelie |first3=Daniel |last4=Vivanco |first4=Jorge M |journal=[[Current Opinion in Biotechnology]] |volume=20 |issue=6 |title=Rhizosphere chemical dialogues: plant–microbe interactions |url=http://www.bicga.org.uk/docs/Rhizosphere_chemical_dialogues_plant.pdf |doi=10.1016/j.copbio.2009.09.014 |pmid=19875278 |year=2009 |pages=642–650 |access-date=13 November 2022 |archive-date=21 September 2022 |archive-url=https://web.archive.org/web/20220921224421/http://www.bicga.org.uk/docs/Rhizosphere_chemical_dialogues_plant.pdf |url-status=dead }}</ref><ref>{{cite journal |last1=Salmon |first1=Sandrine |last2=Ponge |first2=Jean-François |journal=[[Soil Biology and Biochemistry]] |volume=33 |issue=14 |title=Earthworm excreta attract soil springtails: laboratory experiments on Heteromurus nitidus (Collembola: Entomobryidae) |url=https://www.academia.edu/20508985 |doi=10.1016/S0038-0717(01)00129-8 |year=2001 |pages=1959–1969 |bibcode=2001SBiBi..33.1959S |s2cid=26647480 |access-date=13 November 2022}}</ref> playing a decisive role in the stability, dynamics and evolution of soil ecosystems.<ref>{{cite journal |last1=Lambers |first1=Hans |last2=Mougel |first2=Christophe |last3=Jaillard |first3=Benoît |last4=Hinsinger |first4=Philipe |journal=[[Plant and Soil]] |volume=321 |issue=1–2 |title=Plant-microbe-soil interactions in the rhizosphere: an evolutionary perspective |url=https://www.academia.edu/25517742 |doi=10.1007/s11104-009-0042-x |year=2009 |pages=83–115 |bibcode=2009PlSoi.321...83L |s2cid=6840457 |access-date=13 November 2022}}</ref> [[Biogenic substance|Biogenic]] soil [[volatile organic compound]]s are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.<ref>{{cite journal |last1=Peñuelas |first1=Josep |last2=Asensio |first2=Dolores |last3=Tholl |first3=Dorothea |last4=Wenke |first4=Katrin |last5=Rosenkranz |first5=Maaria |last6=Piechulla |first6=Birgit |last7=Schnitzler |first7=Jörg-Petter |journal=[[Plant, Cell and Environment]] |volume=37 |issue=8 |title=Biogenic volatile emissions from the soil |year=2014 |pages=1866–1891 |doi=10.1111/pce.12340 |pmid=24689847 |doi-access=free}}</ref>
===Formation===
The scientists who studied the soil in connection with agricultural practices had considered it mainly as a static substrate. However, soil is the result of evolution from more ancient geological materials, under the action of [[Life|biotic]] and abiotic (not associated with life) processes. After studies of the improvement of the soil commenced, others began to study soil genesis and as a result also soil types and classifications.


Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,<ref>{{cite journal |last1=Buzuleciu |first1=Samuel A. |last2=Crane |first2=Derek P. |last3=Parker |first3=Scott L. |journal=[[Herpetological Conservation and Biology]] |volume=11 |issue=3 |title=Scent of disinterred soil as an olfactory cue used by raccoons to locate nests of diamond-backed terrapins (Malaclemys terrapin) |url=http://www.herpconbio.org/Volume_11/Issue_3/Buzuleciu_etal_2016.pdf |year=2016 |pages=539–551 |access-date=27 November 2022}}</ref> a bulk property attributed in a [[reductionist]] manner to particular biochemical compounds such as [[petrichor]] or [[geosmin]].
In 1860, in Mississippi, [[Eugene W. Hilgard]] studied the relationship among rock material, climate, and vegetation, and the type of soils that were developed. He realised that the soils were dynamic, and considered soil types classification.<ref>{{cite book |last=Hilgard |first=Eugene W. |title=Soils: their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions |year=1921 |publisher=[[The Macmillan Company]] |location=London |url=https://www.biodiversitylibrary.org/item/65783 |accessdate=17 December 2017}}</ref> Unfortunately his work was not continued. At about the same time, [[Friedrich Albert Fallou]] was describing soil profiles and relating soil characteristics to their formation as part of his professional work evaluating forest and farm land for the principality of [[Saxony]]. His 1857 book, Anfangsgründe der Bodenkunde (First principles of soil science) established modern soil science.<ref>{{cite book |language=German |last=Fallou |first=Friedrich Albert |title=Anfangsgründe der Bodenkunde |year=1857 |publisher=G. Schönfeld´s Buchhandlung |location=Dresden |url=http://digital.slub-dresden.de/fileadmin/data/321768043/321768043_tif/jpegs/321768043.pdf}}</ref> Contemporary with Fallou's work, and driven by the same need to accurately assess land for equitable taxation, [[Vasily Dokuchaev]] led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to western Europe until 1914 through a publication in German by [[Konstantin Glinka|Konstantin Dmitrievich Glinka]], a member of the Russian team.<ref>{{cite book |language=German |last=Glinka |first=Konstantin Dmitrievich |title=Die Typen der Bodenbildung: ihre Klassifikation und geographische Verbreitung |year=1914 |publisher=[[Borntraeger]] |location=Berlin}}</ref>


==Solid phase (soil matrix)==
[[Curtis F. Marbut]] was influenced by the work of the Russian team, translated Glinka's publication into English,<ref>{{cite book |last=Glinka |first=Konstantin Dmitrievich |title=The great soil groups of the world and their development |year=1927 |publisher=Edwards Brothers |location=Ann Arbor, Michigan}}</ref> and as he was placed in charge of the U.S. [[National Cooperative Soil Survey]], applied it to a national soil classification system.<ref name="Brady"/>
{{main|Soil matrix}}
Soil particles can be classified by their chemical composition ([[mineralogy]]) as well as their size. The [[Particle-size distribution|particle size distribution]] of a soil, its texture, determines many of the properties of that soil, in particular [[hydraulic conductivity]] and [[water potential]],<ref>{{cite journal |last1=Saxton |first1=Keith E. |last2=Rawls |first2=Walter J. |journal=[[Soil Science Society of America Journal]] |volume=70 |issue=5 |title=Soil water characteristic estimates by texture and organic matter for hydrologic solutions |url=https://hrsl.ba.ars.usda.gov/SPAW/Soil%20Water%20Characteristics-Paper.pdf |archive-url=https://web.archive.org/web/20180902183902/https://pdfs.semanticscholar.org/5e63/c886c4f68af5e5c242c006d2d882f0a65bfe.pdf |url-status=live |archive-date=2 September 2018 |year=2006 |pages=1569–1578 |doi=10.2136/sssaj2005.0117 |access-date=15 January 2023 |bibcode=2006SSASJ..70.1569S |s2cid=16826314}}</ref> but the [[mineralogy]] of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.<ref>{{cite web |last=[[College of Tropical Agriculture and Human Resources]] |title=Soil mineralogy |url=https://www.ctahr.hawaii.edu/mauisoil/a_factor_mineralogy.aspx |publisher=[[University of Hawaiʻi at Mānoa]] |access-date=15 January 2023}}</ref>


== Soil biodiversity ==
==Formation==
Large numbers of [[Microorganism|microbes]], [[animal]]s, [[plant]]s and [[Fungus|fungi]] are living in soil. However, [[biodiversity]] in soil is much harder to study as most of this life is invisible, hence estimates about soil biodiversity have been unsatisfactory. A recent study suggested that soil is likely home to 59 ± 15% of the species on Earth. [[Enchytraeidae]] (worms) have the greatest percentage of species in soil (98.6%), followed by fungi (90%), plants (85.5%), and termites ([[Termite|Isoptera]]) (84.2%). Many other groups of animals have substantial fractions of species living in soil, e.g. about 30% of [[insect]]s, and close to 50% of [[arachnid]]s.<ref>{{Cite journal |last1=Anthony |first1=Mark A. |last2=Bender |first2=S. Franz |last3=van der Heijden |first3=Marcel G. A. |date=2023-08-15 |title=Enumerating soil biodiversity |journal=Proceedings of the National Academy of Sciences |language=en |volume=120 |issue=33 |pages=e2304663120 |doi=10.1073/pnas.2304663120 |pmid=37549278 |pmc=10437432 |bibcode=2023PNAS..12004663A |issn=0027-8424|doi-access=free }}</ref> While most [[vertebrate]]s live above ground (ignoring aquatic species), many species are [[fossorial]], that is, they live in soil, such as most [[Scolecophidia|blind snakes]].
Soil formation, or [[pedogenesis]], is the combined effect of physical, chemical, biological and [[Human impact on the environment|anthropogenic]] processes working on soil parent material. Soil is said to be formed when organic matter has accumulated and [[colloid]]s are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum, producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time. These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive [[soil horizons]]. However, more recent definitions of soil embrace soils without any organic matter, such as those [[regolith]]s that formed on Mars<ref>{{cite journal |last1=Bishop |first1=Janice L. |last2=Murchie |first2=Scott L. |last3=Pieters |first3=Carlé L. |last4=Zent |first4=Aaron P. |lastauthoramp=yes |date=2002 |title=A model for formation of dust, soil, and rock coatings on Mars: physical and chemical processes on the Martian surface |journal=[[Journal of Geophysical Research]] |volume=107 |issue=E11 |pages=1–17 |doi=10.1029/2001JE001581 |bibcode=2002JGRE..107.5097B }}</ref> and analogous conditions in planet Earth deserts.<ref>{{cite journal |last1=Navarro-González |first1=Rafael |last2=Rainey |first2=Fred A. |last3=Molina |first3=Paola |last4=Bagaley |first4=Danielle R. |last5=Hollen |first5=Becky J. |last6=de la Rosa |first6=José |last7=Small |first7=Alanna M. |last8=Quinn |first8=Richard C. |last9=Grunthaner |first9=Frank J. |last10=Cáceres |first10=Luis |last11=Gomez-Silva |first11=Benito |last12=McKay |first12=Christopher P. |lastauthoramp=yes |date=2003 |title=Mars-like soils in the Atacama desert, Chile, and the dry limit of microbial life |journal=[[Science (journal)|Science]] |volume=302 |issue=5647 |pages=1018–21 |doi=10.1126/science.1089143 |pmid=14605363 |url=https://www.researchgate.net/publication/9020258 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2003Sci...302.1018N }}</ref>

An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage [[nitrogen-fixing]] [[lichens]] and [[cyanobacteria]] then [[epilithic]] [[higher plants]]) become established very quickly on [[basalt]]ic lava, even though there is very little organic material. The plants are supported by the porous rock as it is filled with [[nutrient]]-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-[[weathering]] [[Mycorrhiza|mycorrhizal fungi]]<ref name="Van Schöll2006">{{cite journal |last1=Van Schöll |first1=Laura |last2=Smits |first2=Mark M. |last3=Hoffland |first3=Ellis |lastauthoramp=yes |date=2006 |title=Ectomycorrhizal weathering of the soil minerals muscovite and hornblende |journal=[[New Phytologist]] |volume=171 |issue=4 |pages=805–14 |doi=10.1111/j.1469-8137.2006.01790.x |pmid=16918551 }}</ref> that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes,<ref>{{cite journal |last1=Jackson |first1=Togwell A. |last2=Keller |first2=Walter David |lastauthoramp=yes |year=1970 |title=A comparative study of the role of lichens and "inorganic" processes in the chemical weathering of recent Hawaiian lava flows |journal=[[American Journal of Science]] |volume=269 |issue=5 |pages=446–66 |url=http://www.ajsonline.org/content/269/5/446 |subscription=yes |accessdate=17 December 2017 |doi=10.2475/ajs.269.5.446|bibcode=1970AmJS..269..446J }}</ref> inselbergs,<ref>{{cite journal |last1=Dojani |first1=Stephanie |last2=Lakatos |first2=Michael |last3=Rascher |first3=Uwe |last4=Waneck |first4=Wolfgang |last5=Luettge |first5=Ulrich |last6=Büdel |first6=Burkhard |lastauthoramp=yes |year=2007 |title=Nitrogen input by cyanobacterial biofilms of an inselberg into a tropical rainforest in French Guiana |journal=Flora |volume=202 |issue=7 |pages=521–29 |url=https://ac.els-cdn.com/S0367253007000734/1-s2.0-S0367253007000734-main.pdf?_tid=9dc841c6-e315-11e7-b77c-00000aab0f02&acdnat=1513506944_d89cbe7ce2d887a4fb8909019f193e41 |doi=10.1016/j.flora.2006.12.001 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> and glacial moraines.<ref>{{cite journal |last1=Kabala |first1=Cesary |last2=Kubicz |first2=Justyna |lastauthoramp=yes |year=2012 |title=Initial soil development and carbon accumulation on moraines of the rapidly retreating Werenskiold Glacier, SW Spitsbergen, Svalbard archipelago |journal=Geoderma |volume=175/176 |pages=9–20 |url=http://www.sciencedirect.com/science/article/pii/S0016706112000511 |subscription=yes |accessdate=17 December 2017 |doi=10.1016/j.geoderma.2012.01.025|bibcode=2012Geode.175....9K }}</ref>

===Factors===

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil. They are: parent material, climate, topography (relief), organisms, and time.<ref name="Jenny1941">{{cite book |last=Jenny |first=Hans |title=Factors of soil formation: a system of qunatitative pedology |year=1941 |publisher=[[McGraw-Hill]] |location=New York |url=http://netedu.xauat.edu.cn/sykc/hjx/content/ckzl/6/2.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.<ref>{{cite web |url=http://www.earthonlinemedia.com/ebooks/tpe_3e/title_page.html |title=The physical environment: an introduction to physical geography |first=Michael E. |last=Ritter |accessdate=17 December 2017}}</ref>

====Parent material====
The mineral material from which a soil forms is called [[parent material]]. Rock, whether its origin is igneous, sedimentary, or metamorphic, is the source of all soil mineral materials and the origin of all plant nutrients with the exceptions of nitrogen, hydrogen and carbon. As the parent material is chemically and physically weathered, transported, deposited and precipitated, it is transformed into a soil.

Typical soil parent mineral materials are:{{sfn|Donahue|Miller|Shickluna|1977|pp=20–21}}
* [[Quartz]]: SiO<sub>2</sub>
* [[Calcite]]: CaCO<sub>3</sub>
* [[Feldspar]]: KAlSi<sub>3</sub>O<sub>8</sub>
* [[Mica]] (biotite): K(Mg,Fe)<sub>3</sub>AlSi<sub>3</sub>O<sub>10</sub>(OH)<sub>2</sub>

[[File:Lössacker.jpg|thumb|Soil, on an agricultural field in Germany, which has formed on [[loess]] parent material.]]
Parent materials are classified according to how they came to be deposited. Residual materials are mineral materials that have weathered in place from primary [[bedrock]]. Transported materials are those that have been deposited by water, wind, ice or gravity. Cumulose material is organic matter that has grown and accumulates in place.

Residual soils are soils that develop from their underlying parent rocks and have the same general chemistry as those rocks. The soils found on mesas, plateaux, and plains are residual soils. In the United States as little as three percent of the soils are residual.{{sfn|Donahue|Miller|Shickluna|1977|p=21}}

Most soils derive from transported materials that have been moved many miles by wind, water, ice and gravity.
* [[Aeolian processes]] (movement by wind) are capable of moving silt and fine sand many hundreds of miles, forming [[loess]] soils (60–90 percent silt),{{sfn|Donahue|Miller|Shickluna|1977|p=24}} common in the Midwest of North America, north-western Europe, Argentina and Central Asia. Clay is seldom moved by wind as it forms stable aggregates.
* Water-transported materials are classed as either alluvial, lacustrine, or marine. [[Alluvium|Alluvial materials]] are those moved and deposited by flowing water. [[Sediment|Sedimentary deposits]] settled in lakes are called [[Lacustrine plain|lacustrine]]. [[Lake Bonneville]] and many soils around the Great Lakes of the United States are examples. Marine deposits, such as soils along the Atlantic and Gulf Coasts and in the [[Imperial Valley]] of California of the United States, are the beds of ancient seas that have been revealed as the land uplifted.
* Ice moves parent material and makes deposits in the form of terminal and lateral [[moraine]]s in the case of stationary glaciers. Retreating glaciers leave smoother ground moraines and in all cases, outwash plains are left as alluvial deposits are moved downstream from the glacier.
* Parent material moved by gravity is obvious at the base of steep slopes as [[Scree|talus cones]] and is called [[colluvial material]].

Cumulose parent material is not moved but originates from deposited organic material. This includes [[peat]] and [[Muck (soil)|muck soils]] and results from preservation of plant residues by the low oxygen content of a high water table. While peat may form sterile soils, muck soils may be very fertile.

=====Weathering=====
The [[weathering]] of parent material takes the form of physical weathering (disintegration), chemical weathering (decomposition) and chemical transformation. Generally, minerals that are formed under high temperatures and pressures at great depths within the [[Earth's mantle]] are less resistant to weathering, while minerals formed at low temperature and pressure environment of the surface are more resistant to weathering.{{Citation needed|date=June 2017}} Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses and fluctuations generally decrease with depth.<ref>{{cite web |url=http://uregina.ca/~sauchyn/geog323/weather.html |title=Weathering |website=University of Regina |accessdate=17 December 2017}}</ref> Physical disintegration begins as rocks that have solidified deep in the Earth are exposed to lower pressure near the surface and swell and become mechanically unstable. Chemical decomposition is a function of mineral solubility, the rate of which doubles with each 10&nbsp;°C rise in temperature, but is strongly dependent on water to effect chemical changes. Rocks that will decompose in a few years in tropical climates will remain unaltered for millennia in deserts.<ref name="Gilluly1975"/> Structural changes are the result of hydration, oxidation, and reduction. Chemical weathering mainly results from the excretion of [[organic acids]] and [[chelating]] compounds by bacteria<ref>{{cite journal |last1=Uroz |first1=Stéphane |last2=Calvaruso |first2=Christophe |last3=Turpault |first3=Marie-Pierre |last4=Frey-Klett |first4=Pascale |lastauthoramp=yes |year=2009 |title=Mineral weathering by bacteria: ecology, actors and mechanisms |journal=[[Trends in Microbiology]] |volume=17 |issue=8 |pages=378–87 |url=http://www.sciencedirect.com/science/article/pii/S0966842X09001279 |subscription=yes |doi=10.1016/j.tim.2009.05.004 |pmid=19660952 |accessdate=17 December 2017}}</ref> and fungi,<ref name="Landeweert2001">{{cite journal |last1=Landeweert |first1=Renske |last2=Hoffland |first2=Ellis |last3=Finlay |first3=Roger D. |last4=Kuyper |first4=Thom W. |last5=Van Breemen |first5=Nico |lastauthoramp=yes |year=2001 |title=Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals |journal=[[Trends in Ecology and Evolution]] |volume=16 |issue=5 |pages=248–54 |url=https://www.sciencedirect.com/science/article/pii/S016953470102122X |subscription=yes |doi=10.1016/S0169-5347(01)02122-X |pmid=11301154 |accessdate=17 December 2017}}</ref> thought to increase under present-day [[greenhouse effect]].<ref>{{cite journal |last1=Andrews |first1=Jeffrey A. |last2=Schlesinger |first2=William H. |lastauthoramp=yes |year=2001 |title=Soil CO2 dynamics, acidification, and chemical weathering in a temperate forest with experimental CO2 enrichment |journal=Global Biogeochemical Cycles |volume=15 |issue=1 |pages=149–62 |doi=10.1029/2000GB001278 |bibcode=2001GBioC..15..149A }}</ref>
* '''Physical disintegration''' is the first stage in the transformation of parent material into soil. Temperature fluctuations cause expansion and contraction of the rock, splitting it along lines of weakness. Water may then enter the cracks and freeze and cause the physical splitting of material along a path toward the center of the rock, while temperature gradients within the rock can cause exfoliation of "shells". Cycles of wetting and drying cause soil particles to be abraded to a finer size, as does the physical rubbing of material as it is moved by wind, water, and gravity. Water can deposit within rocks minerals that expand upon drying, thereby stressing the rock. Finally, organisms reduce parent material in size and create crevices and pores through the mechanical action of plant roots and the digging activity of animals.{{sfn|Donahue|Miller|Shickluna|1977|pp=28–31}} Grinding of parent material by rock-eating animals also contributes to incipient soil formation.<ref>{{cite journal |last1=Jones |first1=Clive G. |last2=Shachak |first2=Moshe |lastauthoramp=yes |year=1990 |title=Fertilization of the desert soil by rock-eating snails |journal=[[Nature (journal)|Nature]] |volume=346 |issue=6287 |pages=839–41 |url=https://www.researchgate.net/publication/242874418 |doi=10.1038/346839a0 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1990Natur.346..839J }}</ref>
* '''Chemical decomposition''' and '''structural changes''' result when minerals are made soluble by water or are changed in structure. The first three of the following list are solubility changes and the last three are structural changes.{{sfn|Donahue|Miller|Shickluna|1977|pp=31–33}}

# The '''[[solution]]''' of salts in water results from the action of bipolar [[water molecules]] on [[ionic salt]] compounds producing a solution of ions and water, removing those minerals and reducing the rock's integrity, at a rate depending on [[water flow]] and pore channels.<ref>{{cite journal |last1=Li |first1=Li |last2=Steefel |first2=Carl I. |last3=Yang |first3=Li |lastauthoramp=yes |year=2008 |title=Scale dependence of mineral dissolution rates within single pores and fractures |journal=[[Geochimica et Cosmochimica Acta]] |volume=72 |issue=2 |pages=360–77 |url=http://lili.ems.psu.edu/publication/liligca08.pdf |doi=10.1016/j.gca.2007.10.027 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2008GeCoA..72..360L }}</ref>
# '''[[Hydrolysis]]''' is the transformation of minerals into [[Chemical polarity|polar]] molecules by the splitting of intervening water. This results in soluble [[acid-base]] pairs. For example, the hydrolysis of [[orthoclase]]-[[feldspar]] transforms it to acid [[silicate]] clay and basic [[potassium hydroxide]], both of which are more soluble.<ref>{{cite journal |last1=La Iglesia |first1=Ángel |last2=Martin-Vivaldi Jr |first2=Juan Luis |last3=López Aguayo |first3=Francisco |lastauthoramp=yes |year=1976 |title=Kaolinite crystallization at room temperature by homogeneous precipitation. III. Hydrolysis of feldspars |journal=Clays and Clay Minerals |volume=24 |issue=6287 |pages=36–42 |url=http://www.clays.org/journal/archive/volume%2024/24-1-36.pdf |doi=10.1038/346839a0 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1990Natur.346..839J }}</ref>
# In '''[[carbonation]]''', the solution of [[carbon dioxide]] in water forms [[carbonic acid]]. Carbonic acid will transform [[calcite]] into more soluble [[calcium bicarbonate]].<ref>{{cite journal |last1=Al-Hosney |first1=Hashim |last2=Grassian |first2=Vicki H. |lastauthoramp=yes |year=2004 |title=Carbonic acid: an important intermediate in the surface chemistry of calcium carbonate |journal=[[Journal of the American Chemical Society]] |volume=126 |issue=26 |pages=8068–69 |doi=10.1021/ja0490774 |pmid=15225019 }}</ref>
# '''[[hydration reaction|Hydration]]''' is the inclusion of water in a mineral structure, causing it to swell and leaving it stressed and easily [[Chemical decomposition|decomposed]].<ref>{{cite journal |last1=Jiménez-González |first1=Inmaculada |last2=Rodríguez‐Navarro |first2=Carlos |last3=Scherer |first3=George W. |lastauthoramp=yes |year=2008 |title=Role of clay minerals in the physicomechanical deterioration of sandstone |journal=[[Journal of Geophysical Research]] |volume=113 |issue=F02021 |pages=1–17 |doi=10.1029/2007JF000845 |bibcode=2008JGRF..113.2021J }}</ref>
# '''[[Oxidation]]''' of a mineral compound is the inclusion of [[oxygen]] in a mineral, causing it to increase its [[oxidation number]] and swell due to the relatively large size of oxygen, leaving it stressed and more easily attacked by water (hydrolysis) or carbonic acid (carbonation).<ref>{{cite journal |last1=Mylvaganam |first1=Kausala |last2=Zhang |first2=Liangchi |lastauthoramp=yes |year=2002 |title=Effect of oxygen penetration in silicon due to nano-indentation |journal=[[Nanotechnology (journal)|Nanotechnology]] |volume=13 |issue=5 |pages=623–26 |url=https://www.researchgate.net/publication/230680185 |doi=10.1088/0957-4484/13/5/316 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2002Nanot..13..623M }}</ref>
# '''[[redox|Reduction]]''', the opposite of oxidation, means the removal of oxygen, hence the oxidation number of some part of the mineral is reduced, which occurs when oxygen is scarce. The reduction of minerals leaves them electrically unstable, more soluble and internally stressed and easily decomposed. It mainly occurs in [[Waterlogging (agriculture)|waterlogged]] conditions.<ref>{{cite journal |last1=Favre |first1=Fabienne |last2=Tessier |first2=Daniel |last3=Abdelmoula |first3=Mustapha |last4=Génin |first4=Jean-Marie |last5=Gates |first5=Will P. |last6=Boivin |first6=Pascal |lastauthoramp=yes |year=2002 |title=Iron reduction and changes in cation exchange capacity in intermittently waterlogged soil |journal=European Journal of Soil Science |volume=53 |issue=2 |pages=175–83 |doi=10.1046/j.1365-2389.2002.00423.x }}</ref>

Of the above, hydrolysis and carbonation are the most effective, in particular in regions of high rainfall, temperature and physical [[erosion]].<ref>{{cite journal |last1=Riebe |first1=Clifford S. |last2=Kirchner |first2=James W. |last3=Finkel |first3=Robert C. |lastauthoramp=yes |year=2004 |title=Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes |journal=[[Earth and Planetary Science Letters]] |volume=224 |issue=3/4 |pages=547–62 |url=http://www.geog.ucsb.edu/~bodo/Geog295-Fall2012/riebe2004_mineral_weathering.pdf |doi=10.1016/j.epsl.2004.05.019 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2004E&PSL.224..547R }}</ref> [[Chemical weathering]] becomes more effective as the [[surface area]] of the rock increases, thus is favoured by physical disintegration.<ref>{{cite web |url=http://midwaymsscience.weebly.com/uploads/8/2/9/8/8298729/section_2_-_rates_of_weathering.pdf |title=Rates of weathering |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> This stems in latitudinal and altitudinal climate gradients in [[regolith]] formation.<ref>{{cite journal |last1=Dere |first1=Ashlee L. |last2=White |first2=Timothy S. |last3=April |first3=Richard H. |last4=Reynolds |first4=Bryan |last5=Miller |first5=Thomas E. |last6=Knapp |first6=Elizabeth P. |last7=McKay |first7=Larry D. |last8=Brantley |first8=Susan L.|lastauthoramp=yes |year=2013 |title=Climate dependence of feldspar weathering in shale soils along a latitudinal gradient |journal=[[Geochimica et Cosmochimica Acta]] |volume=122 |pages=101–26 |url=https://s3.amazonaws.com/academia.edu.documents/37982793/Dere_et_al._2013_Climate_dependence_feldspar_weathering.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1513511541&Signature=JWinkwnHL53dDoMphE8FfIbYb6c%3D&response-content-disposition=inline%3B%20filename%3DShale_weathering_rates_across_a_continen.pdf |doi=10.1016/j.gca.2013.08.001 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2013GeCoA.122..101D }}</ref><ref>{{cite journal |last1=Kitayama |first1=Kanehiro |last2=Majalap-Lee |first2=Noreen |last3=Aiba |first3=Shin-ichiro |lastauthoramp=yes |year=2000 |title=Soil phosphorus fractionation and phosphorus-use efficiencies of tropical rainforests along altitudinal gradients of Mount Kinabalu, Borneo |journal=[[Oecologia]] |volume=123 |issue=3 |pages=342–49 |doi=10.1007/s004420051020 |pmid=28308588 |bibcode=2000Oecol.123..342K }}</ref>

[[Saprolite]] is a particular example of a residual soil formed from the transformation of granite, metamorphic and other types of bedrock into clay minerals. Often called [weathered granite], saprolite is the result of weathering processes that include: [[hydrolysis]], [[chelation]] from organic compounds, [[hydration reaction|hydration]] (the solution of minerals in water with resulting cation and anion pairs) and physical processes that include [[freezing]] and [[thawing]]. The mineralogical and chemical composition of the primary [[bedrock]] material, its physical features, including grain size and degree of consolidation, and the rate and type of weathering transforms the parent material into a different mineral. The texture, pH and mineral constituents of saprolite are inherited from its parent material. This process is also called ''arenization'', resulting in the formation of sandy soils (granitic arenas), thanks to the much higher resistance of quartz compared to other mineral components of granite ([[micas]], [[amphiboles]], [[feldspars]]).<ref>{{cite journal |last1=Sequeira Braga |first1=Maria Amália |last2=Paquet |first2=Hélène |last3=Begonha |first3=Arlindo |lastauthoramp=yes |year=2002 |title=Weathering of granites in a temperate climate (NW Portugal): granitic saprolites and arenization |journal=Catena |volume=49 |issue=1/2 |pages=41–56 |url=http://home.uevora.pt/~lopes/Artigos/23.PDF |doi=10.1016/S0341-8162(02)00017-6 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

====Climate====
The principal climatic variables influencing soil formation are effective [[precipitation]] (i.e., precipitation minus [[evapotranspiration]]) and temperature, both of which affect the rates of chemical, physical, and biological processes. Temperature and moisture both influence the organic matter content of soil through their effects on the balance between [[primary production]] and [[decomposition]]: the colder or drier the climate the lesser atmospheric carbon is fixed as organic matter while the lesser organic matter is decomposed.<ref>{{cite journal |last1=Epstein |first1=Howard E. |last2=Burke |first2=Ingrid C. |last3=Lauenroth |first3=William K. |lastauthoramp=yes |year=2002 |title=Regional patterns of decomposition and primary production rates in the U.S. Great Plains |journal=[[Ecology (journal)|Ecology]] |volume=83 |issue=2 |pages=320–27 |url=https://www.researchgate.net/publication/233379719 |doi=10.1890/0012-9658(2002)083[0320:RPODAP]2.0.CO;2 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

Climate is the dominant factor in [[soil formation]], and soils show the distinctive characteristics of the [[climate zone]]s in which they form, with a feedback to climate through transfer of carbon stocked in soil horizons back to the atmosphere.<ref name="Davidson"/> If warm temperatures and abundant water are present in the profile at the same time, the processes of [[weathering]], [[Leaching (agriculture)|leaching]], and plant growth will be maximized. According to the climatic determination of [[biomes]], humid climates favor the growth of trees. In contrast, grasses are the dominant native vegetation in subhumid and [[semiarid]] regions, while shrubs and brush of various kinds dominate in arid areas.<ref>{{cite journal |last1=Woodward |first1=F. Ian |last2=Lomas |first2=Mark R. |last3=Kelly |first3=Colleen K. |lastauthoramp=yes |year=2004 |title=Global climate and the distribution of plant biomes |journal=[[Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences]] |volume=359 |issue=1450 |pages=1465–76 |url=https://www.researchgate.net/publication/8200458 |doi=10.1098/rstb.2004.1525 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]] |pmc=1693431 |pmid=15519965}}</ref>

Water is essential for all the major chemical weathering reactions. To be effective in soil formation, water must penetrate the [[regolith]]. The seasonal rainfall distribution, evaporative losses, site [[topography]], and [[soil permeability]] interact to determine how effectively precipitation can influence soil formation. The greater the depth of water penetration, the greater the depth of weathering of the soil and its development. Surplus water percolating through the soil profile transports soluble and suspended materials from the upper layers ([[eluviation]]) to the lower layers ([[illuviation]]), including clay particles<ref>{{cite journal |last=Fedoroff |first=Nicolas |year=1997 |title=Clay illuviation in Red Mediterranean soils |journal=Catena |volume=28 |issue=3/4 |pages=171–89 |url=http://www.sciencedirect.com/science/article/pii/S0341816296000367 |doi=10.1016/S0341-8162(96)00036-7 |accessdate=17 December 2017 |subscription=yes}}</ref> and [[dissolved organic matter]].<ref>{{cite journal |last1=Michalzik |first1=Beate |last2=Kalbitz |first2=Karsten |last3=Park |first3=Ji-Hyung |last4=Solinger |first4=Stephan |last5=Matzner |first5=Egbert |lastauthoramp=yes |year=2001 |title=Fluxes and concentrations of dissolved organic carbon and nitrogen: a synthesis for temperate forests |journal=Biogeochemistry |volume=52 |issue=2 |pages=173–205 |url=https://www.researchgate.net/publication/226356840 |doi=10.1023/A:1006441620810 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> It may also carry away soluble materials in the surface drainage waters. Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons. Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions. Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant<ref>{{cite journal |last=Bernstein |first=Leon |year=1975 |title=Effects of salinity and sodicity on plant growth |journal=[[Annual Review of Phytopathology]] |volume=13 |pages=295–312 |doi=10.1146/annurev.py.13.090175.001455 }}</ref> and microbial growth.<ref>{{cite journal |last1=Yuan |first1=Bing-Cheng |last2=Li |first2=Zi-Zhen |last3=Liu |first3=Hua |last4=Gao |first4=Meng |last5=Zhang |first5=Yan-Yu |lastauthoramp=yes |year=2007 |title=Microbial biomass and activity in salt affected soils under arid conditions |journal=Applied Soil Ecology |volume=35 |issue=2 |pages=319–28 |url=https://www.researchgate.net/publication/222652100 |doi=10.1016/j.apsoil.2006.07.004 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Soil profiles in arid and semi-arid regions are also apt to accumulate carbonates and certain types of expansive clays ([[calcrete]] or [[caliche]] horizons).<ref>{{cite journal |last=Schlesinger |first=William H. |year=1982 |title=Carbon storage in the caliche of arid soils: a case study from Arizona |journal=Soil Science |volume=133 |issue=4 |pages=247–55 |url=http://alliance.la.asu.edu/temporary/students/Phil/ArizonaCarbonStorage.pdf |archive-url=https://web.archive.org/web/20180304054729/http://alliance.la.asu.edu/temporary/students/Phil/ArizonaCarbonStorage.pdf |dead-url=yes |archive-date=4 March 2018 |doi=10.1146/annurev.py.13.090175.001455 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]] }}</ref><ref>{{cite journal |last1=Nalbantoglu |first1=Zalihe |last2=Gucbilmez |first2=Emin |lastauthoramp=yes |year=2001 |title=Improvement of calcareous expansive soils in semi-arid environments |journal=[[Journal of Arid Environments]] |volume=47 |issue=4 |pages=453–63 |url=http://www.sciencedirect.com/science/article/pii/S0140196300907262 |doi=10.1006/jare.2000.0726 |accessdate=17 December 2017 |subscription=yes|bibcode=2001JArEn..47..453N }}</ref> In tropical soils, when the soil has been deprived of vegetation (e.g. by deforestation) and thereby is submitted to intense evaporation, the upward capillary movement of water, which has dissolved iron and aluminum salts, is responsible for the formation of a superficial hard pan of [[laterite]] or [[bauxite]], respectively, which is improper for cutivation, a known case of irreversible [[soil degradation]] ([[lateritization]], bauxitization).<ref>{{cite journal |last=Retallack |first=Gregory J. |year=2010 |title=Lateritization and bauxitization events |journal=[[Economic Geology (journal)|Economic Geology]] |volume=105 |issue=3 |pages=655–67 |url=https://www.researchgate.net/publication/247864948 |doi=10.2113/gsecongeo.105.3.655 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

The direct influences of climate include:{{sfn|Donahue|Miller|Shickluna|1977|p=35}}
* A shallow accumulation of lime in low rainfall areas as [[caliche]]
* Formation of acid soils in humid areas
* Erosion of soils on steep hillsides
* Deposition of eroded materials downstream
* Very intense chemical weathering, leaching, and erosion in warm and humid regions where soil does not freeze

Climate directly affects the rate of weathering and leaching. Wind moves sand and smaller particles (dust), especially in arid regions where there is little plant cover, depositing it close<ref>{{cite book |last1=Pye |first1=Kenneth |last2=Tsoar |first2=Haim |lastauthoramp=yes |date=1987 |chapter=The mechanics and geological implications of dust transport and deposition in deserts with particular reference to loess formation and dune sand diagenesis in the northern Negev, Israel |doi=10.1144/GSL.SP.1987.035.01.10 |title=Desert sediments: ancient and modern |journal=Geological Society of London, Special Publications |volume=35 |issue=1 |editor1-last=Frostick |editor1-first=Lynne |editor2-last=Reid |editor2-first=Ian |pages=139–56|isbn=978-0-632-01905-2 |url=https://www.researchgate.net/publication/238424245 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1987GSLSP..35..139P }}</ref> or far from the entrainment source.<ref>{{cite journal |last=Prospero |first=Joseph M. |year=1999 |title=Long-range transport of mineral dust in the global atmosphere: impact of African dust on the environment of the southeastern United States |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=96 |issue=7 |pages=3396–403 |url=http://www.pnas.org/content/96/7/3396.full.pdf |doi=10.1073/pnas.96.7.3396 |pmid=10097049 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1999PNAS...96.3396P |pmc=34280 }}</ref> The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. Soil profiles are more distinct in wet and cool climates, where organic materials may accumulate, than in wet and warm climates, where organic materials are rapidly consumed.<ref>{{cite journal |last1=Post |first1=Wilfred M. |last2=Emanuel |first2=William R. |last3=Zinke |first3=Paul J. |last4=Stangerberger |first4=Alan G. |lastauthoramp=yes |year=1999 |title=Soil carbon pools and world life zones |journal=[[Nature (journal)|Nature]] |volume=298 |issue=5870 |pages=156–59 |url=https://www.nature.com/nature/journal/v298/n5870/abs/298156a0.html |doi=10.1038/298156a0 |accessdate=17 December 2017 |subscription=yes|bibcode=1982Natur.298..156P }}</ref> The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations, which favour [[tensile stress]]es in rock minerals, and thus their mechanical [[disaggregation]], a process called ''[[thermal fatigue]]''.<ref>{{cite journal |last1=Gómez-Heras |first1=Miguel |last2=Smith |first2=Bernard J. |last3=Fort |first3=Rafael |lastauthoramp=yes |year=2006 |title=Surface temperature differences between minerals in crystalline rocks: implications for granular disaggregation of granites through thermal fatigue |journal=[[Geomorphology (journal)|Geomorphology]] |volume=78 |issue=3/4 |pages=236–49 |url=https://s3.amazonaws.com/academia.edu.documents/9727713/2006_Geomorphology_78_236-249.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1513523203&Signature=ZXRrqJUs8pb1N5kF%2FqBhl9oPK5E%3D&response-content-disposition=inline%3B%20filename%3DSurface_temperature_differences_between.pdf |doi=10.1016/j.geomorph.2005.12.013 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2006Geomo..78..236G }}</ref> By the same process [[freeze-thaw]] cycles are an effective mechanism which breaks up rocks and other consolidated materials.<ref>{{cite journal |last1=Nicholson |first1=Dawn T. |last2=Nicholson |first2=Frank H. |lastauthoramp=yes |year=2000 |title=Physical deterioration of sedimentary rocks subjected to experimental freeze–thaw weathering |journal=[[Earth Surface Processes and Landforms]] |volume=25 |issue=12 |pages=1295–307 |doi=10.1002/1096-9837(200011)25:12<1295::AID-ESP138>3.0.CO;2-E |bibcode=2000ESPL...25.1295N }}</ref>

Climate also indirectly influences soil formation through the effects of vegetation cover and biological activity, which modify the rates of chemical reactions in the soil.<ref>{{cite journal |last=Lucas |first=Yves |year=2001 |title=The role of plants in controlling rates and products of weathering: importance of biological pumping |journal=[[Annual Review of Earth and Planetary Sciences]] |volume=29 |pages=135–63 |url=https://www.researchgate.net/publication/228608786 |doi=10.1146/annurev.earth.29.1.135 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2001AREPS..29..135L }}</ref>

====Topography====
The [[topography]], or [[relief]], is characterized by the inclination ([[slope]]), [[elevation]], and orientation of the terrain. Topography determines the rate of precipitation or [[Surface runoff|runoff]] and rate of formation or erosion of the surface [[soil profile]]. The topographical setting may either hasten or retard the work of climatic forces.

Steep slopes encourage rapid soil loss by [[erosion]] and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles. In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation. For all of these reasons, steep slopes prevent the formation of soil from getting very far ahead of soil destruction. Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites.<ref>{{cite journal |last1=Liu |first1=Baoyuan |last2=Nearing |first2=Mark A. |last3=Risse |first3=L. Mark |lastauthoramp=yes |year=1994 |title=Slope gradient effects on soil loss for steep slopes |journal=Transactions of the American Society of Agricultural and Biological Engineers |volume=37 |issue=6 |pages=1835–40 |url=https://www.researchgate.net/publication/270613706 |doi=10.13031/2013.28273 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

In [[swales]] and depressions where runoff water tends to concentrate, the regolith is usually more deeply weathered and soil profile development is more advanced. However, in the lowest landscape positions, water may saturate the regolith to such a degree that drainage and aeration are restricted. Here, the weathering of some minerals and the decomposition of organic matter are retarded, while the loss of iron and manganese is accelerated. In such low-lying topography, special profile features characteristic of [[wetland]] soils may develop. Depressions allow the accumulation of water, minerals and organic matter and in the extreme, the resulting soils will be [[saline marsh]]es or [[peat bog]]s. Intermediate topography affords the best conditions for the formation of an agriculturally productive soil.

====Organisms====
Soil is the most abundant [[ecosystem]] on Earth, but the vast majority of organisms in soil are [[microbes]], a great many of which have not been described.<ref name="Gans2005">{{cite journal |last1=Gans |first1=Jason |last2=Wolinsky |first2=Murray |last3=Dunbar |first3=John |lastauthoramp=yes |year=2005 |title=Computational improvements reveal great bacterial diversity and high metal toxicity in soil |journal=[[Science (journal)|Science]] |volume=309 |issue=5739 |pages=1387–90 |url=https://www.researchgate.net/publication/7637990 |doi=10.1126/science.1112665 |pmid=16123304 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2005Sci...309.1387G }}</ref><ref name="nature2008">{{cite journal |last=Dance |first=Amber |journal=[[Nature (journal)|Nature]] |title=What lies beneath |year=2008 |volume=455 |issue=7214 |pages=724–25 |pmid=18843336 |doi=10.1038/455724a |url=http://www.nature.com/news/2008/081008/pdf/455724a.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely from 50,000 per gram to over a million per gram of soil.<ref name="Gans2005"/><ref name="roesch">{{cite journal |last1=Roesch |first1=Luiz F.W. |last2=Fulthorpe |first2=Roberta R. |last3=Riva |first3=Alberto |last4=Casella |first4=George |last5=Hadwin |first5=Alison K.M. |last6=Kent |first6=Angela D. |last7=Daroub |first7=Samira H. |last8=Camargo |first8=Flavio A.O. |last9=Farmerie |first9=William G. |last10=Triplett |first10=Eric W. |lastauthoramp=yes |journal=[[The ISME Journal]] |title=Pyrosequencing enumerates and contrasts soil microbial diversity |year=2007 |volume=1 |issue=4 |pages=283–90 |pmc=2970868 |pmid=18043639 |doi=10.1038/ismej.2007.53|url=https://www.researchgate.net/publication/5803531 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> The total number of organisms and species can vary widely according to soil type, location, and depth.<ref name="nature2008"/><ref name="roesch"/>

Plants, [[soil fauna|animals]], fungi, [[bacteria]] and humans affect soil formation (see [[Soil Biomantle|soil biomantle]] and [[stonelayer]]). Soil animals, including soil [[macrofauna]] and [[soil mesofauna]], mix soils as they form [[burrow]]s and [[Porosity|pores]], allowing moisture and gases to move about, a process called [[bioturbation]].<ref>{{cite journal |last1=Meysman |first1=Filip J.R. |last2=Middelburg |first2=Jack J. |last3=Heip |first3=Carlo H.R. |lastauthoramp=yes |year=2006 |title=Bioturbation: a fresh look at Darwin's last idea |journal=[[Trends in Ecology and Evolution]] |volume=21 |issue=12 |pages=688–95 |url=https://www.academia.edu/13631880 |doi=10.1016/j.tree.2006.08.002 |pmid=16901581 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> In the same way, [[plant roots]] penetrate soil horizons and open channels upon decomposition.<ref>{{cite journal |last1=Williams |first1=Stacey M. |last2=Weil |first2=Ray R. |lastauthoramp=yes |year=2004 |title=Crop cover root channels may alleviate soil compaction effects on soybean crop |journal=[[Soil Science Society of America Journal]] |volume=68 |issue=4 |pages=1403–09 |url=https://www.researchgate.net/publication/240789602 |doi=10.2136/sssaj2004.1403 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2004SSASJ..68.1403W }}</ref> Plants with deep [[taproot]]s can penetrate many metres through the different soil layers to bring up [[nutrients]] from deeper in the profile.<ref>{{cite journal |last=Lynch |first=Jonathan |year=1995 |title=Root architecture and plant productivity |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=109 |issue=1 |pages=7–13 |url=https://www.researchgate.net/publication/11160545 |doi=10.1104/pp.109.1.7 |pmid=12228579 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|pmc=157559 }}</ref> Plants have fine roots that excrete organic compounds ([[sugars]], [[organic acids]], [[mucigel]]), slough off cells (in particular at their tip) and are easily decomposed, adding organic matter to soil, a process called ''rhizodeposition''.<ref>{{cite journal |last=Nguyen |first=Christophe |year=2003 |title=Rhizodeposition of organic C by plants: mechanisms and controls |journal=[[Agronomy for Sustainable Development|Agronomie]] |volume=23 |issue=5/6 |pages=375–96 |url=https://hal.archives-ouvertes.fr/file/index/docid/886190/filename/hal-00886190.pdf |doi=10.1051/agro:2003011 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Micro-organisms, including fungi and bacteria, effect chemical exchanges between roots and soil and act as a reserve of nutrients in a soil biological ''hotspot'' called [[rhizosphere]].<ref>{{cite book |last1=Widmer |first1=Franco |last2=Pesaro |first2=Manuel |last3=Zeyer |first3=Josef |last4=Blaser |first4=Peter |lastauthoramp=yes |date=2000 |chapter=Preferential flow paths: biological 'hot spots' in soils |doi=10.3929/ethz-a-004036424 |title=Highways through the soil: properties of preferential flow paths and transport of reactive compounds |editor-first=Maya |editor-last=Bundt |publisher=[[ETH]] Library |location=Zurich |pages=53–75 |url=https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/144808/eth-23683-02.pdf#page=64 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> The growth of roots through the soil stimulates [[microbial]] populations, stimulating in turn the activity of their [[predator]]s (notably [[amoeba]]), thereby increasing the [[mineralization (soil science)|mineralization rate]], and in last turn root growth, a [[positive feedback]] called the soil [[microbial loop]].<ref>{{cite journal |last=Bonkowski |first=Michael |year=2004 |title=Protozoa and plant growth: the microbial loop in soil revisited |journal=[[New Phytologist]] |volume=162 |issue=3 |pages=617–31 |doi=10.1111/j.1469-8137.2004.01066.x }}</ref> Out of root influence, in the [[bulk soil]], most bacteria are in a quiescent stage, forming micro[[aggregate (composite)|aggregates]], i.e. [[mucilage|mucilaginous]] colonies to which clay particles are glued, offering them a protection against [[desiccation]] and [[predation]] by soil [[microfauna]] ([[bacteriophagous]] [[protozoa]] and [[nematodes]]).<ref>{{cite journal |last1=Six |first1=Johan |last2=Bossuyt |first2=Heleen |last3=De Gryze |first3=Steven |last4=Denef |first4=Karolien |lastauthoramp=yes |year=2004 |title=A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics |journal=Soil and Tillage Research |volume=79 |issue=1 |pages=7–31 |url=http://www.sciencedirect.com/science/article/pii/S0167198704000881 |doi=10.1016/j.still.2004.03.008 |accessdate=17 December 2017 |subscription=yes}}</ref> Microaggregates (20-250&nbsp;µm) are ingested by [[soil mesofauna]] and [[macrofauna]], and bacterial bodies are partly or totally digested in their [[guts]].<ref>{{cite journal |last1=Saur |first1=Étienne |last2=Ponge |first2=Jean-François |lastauthoramp=yes |year=1988 |title=Alimentary studies on the collembolan Paratullbergia callipygos using transmission electron microscopy |journal=Pedobiologia |volume=31 |issue=5/6 |pages=355–79 |url=https://www.researchgate.net/publication/240321172 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

Humans impact soil formation by removing vegetation cover with [[erosion]], [[waterlogging (agriculture)|waterlogging]], [[lateritization]] or [[podzolization]] (according to climate and [[topography]]) as the result.<ref>{{cite book |last=Oldeman |first=L. Roel |date=1992 |chapter=Global extent of soil degradation |title=ISRIC Bi-Annual Report 1991/1992 |publisher=[[ISRIC]] |location=Wagenngen, The Netherlands |pages=19–36 |url=http://library.wur.nl/isric/fulltext/isricu_i26803_001.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Their [[tillage]] also mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers, resulting in net increased rate of mineral weathering.<ref>{{cite journal |last1=Karathanasis |first1=Anastasios D. |last2=Wells |first2=Kenneth L. |lastauthoramp=yes |year=2004 |title=A comparison of mineral weathering trends between two management systems on a catena of loess-derived soils |journal=[[Soil Science Society of America Journal]] |volume=53 |issue=2 |pages=582–88 |url=https://dl.sciencesocieties.org/publications/sssaj/abstracts/53/2/SS0530020582 |doi=10.2136/sssaj1989.03615995005300020047x |accessdate=2 January 2019 |subscription=yes|bibcode=1989SSASJ..53..582K }}</ref>

[[Earthworms]], [[ants]], [[termites]], [[Mole (animal)|moles]], [[gophers]], as well as some [[millipedes]] and [[tenebrionid]] beetles mix the soil as they burrow, significantly affecting soil formation.<ref name="Lee1991">{{cite journal |last1=Lee |first1=Kenneth Ernest |last2=Foster |first2=Ralph C. |lastauthoramp=yes |year=2003 |title=Soil fauna and soil structure |journal=[[Australian Journal of Soil Research]] |volume=29 |issue=6 |pages=745–75 |url=http://www.publish.csiro.au/sr/SR9910745 |doi=10.1071/SR9910745 |accessdate=17 December 2017 |subscription=yes}}</ref> Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through their bodies.<ref>{{cite journal |last=Scheu |first=Stefan |year=2003 |title=Effects of earthworms on plant growth: patterns and perspectives |journal=Pedobiologia |volume=47 |issue=5/6 |pages=846–56 |url=https://www.sciencedirect.com/science/article/pii/S0031405604702796 |doi=10.1078/0031-4056-00270 |accessdate=17 December 2017 |subscription=yes}}</ref> They aerate and stir the soil and create stable soil aggregates, after having disrupted links between soil particles during the intestinal transit of ingested soil,<ref>{{cite journal |last1=Zhang |first1=Haiquan |last2=Schrader |first2=Stefan |lastauthoramp=yes |year=1993 |title=Earthworm effects on selected physical and chemical properties of soil aggregates |journal=Biology and Fertility of Soils |volume=15 |issue=3 |pages=229–34 |doi=10.1007/BF00361617 }}</ref> thereby assuring ready infiltration of water.<ref>{{cite journal |last1=Bouché |first1=Marcel B. |last2=Al-Addan |first2=Fathel |lastauthoramp=yes |year=1997 |title=Earthworms, water infiltration and soil stability: some new assessments |journal=Soil Biology and Biochemistry |volume=29 |issue=3/4 |pages=441–52 |url=http://www.sciencedirect.com/science/article/pii/S0038071796002726 |doi=10.1016/S0038-0717(96)00272-6 |accessdate=17 December 2017 |subscription=yes}}</ref> In addition, as ants and termites build mounds, they transport soil materials from one horizon to another.<ref>{{cite journal |last=Bernier |first=Nicolas |year=1998 |title=Earthworm feeding activity and development of the humus profile |journal=Biology and Fertility of Soils |volume=26 |issue=3 |pages=215–23 |doi=10.1007/s003740050370 }}</ref> Other important functions are fulfilled by earthworms in the soil ecosystem, in particular their intense [[mucus]] production, both within the intestine and as a lining in their galleries,<ref>{{cite journal |last=Scheu |first=Stefan |year=1991 |title=Mucus excretion and carbon turnover of endogeic earthworms |journal=Biology and Fertility of Soils |volume=12 |issue=3 |pages=217–20 |url=https://www.researchgate.net/publication/226748808 |doi=10.1007/BF00337206 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> exert a [[Organic matter|priming effect]] on soil microflora,<ref>{{cite journal |last=Brown |first=George G. |year=1995 |title=How do earthworms affect microfloral and faunal community diversity? |journal=Plant and Soil |volume=170 |issue=1 |pages=209–31 |doi=10.1007/BF02183068 }}</ref> giving them the status of [[ecosystem engineer]]s, which they share with ants and termites.<ref>{{cite journal |last1=Jouquet |first1=Pascal |last2=Dauber |first2=Jens |last3=Lagerlöf |first3=Jan |last4=Lavelle |first4=Patrick |last5=Lepage |first5=Michel |lastauthoramp=yes |year=2006 |title=Soil invertebrates as ecosystem engineers: intended and accidental effects on soil and feedback loops |journal=Applied Soil Ecology |volume=32 |issue=2 |pages=153–64 |url=https://www.researchgate.net/publication/222669345 |doi=10.1016/j.apsoil.2005.07.004 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

In general, the mixing of the soil by the activities of animals, sometimes called [[pedoturbation]], tends to undo or counteract the tendency of other soil-forming processes that create distinct horizons.<ref>{{cite journal |last1=Bohlen |first1=Patrick J. |last2=Scheu |first2=Stefan |last3=Hale |first3=Cindy M. |last4=McLean |first4=Mary Ann |last5=Migge |first5=Sonja |last6=Groffman |first6=Peter M. |last7=Parkinson |first7=Dennis |lastauthoramp=yes |year=2004 |title=Non-native invasive earthworms as agents of change in northern temperate forests |journal=[[Frontiers in Ecology and the Environment]] |volume=2 |issue=8 |pages=427–35 |url=https://www.researchgate.net/publication/289148663 |doi=10.2307/3868431 |accessdate=13 August 2017 |format=[[Portable Document Format|PDF]]|jstor=3868431 }}</ref> Termites and ants may also retard soil profile development by denuding large areas of soil around their nests, leading to increased loss of soil by erosion.<ref>{{cite journal |last1=De Bruyn |first1=Lisa Lobry |last2=Conacher |first2=Arthur J. |lastauthoramp=yes |year=1990 |title=The role of termites and ants in soil modification: a review |journal=[[Australian Journal of Soil Research]] |volume=28 |issue=1 |pages=55–93 |url=https://www.researchgate.net/publication/248884324 |doi=10.1071/SR9900055 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Large animals such as gophers, moles, and prairie dogs bore into the lower soil horizons, bringing materials to the surface.<ref>{{cite web |url=http://etd.fcla.edu/UF/UFE0017403/kinlaw_a.pdf |last=Kinlaw |first=Alton Emory |title=Burrows of semi-fossorial vertebrates in upland communities of Central Florida: their architecture, dispersion and ecological consequences |pages=19–45 |year=2006 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Their tunnels are often open to the surface, encouraging the movement of water and air into the subsurface layers. In localized areas, they enhance mixing of the lower and upper horizons by creating, and later refilling, underground tunnels. Old animal burrows in the lower horizons often become filled with soil material from the overlying A horizon, creating profile features known as crotovinas.<ref>{{cite book |last=Borst |first=George |date=1968 |chapter=The occurrence of crotovinas in some southern California soils |title=Transactions of the 9th International Congress of Soil Science, Adelaide, Australia, August 5-15, 1968 |volume=2 |publisher=[[Angus & Robertson]] |location=Sidney |pages=19–27 |url=http://iuss.boku.ac.at/files/9th_international_congress_of_soil_science_transactions_volume_ii_compressed.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that might result from [[surface runoff]].<ref>{{cite journal |last1=Gyssels |first1=Gwendolyn |last2=Poesen |first2=Jean |last3=Bochet |first3=Esther |last4=Li |first4=Yong |lastauthoramp=yes |year=2005 |title=Impact of plant roots on the resistance of soils to erosion by water: a review |journal=[[Progress in Physical Geography]] |volume=29 |issue=2 |pages=189–217 |url=https://www.researchgate.net/publication/240729013 |doi=10.1191/0309133305pp443ra |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Plants shade soils, keeping them cooler<ref>{{cite journal |last1=Balisky |first1=Allen C. |last2=Burton |first2=Philip J. |lastauthoramp=yes |year=1993 |title=Distinction of soil thermal regimes under various experimental vegetation covers |journal=Canadian Journal of Soil Science |volume=73 |issue=4 |pages=411–20 |doi=10.4141/cjss93-043 }}</ref> and slow evaporation of [[soil moisture]],<ref>{{cite journal |last1=Marrou |first1=Hélène |last2=Dufour |first2=Lydie |last3=Wery |first3=Jacques |lastauthoramp=yes |year=2013 |title=How does a shelter of solar panels influence water flows in a soil-crop system? |journal=European Journal of Agronomy |volume=50 |pages=38–51 |url=http://www.sciencedirect.com/science/article/pii/S1161030113000683 |doi=10.1016/j.eja.2013.05.004 |accessdate=17 December 2017 |subscription=yes}}</ref> or conversely, by way of [[transpiration]], plants can cause soils to lose moisture, resulting in complex and highly variable relationships between [[leaf area index]] (measuring light interception) and moisture loss: more generally plants prevent soil from [[desiccation]] during driest months while they dry it during moister months, thereby acting as a buffer against strong moisture variation.<ref>{{cite journal |last1=Heck |first1=Pamela |last2=Lüthi |first2=Daniel |last3=Schär |first3=Christoph |lastauthoramp=yes |year=1999 |title=The influence of vegetation on the summertime evolution of European soil moisture |journal=Physics and Chemistry of the Earth, Part B, Hydrology, Oceans and Atmosphere |volume=24 |issue=6 |pages=609–14 |url=http://www.sciencedirect.com/science/article/pii/S1464190999000520 |doi=10.1016/S1464-1909(99)00052-0 |subscription=yes |accessdate=17 December 2017|bibcode=1999PCEB...24..609H }}</ref> Plants can form new chemicals that can break down minerals, both directly<ref>{{cite journal |last=Jones |first=David L. |year=1998 |title=Organic acids in the rhizospere: a critical review |journal=[[Plant and Soil]] |volume=205 |issue=1 |pages=25–44 |url=https://www.researchgate.net/publication/226305186 |doi=10.1023/A:1004356007312 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> and indirectly through mycorrhizal fungi<ref name="Landeweert2001"/> and rhizosphere bacteria,<ref>{{cite journal |last1=Calvaruso |first1=Christophe |last2=Turpault |first2=Marie-Pierre |last3=Frey-Klett |first3=Pascal |lastauthoramp=yes |year=2006 |title=Root-associated bacteria contribute to mineral weathering and to mineral nutrition in trees: a budgeting analysis |journal=[[Applied and Environmental Microbiology]] |volume=72 |issue=2 |pages=1258–66 |url=https://www.researchgate.net/publication/7312017 |doi=10.1128/AEM.72.2.1258-1266.2006 |pmid=16461674 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|pmc=1392890 }}</ref> and improve the soil structure.<ref>{{cite journal |last1=Angers |first1=Denis A. |last2=Caron |first2=Jean |year=1998 |title=Plant-induced changes in soil structure: processes and feedbacks |journal=Biogeochemistry |volume=42 |issue=1 |pages=55–72 |url=https://www.researchgate.net/publication/226938344 |doi=10.1023/A:1005944025343 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> The type and amount of vegetation depends on climate, topography, soil characteristics and biological factors, mediated or not by human activities.<ref>{{cite journal |last1=Dai |first1=Shengpei |last2=Zhang |first2=Bo |last3=Wang |first3=Haijun |last4=Wang |first4=Yamin |last5=Guo |first5=Lingxia |last6=Wang |first6=Xingmei |last7=Li |first7=Dan |lastauthoramp=yes |year=2011 |title=Vegetation cover change and the driving factors over northwest China |journal=Journal of Arid Land |volume=3 |issue=1 |pages=25–33 |url=https://www.researchgate.net/publication/228841309 |doi=10.3724/SP.J.1227.2011.00025 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Vogiatzakis |first1=Ioannis |last2=Griffiths |first2=Geoffrey H. |last3=Mannion |first3=Antoinette M. |lastauthoramp=yes |year=2003 |title=Environmental factors and vegetation composition, Lefka Ori Massif, Crete, S. Aegean |journal=[[Global Ecology and Biogeography]] |volume=12 |issue=2 |pages=131–46 |doi=10.1046/j.1466-822X.2003.00021.x }}</ref> Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location. Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.<ref>{{cite journal |last1=Brêthes |first1=Alain |last2=Brun |first2=Jean-Jacques |last3=Jabiol |first3=Bernard |last4=Ponge |first4=Jean-François |last5=Toutain |first5=François |lastauthoramp=yes |year=1995 |title=Classification of forest humus forms: a French proposal |journal=Annales des Sciences Forestières |volume=52 |issue=6 |pages=535–46 |url=https://www.researchgate.net/publication/45341270 |doi=10.1051/forest:19950602 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

Human activities widely influence [[soil formation]].<ref>{{cite journal |last=Dudal |first=Rudi |year=2005 |title=The sixth factor of soil formation |journal=Eurasian Soil Science |volume=38 |issue=Supplement 1 |pages=S60–S65 |url=http://www.css.cornell.edu/faculty/dgr2/research/suitma/Dudal_6thFactor.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> For example, it is believed that [[Native Americans in the United States|Native Americans]] regularly set fires to maintain several large areas of [[prairie]] grasslands in [[Indiana]] and [[Michigan]], although climate and mammalian [[grazers]] (e.g. [[bisons]]) are also advocated to explain the maintenance of the [[Great Plains]] of North America.<ref>{{cite journal |last=Anderson |first=Roger C. |year=2006 |title=Evolution and origin of the Central Grassland of North America: climate, fire, and mammalian grazers |journal=[[Journal of the Torrey Botanical Society]] |volume=133 |issue=4 |pages=626–47 |doi=10.3159/1095-5674(2006)133[626:EAOOTC]2.0.CO;2 }}</ref> In more recent times, human destruction of natural vegetation and subsequent [[tillage]] of the soil for [[crop]] production has abruptly modified soil formation.<ref>{{cite journal |last1=Burke |first1=Ingrid C. |last2=Yonker |first2=Caroline M. |last3=Parton |first3=William J. |last4=Cole |first4=C. Vernon |last5=Flach |first5=Klaus |last6=Schimel |first6=David S. |lastauthoramp=yes |year=1989 |title=Texture, climate, and cultivation effects on soil organic matter content in U.S. grassland soils |journal=[[Soil Science Society of America Journal]] |volume=53 |issue=3 |pages=800–05 |url=https://www.researchgate.net/publication/233209856 |doi=10.2136/sssaj1989.03615995005300030029x |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1989SSASJ..53..800B }}</ref> Likewise, [[irrigation|irrigating]] soil in an [[arid]] region drastically influences soil-forming factors,<ref>{{cite journal |last1=Lisetskii |first1=Fedor N. |last2=Pichura |first2=Vitalii I. |lastauthoramp=yes |year=2016 |title=Assessment and forecast of soil formation under irrigation in the steppe zone of Ukraine |journal=Russian Agricultural Sciences |volume=42 |issue=2 |pages=155–59 |url=http://dspace.bsu.edu.ru/bitstream/123456789/16324/1/Lisetskii_Assessment_Forecast_16_D.pdf |doi=10.3103/S1068367416020075 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> as does adding fertilizer and lime to soils of low fertility.<ref>{{cite web |url=https://stud.epsilon.slu.se/3263/1/schon_m_110919.pdf |last=Schön |first=Martina |title=Impact of N fertilization on subsoil properties: soil organic matter and aggregate stability |year=2011 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

====Time====
Time is a factor in the interactions of all the above.<ref name="Jenny1941"/> While a mixture of sand, silt and clay constitute the [[Soil texture|texture]] of a soil and the [[Particle aggregation|aggregation]] of those components produces [[ped]]s, the development of a distinct [[B horizon]] marks the development of a soil or [[pedogenesis]].<ref>{{cite journal |last1=Bormann |first1=Bernard T. |last2=Spaltenstein |first2=Henri |last3=McClellan |first3=Michael H. |last4=Ugolini |first4=Fiorenzo C. |last5=Cromack |first5=Kermit Jr |last6=Nay |first6=Stephan M. |lastauthoramp=yes |year=1995 |title=Rapid soil development after windthrow disturbance in pristine forests |journal=[[Journal of Ecology]] |volume=83 |issue=5 |pages=747–57 |url=http://www.fsl.orst.edu/ltep/Reprints_files/Bormann%20JE1995%20windthrow%20chrono.pdf |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]] |doi=10.2307/2261411|jstor=2261411 }}</ref> With time, soils will evolve features that depend on the interplay of the prior listed soil-forming factors.<ref name="Jenny1941"/> It takes decades<ref>{{cite journal |last1=Crocker |first1=Robert L. |last2=Major |first2=Jack |lastauthoramp=yes |year=1955 |title=Soil development in relation to vegetation and surface age at Glacier Bay, Alaska |journal=[[Journal of Ecology]] |volume=43 |issue=2 |pages=427–48 |url=http://www.britishecologicalsociety.org/100papers/100_Ecological_Papers/100_Influential_Papers_017.pdf |doi=10.2307/2257005 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|jstor=2257005 }}</ref> to several thousand years for a soil to develop a profile,<ref name="Crews1995">{{cite journal |last1=Crews |first1=Timothy E. |last2=Kitayama |first2=Kanehiro |last3=Fownes |first3=James H. |last4=Riley |first4=Ralph H. |last5=Herbert |first5=Darrell A. |last6=Mueller-Dombois |first6=Dieter |last7=Vitousek |first7=Peter M. |lastauthoramp=yes |year=1995 |title=Changes in soil phosphorus and ecosystem dynamics along a long term chronosequence in Hawaii |journal=[[Ecology (journal)|Ecology]] |volume=76 |issue=5 |pages=1407–24 |url=https://www.researchgate.net/publication/259671947 |doi=10.2307/1938144 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|jstor=1938144 }}</ref> although the notion of soil development has been criticized, soil being in a constant state-of-change under the influence of fluctuating soil-forming factors.<ref>{{cite journal |last=Huggett |first=Richard J. |year=1998 |title=Soil chronosequences, soil development, and soil evolution: a critical review |journal=[[Catena (soil)|Catena]] |volume=32 |issue=3/4 |pages=155–72 |url=http://www.sciencedirect.com/science/article/pii/S0341816298000538 |doi=10.1016/S0341-8162(98)00053-8 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]] |subscription=yes}}</ref> That time period depends strongly on climate, parent material, relief, and biotic activity.{{sfn|Simonson|1957|pp=20–21}}{{sfn|Donahue|Miller|Shickluna|1977|p=26}} For example, recently deposited material from a flood exhibits no soil development as there has not been enough time for the material to form a structure that further defines soil.<ref>{{cite journal |last1=Craft |first1=Christopher |last2=Broome |first2=Stephen |last3=Campbell |first3=Carlton |lastauthoramp=yes |year=2002 |title=Fifteen years of vegetation and soil development after brackish‐water marsh creation |journal=[[Restoration Ecology]] |volume=10 |issue=2 |pages=248–58 |url=http://www.marianhs.org/userfiles/1086/Classes/25998/IU%20paper%20NC%20marsh%20restoration.pdf |doi=10.1046/j.1526-100X.2002.01020.x |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> The original soil surface is buried, and the formation process must begin anew for this deposit. Over time the soil will develop a profile that depends on the intensities of biota and climate. While a soil can achieve relative stability of its properties for extended periods,<ref name="Crews1995"/> the soil life cycle ultimately ends in soil conditions that leave it vulnerable to erosion.<ref>{{cite book |last1=Shipitalo |first1=Martin J. |last2=Le Bayon |first2=Renée-Claire |lastauthoramp=yes |date=2004 |chapter=Quantifying the effects of earthworms on soil aggregation and porosity |doi=10.1201/9781420039719.pt5 |title=Earthworm ecology |edition=2nd |editor-first=Clive A. |editor-last=Edwards |publisher=[[CRC Press]] |location=Boca Raton, Florida |pages=183–200|isbn=978-1-4200-3971-9 |url=https://www.researchgate.net/publication/41844767 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Despite the inevitability of soil retrogression and degradation, most soil cycles are long.<ref name="Crews1995"/>

Soil-forming factors continue to affect soils during their existence, even on "stable" landscapes that are long-enduring, some for millions of years.<ref name="Crews1995"/> Materials are deposited on top<ref>{{cite journal |last1=He |first1=Changling |last2=Breuning-Madsen |first2=Henrik |last3=Awadzi |first3=Theodore W. |lastauthoramp=yes |year=2007 |title=Mineralogy of dust deposited during the Harmattan season in Ghana |journal=[[Danish Journal of Geography|Geografisk Tidsskrift]] |volume=107 |issue=1 |pages=9–15 |doi=10.1080/00167223.2007.10801371 |citeseerx=10.1.1.469.8326 }}</ref> or are blown or washed from the surface.<ref>{{cite journal |last1=Pimentel |first1=David |last2=Harvey |first2=C. |last3=Resosudarmo |first3=Pradnja |last4=Sinclair |first4=K. |last5=Kurz |first5=D. |last6=McNair |first6=M. |last7=Crist |first7=S. |last8=Shpritz |first8=Lisa |last9=Fitton |first9=L. |last10=Saffouri |first10=R. |last11=Blair |first11=R. |lastauthoramp=yes |year=1995 |title=Environmental and economic cost of soil erosion and conservation benefits |journal=[[Science (journal)|Science]] |volume=267 |issue=5201 |pages=1117–23 |url=http://www.rachel.org/files/document/Environmental_and_Economic_Costs_of_Soil_Erosi.pdf |doi=10.1126/science.267.5201.1117 |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1995Sci...267.1117P |pmid=17789193}}</ref> With additions, removals and alterations, soils are always subject to new conditions. Whether these are slow or rapid changes depends on climate, topography and biological activity.<ref>{{cite journal |last1=Wakatsuki |first1=Toshiyuki |last2=Rasyidin |first2=Azwar |lastauthoramp=yes |year=1992 |title=Rates of weathering and soil formation |journal=Geoderma |volume=52 |issue=3/4 |pages=251–63 |url=http://kinki-ecotech.jp/download/WakatsukiRasydin1992Geoderma.pdf |doi=10.1016/0016-7061(92)90040-E |accessdate=17 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1992Geode..52..251W }}</ref>

Time as a soil-forming factor may be investigated by studying soil [[chronosequence]]s, in which soils of different ages but with minor differences in other soil-forming factors can be compared.<ref>{{cite journal |last1=Huggett |first1=R.J |title=Soil chronosequences, soil development, and soil evolution: a critical review |journal=Catena |date=1998 |volume=32 |issue=3-4 |pages=155–172 |doi=10.1016/S0341-8162(98)00053-8}}</ref>

==Physical properties==
{{for|the [[academic discipline]]|Soil physics}}
The physical properties of soils, in order of decreasing importance for [[ecosystem services]] such as [[crop production]], are [[Soil texture|texture]], [[Soil structure|structure]], [[bulk density]], [[Pore space in soil|porosity]], consistency, temperature, colour and [[Soil resistivity|resistivity]].<ref>{{cite book |last1=Gardner |first1=Catriona M.K. |last2=Laryea |first2=Kofi Buna |last3=Unger |first3=Paul W. |lastauthoramp=yes |date=1999 |title=Soil physical constraints to plant growth and crop production |edition=1st |location=Rome|publisher=[[Food and Agriculture Organization of the United Nations]] |url=http://plantstress.com/Files/Soil_Physical_Constraints.pdf |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: [[sand]], [[silt]], and [[clay]]. At the next larger scale, soil structures called [[ped]]s or more commonly ''soil aggregates'' are created from the soil separates when [[iron oxide]]s, [[carbonate]]s, clay, [[silica]] and [[humus]], coat particles and cause them to adhere into larger, relatively stable secondary structures.<ref>{{cite journal |last1=Six |first1=Johan |last2=Paustian |first2=Keith |last3=Elliott |first3=Edward T. |last4=Combrink |first4=Clay |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=64 |issue=2 |title=Soil structure and organic matter. I. Distribution of aggregate-size classes and aggregate-associated carbon |url=https://www.researchgate.net/publication/280798601 |year=2000 |pages=681–89 |doi=10.2136/sssaj2000.642681x |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2000SSASJ..64..681S }}</ref> Soil [[bulk density]], when determined at standardized moisture conditions, is an estimate of [[soil compaction]].<ref>{{cite journal |last1=Håkansson |first1=Inge |last2=Lipiec |first2=Jerzy |lastauthoramp=yes |journal=Soil and Tillage Research |volume=53 |issue=2 |title=A review of the usefulness of relative bulk density values in studies of soil structure and compaction |url=https://pdfs.semanticscholar.org/b028/6fcacb6e12473bd1d4796a9a053eb20d5d72.pdf |year=2000 |pages=71–85 |doi=10.1016/S0167-1987(99)00095-1 |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil.<ref>{{cite journal |last=Schwerdtfeger |first=W.J. |journal=[[Journal of Research of the National Bureau of Standards]] |volume=69C |issue=1 |title=Soil resistivity as related to underground corrosion and cathodic protection |url=http://nvlpubs.nist.gov/nistpubs/jres/69c/jresv69cn1p71_a1b.pdf |year=1965 |pages=71–77 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]] |doi=10.6028/jres.069c.012 |archive-url=https://web.archive.org/web/20170809070125/http://nvlpubs.nist.gov/nistpubs/jres/69C/jresv69Cn1p71_A1b.pdf |archive-date=9 August 2017 |dead-url=yes |df=dmy-all }}</ref> These properties vary through the depth of a soil profile, i.e. through [[soil horizons]]. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.<ref>{{cite book |last=Tamboli |first=Prabhakar Mahadeo |date=1961 |title=The influence of bulk density and aggregate size on soil moisture retention |location=Ames, Iowa |publisher=[[Iowa State University]] |url=http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3448&context=rtd |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

{| class="wikitable" style="border-spacing: 5px; margin:auto;"
|+ '''Influence of Soil Texture Separates on Some Properties of Soils'''<ref name="Brady"/>
|-
! scope="col" style="width:100px;"| Property/behavior
! scope="col" style="width:100px;"| Sand
! scope="col" style="width:100px;"| Silt
! scope="col" style="width:100px;"| Clay
|-
|-
| Water-holding capacity || Low || Medium to high || High
|-
| Aeration || Good || Medium || Poor
|-
| Drainage rate || High || Slow to medium || Very slow
|-
| Soil organic matter level || Low || Medium to high || High to medium
|-
| Decomposition of organic matter || Rapid || Medium || Slow
|-
| Warm-up in spring || Rapid || Moderate || Slow
|-
| Compactability || Low || Medium || High
|-
| Susceptibility to wind erosion || Moderate (High if fine sand) || High || Low
|-
| Susceptibility to water erosion || Low (unless fine sand)|| High || Low if aggregated, otherwise high
|-
| Shrink/Swell Potential || Very Low || Low || Moderate to very high
|-
| Sealing of ponds, dams, and landfills || Poor || Poor || Good
|-
| Suitability for tillage after rain || Good || Medium || Poor
|-
| Pollutant leaching potential || High || Medium || Low (unless cracked)
|-
| Ability to store plant nutrients || Poor || Medium to High || High
|-
| Resistance to pH change || Low || Medium || High
|}

===Texture===
{{Main|Soil texture}}
[[File:SoilTexture USDA.png|upright=1.35|thumb|right|[[Soil type]]s by clay, silt, and sand composition as used by the [[United States Department of Agriculture|USDA]]]] [[File:Kootenay National Park - Paint Pots 1.jpg|thumb|Iron-rich soil near Paint Pots in [[Kootenay National Park]], [[Canada]]]]
The mineral components of soil are [[sand]], [[silt]] and [[clay]], and their relative proportions determine a soil's texture. Properties that are influenced by soil texture include [[Pore space in soil|porosity]], [[Permeability (earth sciences)|permeability]], [[Infiltration (hydrology)|infiltration]], [[Shrink–swell capacity|shrink-swell rate]], [[Field capacity|water-holding capacity]], and susceptibility to [[erosion]]. In the illustrated USDA textural classification triangle, the only soil in which neither sand, silt nor clay predominates is called [[loam]]. While even pure sand, silt or clay may be considered a soil, from the perspective of conventional [[agriculture]] a loam soil with a small amount of organic material is considered "ideal", inasmuch as [[fertilizers]] or [[manure]] are currently used to mitigate nutrient losses due to [[crop yields]] in the long term.<ref>{{cite journal |last1=Haynes |first1=Richard J. |last2=Naidu |first2=Ravi |lastauthoramp=yes |journal=Nutrient Cycling in Agroecosystems |volume=51 |issue=2 |title=Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: a review |url=https://www.researchgate.net/publication/225252692 |year=1998 |pages=123–37 |doi=10.1023/A:1009738307837 |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> The mineral constituents of a loam soil might be 40% sand, 40% silt and the balance 20% clay by weight. Soil texture affects soil behaviour, in particular, its retention capacity for nutrients (e.g., [[cation exchange capacity]])<ref>{{cite journal |last1=Silver |first1=Whendee L. |last2=Neff |first2=Jason |last3=McGroddy |first3=Megan |last4=Veldkamp |first4=Ed |last5=Keller |first5=Michael |last6=Cosme |first6=Raimundo |lastauthoramp=yes |journal=Ecosystems |volume=3 |issue=2 |title=Effects of soil texture on belowground carbon and nutrient storage in a lowland Amazonian forest ecosystem |url=https://www.fs.fed.us/global/iitf/pubs/ja_iitf_2000_silver%20eco.pdf |year=2000 |pages=193–209 |doi=10.1007/s100210000019 |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> and [[Ecohydrology#Soil moisture dynamics|water]].

Sand and silt are the products of physical and chemical [[weathering]] of the [[parent rock]];<ref name="Jenny1941"/> clay, on the other hand, is most often the product of the precipitation of the dissolved parent rock as a secondary mineral, except when derived from the weathering of [[mica]].<ref>{{cite journal |last=Jackson |first=Marion L. |journal=Clays and Clay Minerals |volume=6 |issue=1 |title=Frequency distribution of clay minerals in major great soil groups as related to the factors of soil formation |url=http://www.clays.org/journal/archive/volume%206/6-1-133.pdf |year=1957 |pages=133–43 |doi=10.1346/CCMN.1957.0060111 |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1957CCM.....6..133J }}</ref> It is the surface area to volume ratio ([[specific surface area]]) of soil particles and the unbalanced ionic [[electric charges]] within those that determine their role in the [[fertility]] of soil, as measured by its [[cation exchange capacity]].<ref name="Petersen 1996">{{cite journal |last1=Petersen |first1=Lis Wollesen |last2=Moldrup |first2=Per |last3=Jacobsen |first3=Ole Hørbye |last4=Rolston |first4=Dennis E. |lastauthoramp=yes |journal=Soil Science |volume=161 |issue=1 |title=Relations between specific surface area and soil physical and chemical properties |url=https://www.researchgate.net/publication/232162864 |year=1996 |pages=9–21 |doi=10.1097/00010694-199601000-00003 |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite book |last=Lewis |first=D.R. |date=1955 |chapter=Ion exchange reactions of clays |title=Clays and clay technology |editor1-last=Pask |editor1-first=Joseph A. |editor2-last=Turner |editor2-first=Mort D. |publisher=State of California, Department of Natural Resources, Division of Mines |location=San Francisco|pages=54–69 |url=http://www.clays.org/journal/archive/volume%201/1-1-54.pdf |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Sand is least active, having the least specific surface area, followed by silt; clay is the most active. Sand's greatest benefit to soil is that it resists compaction and increases soil porosity, although this property stands only for pure sand, not for sand mixed with smaller minerals which fill the voids among sand grains.<ref>{{cite journal |last=Dexter |first=Anthony R. |journal=Geoderma |volume=120 |issue=3/4 |title=Soil physical quality. I. Theory, effects of soil texture, density, and organic matter, and effects on root growth |url=https://www.sciencedirect.com/science/article/pii/S0016706103002891 |year=2004 |pages=201–14 |doi=10.1016/j.geodermaa.2003.09.005}}</ref> Silt is mineralogically like sand but with its higher specific surface area it is more chemically and physically active than sand. But it is the clay content of soil, with its very high specific surface area and generally large number of negative charges, that gives a soil its high retention capacity for water and nutrients.<ref name="Petersen 1996"/> Clay soils also resist wind and water erosion better than silty and sandy soils, as the particles bond tightly to each other,<ref>{{cite journal |last=Bouyoucos |first=George J. |journal=Journal of the American Society of Agronomy |volume=27 |issue=9 |title=The clay ratio as a criterion of susceptibility of soils to erosion |url=https://dl.sciencesocieties.org/publications/aj/abstracts/27/9/AJ0270090738?access=0&view=pdf |year=1935 |pages=738–41 |subscription=yes |accessdate=25 December 2017 |doi=10.2134/agronj1935.00021962002700090007x}}</ref>
and that with a strong mitigation effect of organic matter.<ref>{{cite journal |last1=Borrelli |first1=Pasquale |last2=Ballabio |first2=Cristiano |last3=Panagos |first3=Panos |last4=Montanarella |first4=Luca |journal=Geoderma |volume=232/234 |title=Wind erosion susceptibility of European soils |url=https://www.researchgate.net/publication/263092389 |year=2014 |pages=471–78 |doi=10.1016/j.geoderma.2014.06.008 |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2014Geode.232..471B }}</ref>

Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily [[quartz]] particles, ranging in size from {{convert|2.0|to|0.05|mm|in|abbr=on}} in diameter. Silt ranges in size from {{convert|0.05|to|0.002|mm|in|abbr=on}}. Clay cannot be resolved by optical microscopes as its particles are {{convert|0.002|mm|in|abbr=on}} or less in diameter and a thickness of only 10 [[angstroms]] (10<sup>−10</sup> m).{{sfn|Russell|1957|pp=32–33}}{{sfn|Flemming|1957|p=331}} In medium-textured soils, clay is often washed downward through the soil profile (a process called [[eluviation]]) and accumulates in the subsoil (a process called [[illuviation]]). There is no clear relationship between the size of soil mineral components and their mineralogical nature: sand and silt particles can be [[calcareous]] as well as [[siliceous]],<ref>{{cite web|url=https://geomaps.wr.usgs.gov/parks/coast/sand/calcsand.html |title=Calcareous Sand |website=U.S. Geological Survey |accessdate=24 December 2017}}</ref> while textural clay ({{convert|0.002|mm|in|abbr=on}}) can be made of very fine quartz particles as well as of multi-layered secondary minerals.<ref>{{cite book|last=Grim |first=Ralph E. |date=1953 |title=Clay mineralogy |publisher=[[McGraw-Hill]] |location=New York |url=http://krishikosh.egranth.ac.in/bitstream/1/2037422/1/1334.pdf |accessdate=24 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Soil mineral components belonging to a given textural class may thus share properties linked to their [[specific surface area]] (e.g. [[moisture retention]]) but not those linked to their chemical composition (e.g. [[cation exchange capacity]]).

Soil components larger than {{convert|2.0|mm|in|abbr=on}} are classed as rock and gravel and are removed before determining the percentages of the remaining components and the textural class of the soil, but are included in the name. For example, a sandy [[loam]] soil with 20% gravel would be called gravelly sandy loam.

When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil. A soil is called organic if:

# Mineral fraction is 0% clay and organic matter is 20% or more
# Mineral fraction is 0% to 50% clay and organic matter is between 20% and 30%
# Mineral fraction is 50% or more clay and organic matter 30% or more.{{sfn|Donahue|Miller|Shickluna|1977|p=53}}

===Structure===
{{Main|Ped|Soil structure|Structural Soil}}
The clumping of the soil textural components of sand, silt and clay causes [[Aggregate (geology)|aggregates]] to form and the further association of those aggregates into larger units creates [[soil structure]]s called peds (a contraction of the word [[pedolith]]). The adhesion of the soil textural components by organic substances, iron oxides, carbonates, clays, and silica, the breakage of those aggregates from expansion-contraction caused by [[Frost weathering|freezing-thawing]] and wetting-drying cycles,<ref>{{cite journal |last1=Sillanpää |first1=Mikko |last2=Webber |first2=L.R. |lastauthoramp=yes |journal=Canadian Journal of Soil Science |volume=41 |issue=2 |title=The effect of freezing-thawing and wetting-drying cycles on soil aggregation |year=1961 |pages=182–87 |doi=10.4141/cjss61-024 }}</ref> and the build-up of aggregates by soil animals, microbial colonies and root tips<ref name="Oades1993">{{cite journal |last=Oades |first=J. Malcolm |journal=Geoderma |volume=56 |issue=1–4 |title=The role of biology in the formation, stabilization and degradation of soil structure |url=http://www.dendrocronologia.cl/pubs/Oades%201992.pdf |year=1993 |pages=377–400 |doi=10.1016/0016-7061(93)90123-3 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1993Geode..56..377O }}</ref> shape soil into distinct geometric forms.<ref name="Bronick2005"/><ref name="Lee1991"/> The peds evolve into units which have various shapes, sizes and degrees of development.<ref>{{cite web |author=Soil Science Division Staff |year=2017 |url=https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_054253#soil_structure |title=Soil structure |website=Soil Survey Manual (issued March 2017), USDA Handbook No. 18 |publisher=United States Department of Agriculture, Natural Researches Conservation Service, Soils |location=Washington, DC |accessdate=25 December 2017}}</ref> A soil clod, however, is not a ped but rather a mass of soil that results from mechanical disturbance of the soil such as [[Tillage|cultivation]]. Soil structure affects [[aeration]], water movement, conduction of heat, plant root growth and resistance to erosion.<ref>{{cite journal |last1=Horn |first1=Rainer |last2=Taubner |first2=Heidi |last3=Wuttke |first3=M. |last4=Baumgartl |first4=Thomas |lastauthoramp=yes |journal=Soil and Tillage Research |volume=30 |issue=2–4 |title=Soil physical properties related to soil structure |url=http://www.sciencedirect.com/science/article/pii/0167198794900051 |year=1994 |pages=187–216 |doi=10.1016/0167-1987(94)90005-1 |subscription=yes |accessdate=25 December 2017}}</ref> Water, in turn, has a strong effect on soil structure, directly via the dissolution and precipitation of minerals, the mechanical destruction of aggregates ([[Slaking (geology)|slaking]])<ref>{{Cite journal|last1=Murray |first1=Robert S. |last2=Grant |first2=Cameron D. |lastauthoramp=yes |year=2007 |title=The impact of irrigation on soil structure |journal=The National Program for Sustainable Irrigation |citeseerx=10.1.1.460.5683 }}</ref> and indirectly by promoting plant, animal and microbial growth.

Soil structure often gives clues to its texture, organic matter content, biological activity, past soil evolution, human use, and the chemical and mineralogical conditions under which the soil formed. While texture is defined by the mineral component of a soil and is an innate property of the soil that does not change with agricultural activities, soil structure can be improved or destroyed by the choice and timing of farming practices.<ref name="Bronick2005"/>

Soil structural classes:{{sfn|Donahue|Miller|Shickluna|1977|pp=55–56}}

# Types: '''Shape''' and arrangement of peds
## Platy: Peds are flattened one atop the other 1–10&nbsp;mm thick. Found in the A-horizon of forest soils and lake sedimentation.
## Prismatic and Columnar: Prismlike peds are long in the vertical dimension, 10–100&nbsp;mm wide. Prismatic peds have flat tops, columnar peds have rounded tops. Tend to form in the B-horizon in high sodium soil where clay has accumulated.
## Angular and subangular: Blocky peds are imperfect cubes, 5–50&nbsp;mm, angular have sharp edges, subangular have rounded edges. Tend to form in the B-horizon where clay has accumulated and indicate poor water penetration.
## Granular and Crumb: Spheroid peds of polyhedrons, 1–10&nbsp;mm, often found in the A-horizon in the presence of organic material. Crumb peds are more porous and are considered ideal.
# Classes: '''Size''' of peds whose ranges depend upon the above type
## Very fine or very thin: <1&nbsp;mm platy and spherical; <5&nbsp;mm blocky; <10&nbsp;mm prismlike.
## Fine or thin: 1–2&nbsp;mm platy, and spherical; 5–10&nbsp;mm blocky; 10–20&nbsp;mm prismlike.
## Medium: 2–5&nbsp;mm platy, granular; 10–20&nbsp;mm blocky; 20–50 prismlike.
## Coarse or thick: 5–10&nbsp;mm platy, granular; 20–50&nbsp;mm blocky; 50–100&nbsp;mm prismlike.
## Very coarse or very thick: >10&nbsp;mm platy, granular; >50&nbsp;mm blocky; >100&nbsp;mm prismlike.
# Grades: Is a measure of the degree of '''development''' or cementation within the peds that results in their strength and stability.
## Weak: Weak cementation allows peds to fall apart into the three textural constituents, sand, silt and clay.
## Moderate: Peds are not distinct in undisturbed soil but when removed they break into aggregates, some broken aggregates and little unaggregated material. This is considered ideal.
## Strong:Peds are distinct before removed from the profile and do not break apart easily.
## Structureless: Soil is entirely cemented together in one great mass such as slabs of clay or no cementation at all such as with sand.

At the largest scale, the forces that shape a soil's structure result from [[Shrink–swell capacity|swelling and shrinkage]] that initially tend to act horizontally, causing vertically oriented prismatic peds. This mechanical process is mainly exemplified in the development of [[vertisols]].<ref>{{cite journal |last1=Dinka |first1=Takele M. |last2=Morgan |first2=Cristine L.S. |last3=McInnes |first3=Kevin J. |last4=Kishné |first4= Andrea Sz. |last5=Harmel |first5=R. Daren |lastauthoramp=yes |journal=[[Journal of Hydrology]] |volume=476 |title=Shrink–swell behavior of soil across a Vertisol catena |url=https://www.academia.edu/13776567 |year=2013 |pages=352–59 |doi=10.1016/j.jhydrol.2012.11.002 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2013JHyd..476..352D }}</ref> Clayey soil, due to its differential drying rate with respect to the surface, will induce horizontal cracks, reducing columns to blocky peds.<ref>{{cite journal |last1=Morris |first1=Peter H. |last2=Graham |first2=James |last3=Williams |first3=David J. |lastauthoramp=yes |journal=[[Canadian Geotechnical Journal]] |volume=29 |issue=2 |title=Cracking in drying soils |url=https://www.researchgate.net/publication/239487071 |year=1992 |pages=263–77 |doi=10.1139/t92-030 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Roots, rodents, worms, and freezing-thawing cycles further break the peds into smaller peds of a more or less spherical shape.<ref name="Oades1993"/>

At a smaller scale, plant roots extend into voids ([[macropores]]) and remove water<ref>{{cite journal |last1=Robinson |first1=Nicole |last2=Harper |first2=R.J. |last3=Smettem |first3=Keith Richard J. |lastauthoramp=yes |journal=[[Plant and Soil]] |volume=286 |issue=1/2 |title=Soil water depletion by Eucalyptus spp. integrated into dryland agricultural systems |url=https://www.researchgate.net/publication/43501164 |year=2006 |pages=141–51 |doi=10.1007/s11104-006-9032-4 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> causing macroporosity to increase and [[microporosity]] to decrease,<ref>{{cite journal |last1=Scholl |first1=Peter |last2=Leitner |first2=Daniel |last3=Kammerer |first3=Gerhard |last4=Loiskandl |first4=Willibald |last5=Kaul |first5=Hans-Peter |last6=Bodner |first6=Gernot |lastauthoramp=yes |journal=[[Plant and Soil]] |volume=381 |issue=1/2 |title=Root induced changes of effective 1D hydraulic properties in a soil column |url=https://www.researchgate.net/publication/271702247 |year=2014 |pages=193–213 |doi=10.1007/s11104-014-2121-x |pmid=25834290 |pmc=4372835 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> thereby decreasing aggregate size.<ref>{{cite journal |last1=Angers |first1=Denis A. |last2=Caron |first2=Jean |lastauthoramp=yes |journal=Biogeochemistry |volume=42 |issue=1 |title=Plant-induced changes in soil structure: processes and feedbacks |url=https://www.researchgate.net/publication/226938344 |year=1998 |pages=55–72 |doi=10.1023/A:1005944025343 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> At the same time, [[root hairs]] and fungal [[hypha]]e create microscopic tunnels that break up peds.<ref>{{cite journal |last1=White |first1=Rosemary G. |last2=Kirkegaard |first2=John A. |lastauthoramp=yes |journal=Plant, Cell and Environment |volume=33 |issue=2 |title=The distribution and abundance of wheat roots in a dense, structured subsoil: implications for water uptake |url=https://www.researchgate.net/publication/38072726 |year=2010 |pages=133–48 |doi=10.1111/j.1365-3040.2009.02059.x |pmid=19895403 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Skinner |first1=Malcolm F. |last2=Bowen |first2=Glynn D. |lastauthoramp=yes |journal=Soil Biology and Biochemistry |volume=6 |issue=1 |title=The penetration of soil by mycelial strands of ectomycorrhizal fungi |url=http://www.sciencedirect.com/science/article/pii/0038071774900121 |year=1974 |pages=57–8 |doi=10.1016/0038-0717(74)90012-1 |subscription=yes |accessdate=25 December 2017}}</ref>

At an even smaller scale, soil aggregation continues as bacteria and fungi exude sticky polysaccharides which bind soil into smaller peds.<ref>{{cite journal |last=Chenu |first=Claire |journal=Geoderma |volume=56 |issue=1–4 |title=Clay- or sand-polysaccharide associations as models for the interface between micro-organisms and soil: water related properties and microstructure |url=https://www.academia.edu/12012172 |year=1993 |pages=143–56 |doi=10.1016/0016-7061(93)90106-U |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1993Geode..56..143C }}</ref> The addition of the raw organic matter that bacteria and fungi feed upon encourages the formation of this desirable soil structure.<ref>{{cite journal |last=Franzluebbers |first=Alan J. |journal=Soil and Tillage Research |volume=66 |issue=2 |title=Water infiltration and soil structure related to organic matter and its stratification with depth |url=https://naldc.nal.usda.gov/download/15662/PDF |year=2002 |pages=197–205 |doi=10.1016/S0167-1987(02)00027-2 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref>

At the lowest scale, the soil chemistry affects the aggregation or [[Dispersion (geology)|dispersal]] of soil particles. The clay particles contain polyvalent cations which give the faces of clay layers localized negative charges.<ref>{{cite journal |last1=Sposito |first1=Garrison |last2=Skipper |first2=Neal T. |last3=Sutton |first3=Rebecca |last4=Park |first4=Sung-Ho |last5=Soper |first5=Alan K. |last6=Greathouse |first6=Jeffery A. |lastauthoramp=yes |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=96 |issue=7 |title=Surface geochemistry of the clay minerals |url=http://www.pnas.org/content/96/7/3358.full.pdf |year=1999 |pages=3358–64 |doi=10.1073/pnas.96.7.3358 |pmid=10097044 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1999PNAS...96.3358S |pmc=34275}}</ref> At the same time, the edges of the clay plates have a slight positive charge, thereby allowing the edges to adhere to the negative charges on the faces of other clay particles or to [[Flocculation|flocculate]] (form clumps).<ref>{{cite journal |last1=Tombácz |first1=Etelka |last2=Szekeres |first2=Márta |lastauthoramp=yes |journal=Applied Clay Science |volume=34 |issue=1–4 |title=Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite |url=https://www.academia.edu/886679 |year=2006 |pages=105–24 |doi=10.1016/j.clay.2006.05.009 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> On the other hand, when monovalent ions, such as sodium, invade and displace the polyvalent cations, they weaken the positive charges on the edges, while the negative surface charges are relatively strengthened. This leaves negative charge on the clay faces that repel other clay, causing the particles to push apart, and by doing so deflocculate clay suspensions.<ref>{{cite journal |last1=Schofield |first1=R. Kenworthy |last2=Samson |first2=H.R. |lastauthoramp=yes |journal=Clay Minerals Bulletin |volume=2 |issue=9 |title=The deflocculation of kaolinite suspensions and the accompanying change-over from positive to negative chloride adsorption |url=http://www.minersoc.org/pages/Archive-CM/Volume_2/2-9-45.pdf |year=1953 |pages=45–51 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]] |doi=10.1180/claymin.1953.002.9.08 |bibcode=1953ClMin...2...45S |archive-url=https://web.archive.org/web/20160527093243/http://www.minersoc.org/pages/Archive-CM/Volume_2/2-9-45.pdf |archive-date=27 May 2016 |dead-url=yes |df=dmy-all }}</ref> As a result, the clay disperses and settles into voids between peds, causing those to close. In this way the open structure of the soil is destroyed and the soil is made impenetrable to air and water.<ref>{{cite journal |last1=Shainberg |first1=Isaac |last2=Letey |first2=John |lastauthoramp=yes |journal=Hilgardia |volume=52 |issue=2 |title=Response of soils to sodic and saline conditions |url=http://hilgardia.ucanr.edu/fileaccess.cfm?article=152852&p=VAFSNP |year=1984 |pages=1–57 |doi=10.3733/hilg.v52n02p057 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Such [[sodic soil]] (also called [[haline]] soil) tends to form columnar peds near the surface.<ref>{{cite journal |last1=Young |first1=Michael H. |last2=McDonald |first2=Eric V. |last3=Caldwell |first3=Todd G. |last4=Benner |first4=Shawn G. |last5=Meadows |first5=Darren G. |lastauthoramp=yes |journal=[[Vadose Zone Journal]] |volume=3 |issue=3 |title=Hydraulic properties of a desert soil chronosequence in the Mojave Desert, USA |url=https://pdfs.semanticscholar.org/c937/e83cd6c3bb8a685de6ae1adf5ba7602907a5.pdf |year=2004 |pages=956–63 |doi=10.2113/3.3.956 |accessdate=16 June 2018 |format=[[Portable Document Format|PDF]]}}</ref>

===Density===
{| class="wikitable floatright" style="text-align:right"
|+ '''Representative bulk densities of soils. The percentage pore space was calculated using 2.7&nbsp;g/cm<sup>3</sup> for particle density except for the peat soil, which is estimated.'''{{sfn|Donahue|Miller|Shickluna|1977|p=60}}
|-
! span="col" | Soil treatment and identification !! Bulk density (g/cm<sup>3</sup>) !! Pore space (%)
|-
! span= "row" | Tilled surface soil of a cotton field
| 1.3 || 51
|-
! span= "row" | Trafficked inter-rows where wheels passed surface
| 1.67 || 37
|-
! span= "row" | Traffic pan at 25&nbsp;cm deep
| 1.7 || 36
|-
! span= "row" | Undisturbed soil below traffic pan, clay loam
| 1.5 || 43
|-
! span= "row" | Rocky silt loam soil under aspen forest
| 1.62 || 40
|-
! span= "row" | Loamy sand surface soil
| 1.5 || 43
|-
! span= "row" | Decomposed peat
| 0.55 || 65
|}
Soil [[particle density (particle count)|particle density]] is typically 2.60 to 2.75 grams per cm<sup>3</sup> and is usually unchanging for a given soil.<ref name="Yu2015"/> Soil particle density is lower for soils with high organic matter content,<ref>{{cite journal |last1=Blanco-Canqui |first1=Humberto |last2=Lal |first2=Rattan |last3=Post |first3=Wilfred M. |last4=Izaurralde |first4=Roberto Cesar |last5=Shipitalo |first5=Martin J. |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=70 |issue=4 |title=Organic carbon influences on soil particle density and rheological properties |url=https://naldc.nal.usda.gov/download/3946/PDF |year=2006 |pages=1407–14 |doi=10.2136/sssaj2005.0355 |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2006SSASJ..70.1407B }}</ref> and is higher for soils with high iron-oxides content.<ref>{{cite book |last1=Cornell |first1=Rochelle M. |last2=Schwertmann |first2=Udo |lastauthoramp=yes |date=2003 |title=The iron oxides: structure, properties, reactions, occurrences and uses |edition=2nd |location=Weinheim, Germany |publisher=[[Wiley-VCH]] |url=http://epsc511.wustl.edu/IronOxide_reading.pdf |accessdate=25 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Soil [[bulk density]] is equal to the dry mass of the soil divided by the volume of the soil; i.e., it includes air space and organic materials of the soil volume. Thereby soil bulk density is always less than soil particle density and is a good indicator of soil compaction.<ref>{{cite journal |last1=Håkansson |first1=Inge |last2=Lipiec |first2=Jerzy |lastauthoramp=yes |journal=Soil and Tillage Research |volume=53 |issue=2 |title=A review of the usefulness of relative bulk density values in studies of soil structure and compaction |url=https://pdfs.semanticscholar.org/b028/6fcacb6e12473bd1d4796a9a053eb20d5d72.pdf |year=2000 |pages=71–85 |doi=10.1016/S0167-1987(99)00095-1 |accessdate=31 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> The soil bulk density of cultivated loam is about 1.1 to 1.4&nbsp;g/cm<sup>3</sup> (for comparison water is 1.0&nbsp;g/cm<sup>3</sup>).{{sfn|Donahue|Miller|Shickluna|1977|pp=59–61}} Contrary to particle density, soil bulk density is highly variable for a given soil, with a strong causal relationship with soil biological activity and management strategies.<ref>{{cite journal |last1=Mäder |first1=Paul |last2=Fließbach |first2=Andreas |last3=Dubois |first3=David |last4=Gunst |first4=Lucie |last5=Fried |first5=Padruot |last6=Liggli |first6=Urs |lastauthoramp=yes |journal=[[Science (journal)|Science]] |volume=296 |issue=1694 |title=Soil fertility and biodiversity in organic farming |url=http://www.ask-force.org/web/Organic/Maeder-Organicfarming-2002.pdf |year=2002 |pages=1694–97 |doi=10.1126/science.1071148 |pmid=12040197 |accessdate=30 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=2002Sci...296.1694M }}</ref> However, it has been shown that, depending on species and the size of their aggregates (faeces), earthworms may either increase or decrease soil bulk density.<ref>{{cite book |last1=Blanchart |first1=Éric |last2=Albrecht |first2=Alain |last3=Alegre |first3=Julio |last4=Duboisset |first4=Arnaud |last5=Gilot |first5=Cécile |last6=Pashanasi |first6=Beto |last7=Lavelle |first7=Patrick |last8=Brussaard |first8=Lijbert |lastauthoramp=yes |date=1999 |chapter=Effects of earthworms on soil structure and physical properties |title=Earthworm management in tropical agroecosystems |edition=1st |editor1-first=Patrick |editor1-last=Lavelle |editor2-first=Lijbert |editor2-last=Brussaard |editor3-first=Paul F. |editor3-last=Hendrix |publisher=[[CAB International]] |location=Wallingford, UK |pages=149–72|isbn=978-0-85199-270-9 |url=http://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers16-03/010021558.pdf |accessdate=31 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> A lower bulk density by itself does not indicate suitability for plant growth due to the confounding influence of soil texture and structure.<ref>{{cite journal |last1=Rampazzo |first1=Nicola |last2=Blum |first2=Winfried E.H. |last3=Wimmer |first3=Bernhard |lastauthoramp=yes |journal=Die Bodenkultur |volume=49 |issue=2 |title=Assessment of soil structure parameters and functions in agricultural soils |url=https://diebodenkultur.boku.ac.at/volltexte/band-49/heft-2/rampazzo.pdf |year=1998 |pages=69–84 |accessdate=30 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> A high bulk density is indicative of either soil compaction or a mixture of soil textural classes in which small particles fill the voids among coarser particles.<ref>{{cite journal |last1=Bodman |first1=Geoffrey Baldwin |last2=Constantin |first2=Winfried G.K. |lastauthoramp=yes |journal=Hilgardia |volume=36 |issue=15 |title=Influence of particle size distribution in soil compaction |doi=10.3733/hilg.v36n15p567 |url=http://ucanr.edu/sites/UCCE_LR/files/203094.pdf |year=1965 |pages=567–91 |accessdate=30 December 2017 |format=[[Portable Document Format|PDF]]}}</ref> Hence the positive correlation between the [[fractal dimension]] of soil, considered as a [[porous medium]], and its bulk density,<ref>{{cite journal |last1=Zeng |first1=Y. |last2=Gantzer |first2=Clark |last3=Payton |first3=R.L. |last4=Anderson |first4=Stephen H. |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=60 |issue=6 |title=Fractal dimension and lacunarity of bulk density determined with X-ray computed tomography |doi=10.2136/sssaj1996.03615995006000060016x |url=https://www.researchgate.net/publication/200750939 |year=1996 |pages=1718–24 |accessdate=30 December 2017 |format=[[Portable Document Format|PDF]]|bibcode=1996SSASJ..60.1718Z }}</ref> that explains the poor hydraulic conductivity of silty clay loam in the absence of a faunal structure.<ref>{{cite journal |last1=Rawls |first1=Walter J. |last2=Brakensiek |first2=Donald L. |last3=Saxton |first3=Keith E. |lastauthoramp=yes |journal=Transactions of the American Society of Agricultural Engineers |volume=25 |issue=5 |title=Estimation of soil water properties |doi=10.13031/2013.33720 |url=http://www.envsci.rutgers.edu/~gimenez/SoilPhysics/HomeworkCommonFiles/Rawls%20et%20al%201982.pdf |year=1982 |pages=1316–20 |accessdate=30 December 2017 |format=[[Portable Document Format|PDF]] |archive-url=https://web.archive.org/web/20170517020519/http://www.envsci.rutgers.edu/~gimenez/SoilPhysics/HomeworkCommonFiles/Rawls%20et%20al%201982.pdf |archive-date=17 May 2017 |dead-url=yes |df=dmy-all }}</ref>

===Porosity===
{{main|Pore space in soil}}
[[Pore space]] is that part of the bulk volume of soil that is not occupied by either mineral or organic matter but is open space occupied by either gases or water. In a productive, medium-textured soil the total pore space is typically about 50% of the soil volume.<ref>{{cite web |title=Physical aspects of crop productivity |url=http://www.fao.org/docrep/v9926e/v9926e04.htm |website=www.fao.org |publisher=[[Food and Agriculture Organization of the United Nations]] |location=Rome|accessdate=1 January 2018}}</ref> [[Pore space in soil#Pore types|Pore size]] varies considerably; the smallest pores ([[Pore space in soil#cryptopores|cryptopores]]; <0.1 [[micrometre|µm]]) hold water too tightly for use by plant roots; [[Available water capacity|plant-available water]] is held in [[Pore space in soil#ultramicropores|ultramicropores]], [[Pore space in soil#micropores|micropores]] and [[Pore space in soil#mesopores|mesopores]] (0.1–75 [[µm]]); and [[Pore space in soil#macropores|macropores]] (>75 [[µm]]) are generally air-filled when the soil is at [[field capacity]].

Soil texture determines total volume of the smallest pores;<ref>{{cite journal |last1=Rutherford |first1=P. Michael |last2=Juma |first2=Noorallah G. |lastauthoramp=yes |journal=Biology and Fertility of Soils |volume=12 |issue=4 |title=Influence of texture on habitable pore space and bacterial-protozoan populations in soil |year=1992 |pages=221–27 |doi=10.1007/BF00336036 }}</ref> clay soils have smaller pores, but more total pore space than sands,<ref>{{cite journal |last=Diamond |first=Sidney |journal=Clays and Clay Minerals |volume=18 |issue=1 |title=Pore size distributions in clays |url=https://www.researchgate.net/publication/255602213 |year=1970 |pages=7–23 |doi=10.1346/CCMN.1970.0180103 |accessdate=1 January 2018 |format=[[Portable Document Format|PDF]]|bibcode=1970CCM....18....7D }}</ref> despite of a much lower [[Hydraulic conductivity|permeability]].<ref>{{cite web |title=Permeability of different soils |url=https://nptel.ac.in/courses/105103097/27 |website=nptel.ac.in |publisher=NPTEL, Government of India |location=Chennai, India |accessdate=1 January 2018}}</ref> Soil structure has a strong influence on the larger pores that affect soil aeration, water infiltration and drainage.{{sfn|Donahue|Miller|Shickluna|1977|pp=62–63}} Tillage has the short-term benefit of temporarily increasing the number of pores of largest size, but these can be rapidly degraded by the destruction of soil aggregation.<ref>{{cite web |url=http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447039&topicorder=8&maxto=10 |title=Physical properties of soil and soil water |website=passel.unl.edu |publisher=Plant and Soil Sciences eLibrary |location=Lincoln, Nebraska |accessdate=1 January 2018}}</ref>

The pore size distribution affects the ability of plants and other organisms to access water and oxygen; large, continuous pores allow rapid transmission of air, water and dissolved nutrients through soil, and small pores store water between rainfall or irrigation events.<ref>{{cite book |last=Nimmo |first=John R. |date=2004 |chapter=Porosity and pore size distribution |title=Encyclopedia of soils in the environment, volume 3 |edition=1st |editor1-first=Daniel |editor1-last=Hillel |editor2-first=Cynthia |editor2-last=Rosenzweig |editor3-first=David |editor3-last=Powlson |editor4-first=Kate |editor4-last=Scow|editorlink4=Kate Scow |editor5-first=Michail |editor5-last=Singer |editor6-first=Donald |editor6-last=Sparks |publisher=[[Academic Press]] |location=London |pages=295–303 |isbn=978-0-12-348530-4 |url=https://wwwrcamnl.wr.usgs.gov/uzf/abs_pubs/papers/nimmo.04.encyc.por.ese.pdf |accessdate=7 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> Pore size variation also compartmentalizes the soil pore space such that many microbial and faunal organisms are not in direct competition with one another, which may explain not only the large number of species present, but the fact that functionally redundant organisms (organisms with the same ecological niche) can co-exist within the same soil.<ref>{{cite journal |last=Giller |first=Paul S. |journal=Biodiversity and Conservation |volume=5 |issue=2 |title=The diversity of soil communities, the 'poor man's tropical rainforest' |url=https://www.researchgate.net/publication/226978038 |year=1996 |pages=135–68 |doi=10.1007/BF00055827 |accessdate=1 January 2018 |format=[[Portable Document Format|PDF]]}}</ref>

===Consistency===
Consistency is the ability of soil to stick to itself or to other objects ([[Cohesion (geology)|cohesion]] and [[adhesion]], respectively) and its ability to resist deformation and rupture. It is of approximate use in predicting cultivation problems<ref>{{cite journal |last1=Boekel |first1=P. |last2=Peerlkamp |first2=Petrus K. |lastauthoramp=yes |journal=Netherlands Journal of Agricultural Science |volume=4 |issue=1 |title=Soil consistency as a factor determining the soil structure of clay soils |url=http://edepot.wur.nl/211680 |year=1956 |pages=122–25 |accessdate=7 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> and the engineering of foundations.<ref>{{cite book |last=Day |first=Robert W. |date=2000 |chapter=Soil mechanics and foundations |title=Building design and construction handbook |edition=6th |editor1-first=Frederick S. |editor1-last=Merritt |editor2-first=Jonathan T. |editor2-last=Rickett |publisher=[[McGraw-Hill Professional]] |location=New York|isbn=978-0-07-041999-5 |url=http://my.fit.edu/~locurcio/14-Civil%20&%20Const%20handbooks/McGraw%20Hill%20-%20Design%20&%20Const%20Handbook/06-Soils%20&%20Foundations.pdf |accessdate=7 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> Consistency is measured at three moisture conditions: air-dry, moist, and wet.<ref>{{cite web |url=http://www.fao.org/fishery/docs/CDrom/FAO_Training/FAO_Training/General/x6706e/x6706e08.htm |title=Soil consistency |publisher=[[Food and Agriculture Organization of the United Nations]] |location=Rome |accessdate=7 January 2018}}</ref> In those conditions the consistency quality depends upon the clay content. In the wet state, the two qualities of stickiness and plasticity are assessed. A soil's resistance to fragmentation and crumbling is assessed in the dry state by rubbing the sample. Its resistance to shearing forces is assessed in the moist state by thumb and finger pressure. Additionally, the cemented consistency depends on cementation by substances other than clay, such as calcium carbonate, silica, oxides and salts; moisture content has little effect on its assessment. The measures of consistency border on subjective compared to other measures such as pH, since they employ the apparent feel of the soil in those states.

The terms used to describe the soil consistency in three moisture states and a last not affected by the amount of moisture are as follows:

# Consistency of Dry Soil: loose, soft, slightly hard, hard, very hard, extremely hard
# Consistency of Moist Soil: loose, very friable, friable, firm, very firm, extremely firm
# Consistency of Wet Soil: nonsticky, slightly sticky, sticky, very sticky; nonplastic, slightly plastic, plastic, very plastic
# Consistency of Cemented Soil: weakly cemented, strongly cemented, indurated (requires hammer blows to break up){{sfn|Donahue|Miller|Shickluna|1977|pp=62–63, 565–67}}

Soil consistency is useful in estimating the ability of soil to support buildings and roads. More precise measures of soil strength are often made prior to construction.

===Temperature===
{{Further|Soil thermal properties|Heat capacity|Thermal conduction}}

Soil [[temperature]] depends on the ratio of the [[energy]] absorbed to that lost.<ref>{{cite journal |last=Deardorff |first=James W. |journal=[[Journal of Geophysical Research]] |volume=83 |issue=C4 |title=Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation |url=http://patarnott.com/atms411/pdf/Deardorff1978GroundTemperature.pdf |year=1978 |pages=1889–903 |doi=10.1029/JC083iC04p01889 |accessdate=28 January 2018 |bibcode=1978JGR....83.1889D |citeseerx=10.1.1.466.5266 }}</ref> Soil has a temperature range between -20 to 60&nbsp;°C,{{Citation needed|date=January 2018}} with a mean annual temperature from -10 to 26&nbsp;°C according to [[biomes]].<ref>{{cite journal |last1=Hursh |first1=Andrew |last2=Ballantyne |first2=Ashley |last3=Cooper |first3=Leila |last4=Maneta |first4=Marco |last5=Kimball |first5=John |last6=Watts |first6=Jennifer |lastauthoramp=yes |journal=[[Global Change Biology]] |volume=23 |issue=5 |title=The sensitivity of soil respiration to soil temperature, moisture, and carbon supply at the global scale |url=https://pdfs.semanticscholar.org/cd03/8a35140615dfe70b706fac68cfde5b5fef31.pdf |year=2017 |pages=2090–103 |doi=10.1111/gcb.13489 |pmid=27594213 |accessdate=28 January 2018 |bibcode=2017GCBio..23.2090H }}</ref> Soil temperature regulates [[seed germination]],<ref>{{cite journal |last1=Forcella |first1=Frank |last2=Benech Arnold |first2=Roberto L. |last3=Sanchez |first3=Rudolfo |last4=Ghersa |first4=Claudio M. |lastauthoramp=yes |journal=Field Crops Research |volume=67 |issue=2 |title=Modeling seedling emergence |url=https://pubag.nal.usda.gov/download/25689/PDF |year=2000 |pages=123–39 |doi=10.1016/S0378-4290(00)00088-5 |accessdate=28 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> breaking of [[seed dormancy]],<ref>{{cite journal |last1=Benech-Arnold |first1=Roberto L. |last2=Sánchez |first2=Rodolfo A. |last3=Forcella |first3=Frank |last4=Kruk |first4=Betina C. |last5=Ghersa |first5=Claudio M. |lastauthoramp=yes |journal=Field Crops Research |volume=67 |issue=2 |title=Environmental control of dormancy in weed seed banks in soil |url=https://naldc.nal.usda.gov/download/14449/PDF |year=2000 |pages=105–22 |doi=10.1016/S0378-4290(00)00087-3 |accessdate=28 January 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Herranz |first1=José M. |last2=Ferrandis |first2=Pablo |last3=Martínez-Sánchez |first3=Juan J. |lastauthoramp=yes |journal=[[Plant Ecology (journal)|Plant Ecology]] |volume=136 |issue=1 |title=Influence of heat on seed germination of seven Mediterranean Leguminosae species |url=https://www.researchgate.net/publication/226645015 |year=1998 |pages=95–103 |doi=10.1023/A:1009702318641 |accessdate=28 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> plant and root growth<ref>{{cite journal |last1=McMichael |first1=Bobbie L. |last2=Burke |first2=John J. |lastauthoramp=yes |journal=[[HortScience]] |volume=33 |issue=6 |title=Soil temperature and root growth |url=http://hortsci.ashspublications.org/content/33/6/947.full.pdf |year=1998 |pages=947–51 |accessdate=28 January 2018 |doi=10.21273/HORTSCI.33.6.947 }}</ref> and the availability of [[nutrients]].<ref>{{cite journal |last1=Tindall |first1=James A. |last2=Mills |first2=Harry A. |last3=Radcliffe |first3=David E. |lastauthoramp=yes |journal=Journal of Plant Nutrition |volume=13 |issue=8 |title=The effect of root zone temperature on nutrient uptake of tomato |url=https://eurekamag.com/ftext.php?pdf=002248024 |year=1990 |pages=939–56 |doi=10.1080/01904169009364127 |accessdate=28 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> Soil temperature has important seasonal, monthly and daily variations, fluctuations in soil temperature being much lower with increasing soil depth.<ref>{{cite web |url=http://www.halesowenweather.co.uk/soil_temperatures.htm |publisher=[[Met Office]]|location=Exeter, UK |title=Soil temperatures |accessdate=3 February 2018}}</ref> Heavy [[mulch]]ing (a type of soil cover) can slow the warming of soil in summer, and, at the same time, reduce fluctuations in surface temperature.<ref name="Lal1974">{{cite journal |last=Lal |first=Ratan |journal=[[Plant and Soil]] |volume=40 |issue=1 |title=Soil temperature, soil moisture and maize yield from mulched and unmulched tropical soils |url=https://eurekamag.com/ftext.php?pdf=000195083 |year=1974 |pages=129–43 |doi=10.1007/BF00011415 |accessdate=3 February 2018 |format=[[Portable Document Format|PDF]]}}</ref>

Most often, agricultural activities must adapt to soil temperatures by:

# maximizing germination and growth by timing of planting (also determined by [[photoperiod]])<ref>{{cite book |last1=Ritchie |first1=Joe T. |last2=NeSmith |first2=D. Scott |lastauthoramp=yes |date=1991 |chapter=Temperature and crop development |title=Modeling plant and soil systems |edition=1st |editor1-first=John |editor1-last=Hanks |editor2-first=Joe T. |editor2-last=Ritchie |publisher=[[American Society of Agronomy]] |location=Madison, Wisconsin |pages=5–29|isbn=978-0-89118-106-4 |chapter-url=https://www.researchgate.net/publication/286506189 |accessdate=4 February 2018 |chapter-format=[[Portable Document Format|PDF]]}}</ref>
# optimizing use of [[anhydrous ammonia]] by applying to soil below {{convert|10|°C|°F|abbr=on}}<ref>{{cite journal|last1=Vetsch |first1=Jeffrey A. |last2=Randall |first2=Gyles W. |lastauthoramp=yes |journal=[[Agronomy Journal]] |volume=96 |issue=2 |title=Corn production as affected by nitrogen application timing and tillage |url=http://nue.okstate.edu/Index_Publications/A_split.pdf |year=2004 |pages=502–09 |doi=10.2134/agronj2004.5020 |accessdate=4 February 2018 }}</ref>
# preventing [[heaving]] and [[thawing]] due to frosts from damaging shallow-rooted crops<ref>{{cite journal |last1=Holmes |first1=R.M. |last2=Robertson |first2=G.W. |lastauthoramp=yes |journal=Canadian Journal of Soil Science |volume=40 |issue=2 |title=Soil heaving in alfalfa plots in relation to soil and air temperature |year=1960 |pages=212–18 |doi=10.4141/cjss60-027 }}</ref>
# preventing damage to desirable soil structure by freezing of saturated soils<ref>{{cite journal |last=Dagesse |first=Daryl F. |journal=Canadian Journal of Soil Science |volume=93 |issue=4 |title=Freezing cycle effects on water stability of soil aggregates |year=2013 |pages=473–83 |doi=10.4141/cjss2012-046 }}</ref>
# improving uptake of phosphorus by plants<ref>{{cite journal |last1=Dormaar |first1=Johan F. |last2=Ketcheson |first2=John W. |lastauthoramp=yes |journal=Canadian Journal of Soil Science |volume=40 |issue=2 |title=The effect of nitrogen form and soil temperature on the growth and phosphorus uptake of corn plants grown in the greenhouse |year=1960 |pages=177–84 |doi=10.4141/cjss60-023 }}</ref>

Soil temperatures can be raised by drying soils<ref>{{cite journal |last1=Fuchs |first1=Marcel |last2=Tanner |first2=Champ B. |lastauthoramp=yes |journal=[[Journal of Applied Meteorology]] |volume=6 |issue=5 |title=Evaporation from a drying soil |year=1967 |pages=852–57 |doi=10.1175/1520-0450(1967)006<0852:EFADS>2.0.CO;2 }}</ref> or the use of clear plastic mulches.<ref>{{cite journal |last1=Waggoner |first1=Paul E. |last2=Miller |first2=Patrick M. |last3=De Roo |first3=Henry C. |lastauthoramp=yes |journal=Bulletin of the Connecticut Agricultural Experiment Station |volume=634 |title=Plastic mulching: principles and benefits |url=https://archive.org/details/plasticmulchingp00wagg |year=1960 |pages=1–44 |accessdate=10 February 2018 |format=[[Portable Document Format|PDF]]}}</ref> Organic mulches slow the warming of the soil.<ref name="Lal1974"/>

There are various factors that affect soil temperature, such as water content,<ref>{{cite journal |last=Beadle |first=Noel C.W. |journal=[[Journal of Ecology]] |volume=28 |issue=1 |title=Soil temperatures during forest fires and their effect on the survival of vegetation |url=http://firearchaeology.com/Direct_Effects_files/Beadle_1940.pdf |year=1940 |pages=180–92 |doi=10.2307/2256168 |accessdate=18 February 2018 |jstor=2256168 }}</ref> soil color,<ref name="Post">{{cite journal |last1=Post |first1=Donald F. |last2=Fimbres |first2=Adan |last3=Matthias |first3=Allan D. |last4=Sano |first4=Edson E. |last5=Accioly |first5=Luciano |last6=Batchily |first6=A. Karim |last7=Ferreira |first7=Laerte G. |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=64 |issue=3 |title=Predicting soil albedo from soil color and spectral reflectance data |url=https://www.researchgate.net/publication/237751086 |year=2000 |pages=1027–34 |doi=10.2136/sssaj2000.6431027x |accessdate=25 February 2018 |format=[[Portable Document Format|PDF]]|bibcode=2000SSASJ..64.1027P }}</ref> and relief (slope, orientation, and elevation),<ref>{{cite journal |last1=Macyk |first1=T.M. |last2=Pawluk |first2=S. |last3=Lindsay |first3=J.D. |lastauthoramp=yes |journal=Canadian Journal of Soil Science |volume=58 |issue=3 |title=Relief and microclimate as related to soil properties |year=1978 |pages=421–38 |doi=10.4141/cjss78-049 }}</ref> and soil cover (shading and insulation), in addition to air temperature.<ref>{{cite journal |last1=Zheng |first1=Daolan |last2=Hunt Jr |first2=E. Raymond |last3=Running |first3=Steven W. |lastauthoramp=yes |journal=[[Climate Research (journal)|Climate Research]] |volume=2 |issue=3 |title=A daily soil temperature model based on air temperature and precipitation for continental applications |url=https://www.int-res.com/articles/cr/2/c002p183.pdf |year=1993 |pages=183–91 |doi=10.3354/cr002183 |bibcode=1993ClRes...2..183Z |accessdate=10 March 2018 |format=[[Portable Document Format|PDF]]}}</ref> The color of the ground cover and its insulating properties have a strong influence on soil temperature.<ref>{{cite journal |last1=Kang |first1=Sinkyu |last2=Kim |first2=S. |last3=Oh |first3=S. |last4=Lee |first4=Dowon |lastauthoramp=yes |journal=[[Forest Ecology and Management]] |volume=136 |issue=1–3 |title=Predicting spatial and temporal patterns of soil temperature based on topography, surface cover and air temperature |url=https://www.academia.edu/9410216 |year=2000 |pages=173–84 |doi=10.1016/S0378-1127(99)00290-X |accessdate=4 March 2018 |format=[[Portable Document Format|PDF]]}}</ref> Whiter soil tends to have a higher [[albedo]] than blacker soil cover, which encourages whiter soils to have lower soil temperatures.<ref name="Post"/> The [[specific heat]] of soil is the energy required to raise the temperature of soil by 1&nbsp;°C. The specific heat of soil increases as water content increases, since the heat capacity of water is greater than that of dry soil.<ref>{{cite journal |last=Bristow |first=Keith L. |journal=[[Agricultural and Forest Meteorology]] |volume=89 |issue=2 |title=Measurement of thermal properties and water content of unsaturated sandy soil using dual-probe heat-pulse probes |url=https://eurekamag.com/ftext.php?pdf=003197845 |year=1998 |pages=75–84 |doi=10.1016/S0168-1923(97)00065-8 |accessdate=4 March 2018 |format=[[Portable Document Format|PDF]]|bibcode=1998AgFM...89...75B }}</ref> The specific heat of pure water is ~ 1 calorie per gram, the specific heat of dry soil is ~ 0.2 calories per gram, hence, the specific heat of wet soil is ~ 0.2 to 1 calories per gram (0.8 to 4.2 kJ per kilogram).<ref>{{cite journal |last=Abu-Hamdeh |first=Nidal H. |journal=Biosystems Engineering |volume=86 |issue=1 |title=Thermal properties of soils as affected by density and water content |url=https://www.academia.edu/1319876 |year=2003 |pages=97–102 |doi=10.1016/S1537-5110(03)00112-0 |accessdate=4 March 2018 |format=[[Portable Document Format|PDF]]}}</ref> Also, a tremendous energy (~584 cal/g or 2442 kJ/kg at 25 ℃) is required to evaporate water (known as the [[Enthalpy of vaporization|heat of vaporization]]). As such, wet soil usually warms more slowly than dry soil – wet surface soil is typically 3 to 6&nbsp;°C colder than dry surface soil.<ref>{{cite journal |last=Beadle |first=N.C.W. |journal=[[Journal of Ecology]] |volume=28 |issue=1 |title=Soil temperatures during forest fires and their effect on the survival of vegetation |url=http://firearchaeology.com/Direct_Effects_files/Beadle_1940.pdf |year=1940 |pages=180–92 |doi=10.2307/2256168 |accessdate=11 March 2018 |jstor=2256168 }}</ref>

Soil [[heat flux]] refers to the rate at which [[heat energy]] moves through the soil in response to a temperature difference between two points in the soil. The heat [[flux density]] is the amount of energy that flows through soil per unit area per unit time and has both magnitude and direction. For the simple case of conduction into or out of the soil in the vertical direction, which is most often applicable the heat flux density is:

: <math>q_x = - k \frac{\delta T}{\delta x}</math>

In [[SI]] units
: <math>q</math> is the heat flux density, in SI the units are [[Watt|W]]·m<sup>−2</sup>
: <math>k</math> is the soils' [[thermal conductivity|conductivity]], [[Watt|W]]·m<sup>−1</sup>·[[Kelvin|K]]<sup>−1</sup>. The thermal conductivity is sometimes a constant, otherwise an average value of conductivity for the soil condition between the surface and the point at depth is used.
: <math>\delta T</math> is the temperature difference ([[temperature gradient]]) between the two points in the soil between which the heat flux density is to be calculated. In SI the units are kelvin, [[Kelvin|K]].
: <math>\delta x</math> is the distance between the two points within the soil, at which the temperatures are measured and between which the heat flux density is being calculated. In SI the units are meters [[meter|m]], and where x is measured positive downward.

Heat flux is in the direction opposite the temperature gradient, hence the minus sign. That is to say, if the temperature of the surface is higher than at depth x the negative sign will result in a positive value for the heat flux q, and which is interpreted as the heat being conducted into the soil.

{| class="wikitable"
|+<ref name="Brady"/>
|-
! Component!! Thermal Conductivity (W·m‐1·K‐1)
|-
| Quartz || 8.8
|-
| Clay || 2.9
|-
| Organic matter || 0.25
|-
| Water || 0.57
|-
| Ice || 2.4
|-
| Air || 0.025
|-
| Dry soil || 0.2‐0.4
|-
| Wet soil || 1–3
|-
|}

Soil temperature is important for the survival and early growth of [[seedling]]s.<ref>{{cite journal |last=Barney |first=Charles W. |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=26 |issue=1 |title=Effects of soil temperature and light intensity on root growth of loblolly pine seedlings |year=1951 |pages=146–63 |doi=10.1104/pp.26.1.146 |pmid=16654344 |pmc=437627 }}</ref> Soil temperatures affect the anatomical and morphological character of root systems.<ref>{{cite journal |last1=Equiza |first1=Maria A. |last2=Miravé |first2=Juan P. |last3=Tognetti |first3=Jorge A. |lastauthoramp=yes |journal=[[Annals of Botany]] |volume=87 |issue=1 |title=Morphological, anatomical and physiological responses related to differential shoot vs. root growth inhibition at low temperature in spring and winter wheat |url=https://eurekamag.com/ftext.php?pdf=003504275 |year=2001 |pages=67–76 |doi=10.1006/anbo.2000.1301 |accessdate=17 March 2018 |format=[[Portable Document Format|PDF]]}}</ref> All physical, chemical, and biological processes in soil and roots are affected in particular because of the increased viscosities of water and [[protoplasm]] at low temperatures.<ref>{{cite journal |last1=Babalola |first1=Olubukola |last2=Boersma |first2=Larry |last3=Youngberg |first3=Chester T. |lastauthoramp=yes |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=43 |issue=4 |title=Photosynthesis and transpiration of Monterey pine seedlings as a function of soil water suction and soil temperature |url=http://www.plantphysiol.org/content/plantphysiol/43/4/515.full.pdf |year=1968 |pages=515–21 |doi=10.1104/pp.43.4.515 |pmid=16656800 |accessdate=17 March 2018 |pmc=1086880 }}</ref> In general, climates that do not preclude survival and growth of [[white spruce]] above ground are sufficiently benign to provide soil temperatures able to maintain white spruce root systems. In some northwestern parts of the range, white spruce occurs on [[permafrost]] sites<ref>{{cite journal |last=Gill |first=Don |journal=[[Canadian Journal of Earth Sciences]] |volume=12 |issue=2 |title=Influence of white spruce trees on permafrost-table microtopography, Mackenzie River Delta |url=https://eurekamag.com/ftext.php?pdf=000118451 |year=1975 |pages=263–72 |doi=10.1139/e75-023 |accessdate=18 March 2018 |format=[[Portable Document Format|PDF]]|bibcode=1975CaJES..12..263G }}</ref> and although young unlignified roots of [[Pinophyta|conifers]] may have little resistance to freezing,<ref>{{cite journal |last1=Coleman |first1=Mark D. |last2=Hinckley |first2=Thomas M. |last3=McNaughton |first3=Geoffrey |last4=Smit |first4=Barbara A. |lastauthoramp=yes |journal=[[Canadian Journal of Forest Research]] |volume=22 |issue=7 |title=Root cold hardiness and native distribution of subalpine conifers |url=https://www.researchgate.net/publication/235695452 |year=1992 |pages=932–38 |doi=10.1139/x92-124 |accessdate=25 March 2018 |format=[[Portable Document Format|PDF]]}}</ref> the root system of containerized white spruce was not affected by exposure to a temperature of 5 to 20&nbsp;°C.<ref>{{cite journal |last1=Binder |first1=Wolfgang D. |last2=Fielder |first2=Peter |lastauthoramp=yes |journal=New Forests |volume=9 |issue=3 |title=Heat damage in boxed white spruce (Picea glauca [Moench.] Voss) seedlings: its pre-planting detection and effect on field performance |url=https://eurekamag.com/ftext.php?pdf=002630271 |year=1995 |pages=237–59 |doi=10.1007/BF00035490 |accessdate=25 March 2018 |format=[[Portable Document Format|PDF]]}}</ref>

Optimum temperatures for tree root growth range between 10&nbsp;°C and 25&nbsp;°C in general<ref>{{cite journal |last1=McMichael |first1=Bobby L. |last2=Burke |first2=John J. |lastauthoramp=yes |journal=[[HortScience]] |volume=33 |issue=6 |title=Soil temperature and root growth |url=http://hortsci.ashspublications.org/content/33/6/947.full.pdf |year=1998 |pages=947–51 |accessdate=1 April 2018 |doi=10.21273/HORTSCI.33.6.947 }}</ref> and for spruce in particular.<ref name="Landhäusser">{{cite journal |last1=Landhäusser |first1=Simon M. |last2=DesRochers |first2=Annie |last3=Lieffers |first3=Victor J. |lastauthoramp=yes |journal=[[Canadian Journal of Forest Research]] |volume=31 |issue=11 |title=A comparison of growth and physiology in Picea glauca and Populus tremuloides at different soil temperatures |url=https://www.academia.edu/15511627 |year=2001 |pages=1922–29 |doi=10.1139/x01-129 |accessdate=1 April 2018 |format=[[Portable Document Format|PDF]]}}</ref> In 2-week-old white spruce seedlings that were then grown for 6 weeks in soil at temperatures of 15&nbsp;°C, 19&nbsp;°C, 23&nbsp;°C, 27&nbsp;°C, and 31&nbsp;°C; shoot height, shoot dry weight, stem diameter, root penetration, root volume, and root dry weight all reached maxima at 19&nbsp;°C.<ref>{{cite journal |last1=Heninger |first1=Ronald L. |last2=White |first2=D.P. |lastauthoramp=yes |journal=Forest Science |volume=20 |issue=4 |title=Tree seedling growth at different soil temperatures |url=https://academic.oup.com/forestscience/article/20/4/363/4675565 |year=1974 |pages=363–67 |doi=10.1093/forestscience/20.4.363 |accessdate=1 April 2018 |format=[[Portable Document Format|PDF]]|doi-broken-date=2019-03-10 }}</ref>

However, whereas strong positive relationships between soil temperature (5&nbsp;°C to 25&nbsp;°C) and growth have been found in [[Populus tremuloides|trembling aspen]] and [[Populus balsamifera|balsam poplar]], white and other spruce species have shown little or no changes in growth with increasing soil temperature.<ref name="Landhäusser"/><ref>{{cite journal |last1=Tryon |first1=Peter R. |last2=Chapin |first2=F. Stuart III |lastauthoramp=yes |journal=[[Canadian Journal of Forest Research]] |volume=13 |issue=5 |title=Temperature control over root growth and root biomass in taiga forest trees |year=1983 |pages=827–33 |doi=10.1139/x83-112 }}</ref><ref>{{cite journal |last1=Landhäusser |first1=Simon M. |last2=Silins |first2=Uldis |last3=Lieffers |first3=Victor J. |last4=Liu |first4=Wei |lastauthoramp=yes |journal=Scandinavian Journal of Forest Research |volume=18 |issue=5 |title=Response of Populus tremuloides, Populus balsamifera, Betula papyrifera and Picea glauca seedlings to low soil temperature and water-logged soil conditions |url=https://www.researchgate.net/publication/41107813 |year=2003 |pages=391–400 |doi=10.1080/02827580310015044 |accessdate=1 April 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Turner |first1=N.C. |last2=Jarvis |first2=Paul G. |lastauthoramp=yes |journal=[[Journal of Applied Ecology]] |volume=12 |issue=2 |title=Photosynthesis in Sitka spruce (Picea sitchensis (Bong.) Carr. IV. Response to soil temperature |jstor=2402174 |year=1975 |pages=561–76 |doi=10.2307/2402174 }}</ref><ref>{{cite journal |last1=Day |first1=Tolly A. |last2=DeLucia |first2=Evan H. |last3=Smith |first3=William K. |lastauthoramp=yes |year=1990 |title=Effect of soil temperature on stem flow, shoot gas exchange and water potential of ''Picea engelmannii'' (Parry) during snowmelt |jstor=4219453 |journal=[[Oecologia]] |volume=84 |issue=4 |pages=474–81 |doi=10.1007/bf00328163 |bibcode=1990Oecol..84..474D |pmid=28312963 }}</ref> Such insensitivity to soil low temperature may be common among a number of western and boreal conifers.<ref>{{cite journal |last=Green |first=D. Scott |year=2004 |title=Describing condition-specific determinants of competition in boreal and sub-boreal mixedwood stands |journal=Forestry Chronicle |volume=80 |issue=6 |pages=736–42 |doi=10.5558/tfc80736-6 }}</ref>

Soil temperatures are increasing worldwide under the influence of present-day global [[climate warming]], with opposing views about expected effects on [[carbon capture and storage]] and [[feedback loops]] to [[climate change]]<ref>{{cite journal |last1=Davidson |first1=Eric A. |last2=Janssens |first2=Ivan A. |lastauthoramp=yes |year=2006 |title=Temperature sensitivity of soil carbon decomposition and feedbacks to climate change |url=https://pdfs.semanticscholar.org/a821/54357b012bd2b159d5edea949ffc2398561d.pdf |journal=[[Nature (journal)|Nature]] |volume=440 |issue=7081 |pages=165–73 |doi=10.1038/nature04514 |pmid=16525463 |accessdate=8 April 2018 |bibcode=2006Natur.440..165D }}</ref> Most threats are about [[permafrost]] thawing and attended effects on carbon destocking<ref>{{cite journal |last1=Schaefer |first1=Kevin |last2=Zhang |first2=Tingjun |last3=Bruhwiler |first3=Lori |last4=Barrett |first4=Andrew P. |lastauthoramp=yes |year=2011 |title=Amount and timing of permafrost carbon release in response to climate warming |url=https://www.tandfonline.com/doi/pdf/10.1111/j.1600-0889.2010.00527.x |journal=[[Tellus B]] |volume=63 |issue=2 |pages=165–80 |doi=10.1111/j.1600-0889.2011.00527.x |accessdate=8 April 2018 |format=[[Portable Document Format|PDF]]|bibcode=2011TellB..63..165S }}</ref> and ecosystem collapse.<ref>{{cite journal |last1=Jorgenson |first1=M. Torre |last2=Racine |first2=Charles H. |last3=Walters |first3=James C. |last4=Osterkamp |first4=Thomas E. |lastauthoramp=yes |year=2001 |title=Permafrost degradation and ecological changes associated with a warming climate in Central Alaska |journal=[[Climatic Change (journal)|Climatic Change]] |volume=48 |issue=4 |pages=551–79 |doi=10.1023/A:1005667424292 |citeseerx=10.1.1.420.5083}}</ref>

===Color===
{{Main|Soil color}}
Soil colour is often the first impression one has when viewing soil. Striking colours and contrasting patterns are especially noticeable. The [[Red River of the South]] carries sediment eroded from extensive reddish soils like [[Port Silt Loam]] in Oklahoma. The [[Yellow River]] in China carries yellow sediment from eroding loess soils. [[Mollisols]] in the [[Great Plains]] of North America are darkened and enriched by organic matter. [[Podsol]]s in [[Taiga|boreal forests]] have highly contrasting layers due to acidity and leaching.

In general, color is determined by the organic matter content, drainage conditions, and degree of oxidation. Soil color, while easily discerned, has little use in predicting soil characteristics.{{sfn|Donahue|Miller|Shickluna|1977|p=71}} It is of use in distinguishing boundaries of [[Soil horizon|horizons]] within a soil profile,<ref>{{cite web |url=https://blogs.egu.eu/divisions/sss/2014/03/30/soil-color-never-lies/ |publisher=[[European Geosciences Union]] |title=Soil color never lies |accessdate=25 February 2018}}</ref> determining the origin of a soil's [[parent material]],<ref>{{cite journal |last1=Viscarra Rossel |first1=Raphael A. |last2=Cattle |first2=Stephen R. |last3=Ortega |first3=A. |last4=Fouad |first4=Youssef |lastauthoramp=yes |journal=Geoderma |volume=150 |title=In situ measurements of soil colour, mineral composition and clay content by vis–NIR spectroscopy |year=2009 |pages=253–66 |df=dmy-all |citeseerx=10.1.1.462.5659 }}</ref> as an indication of wetness and [[Waterlogging (agriculture)|waterlogged]] conditions,<ref name="Blavet">{{cite journal |last1=Blavet |first1=Didier |last2=Mathe |first2=E. |last3=Leprun |first3=Jean-Claude |lastauthoramp=yes |journal=Catena |volume=39 |issue=3 |title=Relations between soil colour and waterlogging duration in a representative hillside of the West African granito-gneissic bedrock |url=http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_55-56/010021572.pdf |year=2000 |pages=187–210 |doi=10.1016/S0341-8162(99)00087-9 |accessdate=13 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> and as a qualitative means of measuring organic,<ref>{{cite journal |last1=Shields |first1=J.A. |last2=Paul |first2=Eldor A. |last3=St. Arnaud |first3=Roland J. |last4=Head |first4=W.K. |lastauthoramp=yes |journal=Canadian Journal of Soil Science |volume=48 |issue=3 |title=Spectrophotometric measurement of soil color and its relationship to moisture and organic matter |year=1968 |pages=271–80 |doi=10.4141/cjss68-037 }}</ref> iron oxide<ref name="Barrón">{{cite journal |last1=Barrón |first1=Vidal |last2=Torrent |first2=José |lastauthoramp=yes |journal=Journal of Soil Science |volume=37 |issue=4 |title=Use of the Kubelka-Munk theory to study the influence of iron oxides on soil colour |url=http://www.uco.es/organiza/departamentos/decraf/pdf-edaf/JSS1986.pdf |year=1986 |pages=499–510 |doi=10.1111/j.1365-2389.1986.tb00382.x |accessdate=5 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> and clay contents of soils.<ref>{{cite journal |last1=Viscarra Rossel |first1=Raphael A. |last2=Cattle |first2=Stephen R. |last3=Ortega |first3=Andres |last4=Fouad |first4=Youssef |lastauthoramp=yes |journal=Geoderma |volume=150 |issue=3/4 |title=In situ measurements of soil colour, mineral composition and clay content by vis–NIR spectroscopy |year=2009 |pages=253–66 |doi=10.1016/j.geoderma.2009.01.025 |bibcode=2009Geode.150..253V |citeseerx=10.1.1.462.5659 }}</ref> Color is recorded in the [[Munsell color system]] as for instance 10YR3/4 ''Dusky Red'', with 10YR as ''[[hue]]'', 3 as ''[[lightness|value]]'' and 4 as ''[[colorfulness|chroma]]''. Munsell color dimensions (hue, value and chroma) can be averaged among samples and treated as quantitative parameters, displaying significant correlations with various soil<ref>{{cite journal |last1=Ponge |first1=Jean-François |last2=Chevalier |first2=Richard |last3=Loussot |first3=Philippe |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=66 |issue=6 |title=Humus Index: an integrated tool for the assessment of forest floor and topsoil properties |url=https://www.researchgate.net/publication/240789573 |year=2002 |pages=1996–2001 |doi=10.2136/sssaj2002.1996 |accessdate=14 January 2018 |format=[[Portable Document Format|PDF]]|bibcode=2002SSASJ..66.1996P }}</ref> and vegetation properties.<ref>{{cite journal |last1=Maurel |first1=Noelie |last2=Salmon |first2=Sandrine |last3=Ponge |first3=Jean-François |last4=Machon |first4=Nathalie |last5=Moret |first5=Jacques |last6=Muratet |first6=Audrey |lastauthoramp=yes |journal=Biological Invasions |volume=12 |issue=6 |title=Does the invasive species Reynoutria japonica have an impact on soil and flora in urban wastelands? |url=https://www.researchgate.net/publication/234058727 |year=2010 |pages=1709–19 |doi=10.1007/s10530-009-9583-4 |accessdate=14 January 2018 |format=[[Portable Document Format|PDF]]}}</ref>

Soil color is primarily influenced by soil mineralogy. Many soil colours are due to various iron minerals.<ref name="Barrón"/> The development and distribution of colour in a soil profile result from chemical and biological weathering, especially [[redox]] reactions.<ref name="Blavet"/> As the primary minerals in soil parent material weather, the elements combine into new and colourful [[Chemical compound|compounds]]. Iron forms secondary minerals of a yellow or red colour,<ref>{{cite journal |last1=Davey |first1=B.G. |last2=Russell |first2=J.D. |last3=Wilson |first3=M. Jeff |lastauthoramp=yes |journal=Geoderma |volume=14 |issue=2 |title=Iron oxide and clay minerals and their relation to colours of red and yellow podzolic soils near Sydney, Australia |url=https://eurekamag.com/ftext.php?pdf=000415486 |year=1975 |pages=125–38 |doi=10.1016/0016-7061(75)90071-3 |accessdate=21 January 2018 |format=[[Portable Document Format|PDF]]|bibcode=1975Geode..14..125D }}</ref> organic matter decomposes into black and brown [[Humus|humic]] compounds,<ref>{{cite journal |last=Anderson |first=Darwin W. |journal=European Journal of Soil Science |volume=30 |issue=1 |title=Processes of humus formation and transformation in soils of the Canadian Great Plains |year=1979 |pages=77–84 |doi=10.1111/j.1365-2389.1979.tb00966.x }}</ref> and [[manganese]]<ref>{{cite journal |last1=Vodyanitskii |first1=Yu. N. |last2=Vasil'ev |first2=A.A. |last3=Lessovaia |first3=Sofia N. |last4=Sataev |first4=E.F. |last5=Sivtsov |first5=A.V.|lastauthoramp=yes |journal=Eurasian Soil Science |volume=37 |issue=6 |title=Formation of manganese oxides in soils |url=https://www.researchgate.net/publication/279708542 |year=2004 |pages=572–84 |accessdate=21 January 2018 |format=[[Portable Document Format|PDF]]}}</ref> and [[sulfur]]<ref>{{cite book |last1=Fanning |first1=D.S. |last2=Rabenhorst |first2=M.C. |last3=Bigham |first3=J.M. |lastauthoramp=yes |date=1993 |chapter=Colors of acid sulfate soils |title=Soil color |edition=1st |editor1-first=J.M. |editor1-last=Bigham |editor2-first=E.J. |editor2-last=Ciolkosz |publisher=[[Soil Science Society of America]] |location=Fitchburg, Wisconsin |pages=91–108 |isbn=978-0-89118-926-8 |url=https://dl.sciencesocieties.org/publications/books/abstracts/sssaspecialpubl/soilcolor/91 |accessdate=21 January 2018 |subscription=yes}}</ref> can form black mineral deposits. These pigments can produce various colour patterns within a soil. [[Oxygen|Aerobic]] conditions produce uniform or gradual colour changes, while [[Hypoxia (environmental)|reducing environments]] ([[:wikt:anaerobic|anaerobic]]) result in rapid colour flow with complex, mottled patterns and points of colour concentration.<ref>{{cite web |url=https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=nrcs142p2_054286 | publisher=U.S. Department of Agriculture – Natural Resources Conservation Service |title=The color of soil |accessdate=7 January 2018}}</ref>

===Resistivity===
{{main|Soil resistivity}}
Soil resistivity is a measure of a soil's ability to retard the [[electrical conduction|conduction]] of an [[electric current]]. The electrical [[resistivity]] of soil can affect the rate of [[galvanic corrosion]] of metallic structures in contact with the soil.<ref>{{cite journal |last1=Bansode |first1=Vishal M. |last2=Vagge |first2=Shashikant T. |last3=Kolekar |first3=Aniket B. |lastauthoramp=yes |journal=International Journal of Research and Scientific Innovation |volume=2 |issue=11 |title=Relationship between soil properties and corrosion of steel pipe in alkaline soils |url=http://www.rsisinternational.org/Issue20/57-61.pdf |year=2015 |pages=57–61 |accessdate=22 April 2018 |format=[[Portable Document Format|PDF]]}}</ref> Higher moisture content or increased [[electrolyte]] concentration can lower resistivity and increase conductivity, thereby increasing the rate of corrosion.<ref>{{cite journal |last1=Noor |first1=Ehteram A. |last2=Al-Moubaraki |first2=Aisha |lastauthoramp=yes |journal=Arabian Journal for Science and Engineering |volume=39 |issue=7 |title=Influence of soil moisture content on the corrosion behavior of X60 steel in different soils |url=https://www.researchgate.net/publication/272039484 |year=2014 |pages=5421–35 |doi=10.1007/s13369-014-1135-2 |accessdate=22 April 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Amrheln |first1=Christopher |last2=Strong |first2=James E. |last3=Mosher |first3=Paul A.
|lastauthoramp=yes |journal=[[Environmental Science and Technology]] |volume=26 |issue=4 |title=Effect of deicing salts on metal and organic matter mobility in roadside soils |url=https://eurekamag.com/ftext.php?pdf=002354564 |year=1992 |pages=703–09 |doi=10.1021/es00028a006 |accessdate=22 April 2018 |format=[[Portable Document Format|PDF]]|bibcode=1992EnST...26..703A }}</ref> Soil resistivity values typically range from about 1 to 100000&nbsp;[[Ohm|Ω]]·m, extreme values being for saline soils and dry soils overlaying cristalline rocks, respectively.<ref>{{cite journal |last1=Samouëlian |first1=Anatja |last2=Cousin |first2=Isabelle |last3=Tabbagh |first3=Alain |last4=Bruand |first4=Ary |last5=Richard |first5=Guy |lastauthoramp=yes |journal=Soil and Tillage Research |volume=83 |issue=2 |title=Electrical resistivity survey in soil science: a review |url=https://hal-insu.archives-ouvertes.fr/hal-00023493/document |year=2005 |pages=173–93 |doi=10.1016/j.still.2004.10.004 |accessdate=29 April 2018 |format=[[Portable Document Format|PDF]]|citeseerx=10.1.1.530.686 }}</ref>

==Water==
{{Further|Water content|Water potential}}

Water that enters a field is removed from a field by [[Surface runoff|runoff]], [[drainage]], [[evaporation]] or [[transpiration]].<ref>{{cite journal |last1=Wallace |first1=James S. |last2=Batchelor |first2=Charles H. |lastauthoramp=yes |journal=[[Philosophical Transactions of the Royal Society B: Biological Sciences]] |volume=352 |issue=1356 |title=Managing water resources for crop production |year=1997 |pages=937–47 |doi=10.1098/rstb.1997.0073 |pmc=1691982 }}</ref> Runoff is the water that flows on the surface to the edge of the field; drainage is the water that flows through the soil downward or toward the edge of the field underground; evaporative water loss from a field is that part of the water that evaporates into the atmosphere directly from the field's surface; transpiration is the loss of water from the field by its evaporation from the plant itself.

Water affects [[soil formation]], [[soil structure|structure]], stability and [[erosion]] but is of primary concern with respect to plant growth.<ref>{{cite journal |last1=Veihmeyer |first1=Frank J. |last2=Hendrickson |first2=Arthur H. |lastauthoramp=yes |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=2 |issue=1 |title=Soil-moisture conditions in relation to plant growth |year=1927 |pages=71–82 |doi=10.1104/pp.2.1.71 |pmid=16652508 |pmc=439946 }}</ref> Water is essential to plants for four reasons:

# It constitutes 80%-95% of the plant's [[protoplasm]].
# It is essential for [[photosynthesis]].
# It is the solvent in which [[nutrients]] are carried to, into and throughout the plant.
# It provides the [[turgidity]] by which the plant keeps itself in proper position.{{sfn|Donahue|Miller|Shickluna|1977|p=72}}

In addition, water alters the soil profile by dissolving and re-depositing minerals, often at lower levels.<ref>{{cite book |last1=Van Breemen |first1=Nico |last2=Buurman |first2=Peter |lastauthoramp=yes |date=2003 |title=Soil formation |edition=2nd |isbn=978-0-306-48163-5 |publisher=[[Kluwer Academic Publishers]] |location=Dordrecht, The Netherlands |url=https://www.researchgate.net/publication/40190754 |accessdate=29 April 2018 |format=[[Portable Document Format|PDF]]}}</ref> In a loam soil, solids constitute half the volume, gas one-quarter of the volume, and water one-quarter of the volume<ref name="McClellan2017"/> of which only half will be available to most plants, with a strong variation according to [[matric potential]].<ref>{{cite journal |last1=Ratliff |first1=Larry F. |last2=Ritchie |first2=Jerry T. |last3=Cassel |first3=D. Keith |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=47 |issue=4 |title=Field-measured limits of soil water availability as related to laboratory-measured properties |url=https://www.researchgate.net/publication/250125818 |year=1983 |pages=770–75 |doi=10.2136/sssaj1983.03615995004700040032x |accessdate=29 April 2018 |format=[[Portable Document Format|PDF]]|bibcode=1983SSASJ..47..770R }}</ref>

A flooded field will drain the gravitational water under the influence of [[gravity]] until water's adhesive and cohesive forces resist further drainage at which point it is said to have reached [[field capacity]].{{sfn|Wadleigh|1957|p=48}} At that point, plants must apply [[suction]]{{sfn|Wadleigh|1957|p=48}}{{sfn|Richards|Richards|1957|p=50}} to draw water from a soil. The water that plants may draw from the soil is called the [[available water]].{{sfn|Wadleigh|1957|p=48}}{{sfn|Richards|Richards|1957|p=56}} Once the available water is used up the remaining moisture is called unavailable water as the plant cannot produce sufficient suction to draw that water in. At 15&nbsp;bar suction, [[wilting point]], seeds will not germinate,{{sfn|Wadleigh|1957|p=39}}{{sfn|Wadleigh|1957|p=48}}{{sfn|Richards|Richards|1957|p=52}} plants begin to wilt and then die. Water moves in soil under the influence of [[gravity]], [[osmosis]] and [[capillarity]].<ref>{{cite web |url=http://www.soilphysics.okstate.edu/software/water/infil.html |title=Water movement in soils |website=[[Oklahoma State University]] |accessdate=1 May 2018}}</ref> When water enters the soil, it displaces air from interconnected [[macropores]] by [[buoyancy]], and breaks aggregates into which air is entrapped, a process called [[Slaking (geology)|slaking]].<ref>{{cite journal |last=Le Bissonnais |first=Yves |journal=European Journal of Soil Science |volume=67 |issue=1 |title=Aggregate stability and assessment of soil crustability and erodibility. I. Theory and methodology |url=https://eurekamag.com/pdf/002/002748456.pdf |year=2016 |pages=11–21 |doi=10.1111/ejss.4_12311 |accessdate=5 May 2018 |format=[[Portable Document Format|PDF]]}}</ref>

The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores ([[macropores]]) first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores ([[micropores]]). The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away. Consequently, not all soil water is available to plants, with a strong dependence on [[Soil texture|texture]].<ref name="Easton">{{cite web |url=https://vtechworks.lib.vt.edu/bitstream/handle/10919/75545/BSE-194.pdf |last1=Easton |first1=Zachary M. |last2=Bock |first2=Emily |lastauthoramp=yes |title=Soil and soil water relationships |website=[[Virginia Tech]] |accessdate=5 May 2018}}</ref> When saturated, the soil may lose nutrients as the water drains.<ref>{{cite journal |last1=Sims |first1=J. Thomas |last2=Simard |first2=Régis R. |last3=Joern |first3=Brad Christopher |lastauthoramp=yes |journal=[[Journal of Environmental Quality]] |volume=27 |issue=2 |title=Phosphorus loss in agricultural drainage: historical perspective and current research |url=https://www.researchgate.net/publication/247175178 |year=1998 |pages=277–93 |doi=10.2134/jeq1998.00472425002700020006x |accessdate=6 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> Water moves in a draining field under the influence of pressure where the soil is locally saturated and by capillarity pull to drier parts of the soil.<ref>{{cite journal |last1=Brooks |first1=Royal H. |last2=Corey |first2=Arthur T. |lastauthoramp=yes |journal=Journal of the Irrigation and Drainage Division |volume=92 |issue=2 |title=Properties of porous media affecting fluid flow |url=http://www.discovery-group.com/pdfs/Brooks_Corey_1966.pdf |year=1966 |pages=61–90 |accessdate=6 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> Most plant water needs are supplied from the suction caused by evaporation from plant leaves ([[transpiration]]) and a lower fraction is supplied by suction created by [[osmotic pressure]] differences between the plant interior and the soil solution.<ref>{{cite web |url=https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037 |last1=McElrone |first1=Andrew J. |last2=Choat |first2=Brendan |last3=Gambetta |first3=Greg A. |last4=Brodersen |first4=Craig R. |lastauthoramp=yes |title=Water uptake and transport in vascular plants |website=The Nature Education Knowledge Project |accessdate=6 May 2018}}</ref><ref>{{cite journal |last=Steudle |first=Ernst |journal=[[Plant and Soil]] |volume=226 |issue=1 |title=Water uptake by plant roots: an integration of views |url=https://eurekamag.com/pdf/011/011648732.pdf |year=2000 |pages=45–56 |doi=10.1023/A:1026439226716 |accessdate=6 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> Plant roots must seek out water and grow preferentially in moister soil microsites,<ref>{{cite journal |last1=Wilcox |first1=Carolyn S. |last2=Ferguson |first2=Joseph W. |last3=Fernandez |first3=George C.J. |last4=Nowak |first4=Robert S. |lastauthoramp=yes |journal=[[Journal of Arid Environments]] |volume=56 |issue=1 |title=Fine root growth dynamics of four Mojave Desert shrubs as related to soil moisture and microsite |url=https://eurekamag.com/pdf/004/004162907.pdf |year=2004 |pages=129–48 |doi=10.1016/S0140-1963(02)00324-5 |accessdate=6 May 2018 |format=[[Portable Document Format|PDF]]|bibcode=2004JArEn..56..129W }}</ref> but some parts of the root system are also able to remoisten dry parts of the soil.<ref>{{cite journal |last1=Hunter |first1=Albert S. |last2=Kelley |first2=Omer J. |lastauthoramp=yes |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=21 |issue=4 |title=The extension of plant roots into dry soil |year=1946 |pages=445–51 |doi=10.1104/pp.21.4.445 |pmid=16654059 |pmc=437296 }}</ref> Insufficient water will damage the yield of a crop.<ref>{{cite journal |last1=Zhang |first1=Yongqiang |last2=Kendy |first2=Eloise |last3=Qiang |first3=Yu |last4=Liu |first4=Changming |last5=Shen |first5=Yanjun |last6=Sun |first6=Hongyong |lastauthoramp=yes |journal=Agricultural Water Management |volume=64 |issue=2 |title=Effect of soil water deficit on evapotranspiration, crop yield, and water use efficiency in the North China Plain |url=https://www.academia.edu/21970856 |year=2004 |pages=107–22 |doi=10.1016/S0378-3774(03)00201-4 |accessdate=6 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> Most of the available water is used in transpiration to pull nutrients into the plant.<ref>{{cite journal |last1=Oyewole |first1=Olusegun Ayodeji |last2=Inselsbacher |first2=Erich |last3=Näsholm |first3=Torgny |lastauthoramp=yes |journal=[[New Phytologist]] |volume=201 |issue=3 |title=Direct estimation of mass flow and diffusion of nitrogen compounds in solution and soil |url=https://www.academia.edu/23273727 |year=2014 |pages=1056–64 |doi=10.1111/nph.12553 |pmid=24134319 |accessdate=10 May 2018 |format=[[Portable Document Format|PDF]]}}</ref>

Soil water is also important for climate modeling and numerical weather prediction. [[Global Climate Observing System]] specified soil water as one of the 50 Essential Climate Variables (ECVs)<ref>{{cite news |last= |first= |title= the GCOS Essential Climate Variables |pages= |publisher=GCOS |year= 2013 |url= http://www.wmo.int/pages/prog/gcos/index.php?name=EssentialClimateVariables|accessdate= 2013-11-05}}</ref>. Soil water can be measured in situ with [[soil moisture sensor]] or can be estimated from satellite data and hydrological models. Each method exhibits pros and cons, and hence, the integration of different techniques may decrease the drawbacks of a single given method.<ref>{{cite journal |last1=Brocca |first1=L. |last2=Hasenauer |first2=S. |last3=Lacava |first3=T. |last4=oramarco |first4=T. |last5=Wagner |first5=W. |last6=Dorigo |first6=W. |last7=Matgen |first7=P. |last8=Martínez-Fernández |first8=J. |last9=Llorens |first9=P. |last10=Latron |first10=C. |last11=Martin |first11=C. |last12=Bittelli |first12=M. |journal=Remote Sensing of Environment |volume=115 |issue=12 |title=Soil moisture estimation through ASCAT and AMSR-E sensors: An intercomparison and validation study across Europe |year=2011 |pages=3390-3408 |doi=10.1016/j.rse.2011.08.003}}</ref>

===Water retention===
{{Further|Soil water (retention)|Water retention curve}}
Water is retained in a soil when the [[adhesive force]] of attraction that water's [[hydrogen]] atoms have for the [[oxygen]] of soil particles is stronger than the cohesive forces that water's hydrogen feels for other water oxygen atoms.{{sfn|Donahue|Miller|Shickluna|1977|pp=72–74}} When a field is flooded, the soil [[pore space]] is completely filled by water. The field will drain under the force of gravity until it reaches what is called [[field capacity]], at which point the smallest pores are filled with water and the largest with water and gases.<ref>{{cite web |url=http://www.fao.org/docrep/r4082e/r4082e03.htm |title=Soil and water |publisher=[[Food and Agriculture Organization of the United Nations]] |accessdate=10 May 2018}}</ref> The total amount of water held when field capacity is reached is a function of the [[specific surface area]] of the soil particles.<ref>{{cite journal |last1=Petersen |first1=Lis Wollesen |last2=Møldrup |first2=Per |last3=Jacobsen |first3=Ole H. |last4=Rolston |first4=Dennis E. |lastauthoramp=yes |journal=Soil Science |volume=161 |issue=1 |title=Relations between specific surface area and soil physical and chemical properties |url=https://www.researchgate.net/publication/232162864 |year=1996 |pages=9–21 |doi=10.1097/00010694-199601000-00003 |accessdate=10 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> As a result, high clay and high organic soils have higher field capacities.<ref name="Gupta1979">{{cite journal |last1=Gupta |first1=Satish C. |last2=Larson |first2=William E. |lastauthoramp=yes |journal=[[Water Resources Research]] |volume=15 |issue=6 |title=Estimating soil water retention characteristics from particle size distribution, organic matter percent, and bulk density |year=1979 |pages=1633–35 |doi=10.1029/WR015i006p01633 |bibcode=1979WRR....15.1633G |citeseerx=10.1.1.475.497 }}</ref> The potential energy of water per unit volume relative to pure water in reference conditions is called [[water potential]]. Total water potential is a sum of matric potential which results from [[capillary action]], osmotic potential for saline soil, and gravitational potential when dealing with vertical direction of water movement. Water potential in soil usually has negative values, and therefore it is also expressed in [[suction]], which is defined as the minus of water potential. Suction has a positive value and can be regarded as the total force required to pull or push water out of soil. Water potential or suction is expressed in units of kPa (10<sup>3</sup> [[Pascal_(unit)|pascal]]), [[Bar_(unit)|bar]] (100 kPa), or [[Centimetre of water|cm H<sub>2</sub>O]] (approximately 0.098 kPa). [[Common logarithm]] of suction in cm H<sub>2</sub>O is called pF.<ref>{{Cite web|url=https://agriinfo.in/?page=topic&superid=4&topicid=277| title=Soil Water Potential| publisher=AgriInfo.in |accessdate=March 15, 2019}}</ref> Therefore pF 3 = 1000 cm = 98 kPa = 0.98 bar.

The forces with which water is held in soils determine its availability to plants. Forces of [[adhesion]] hold water strongly to mineral and humus surfaces and less strongly to itself by cohesive forces. A plant's root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. But as the droplet is drawn down, the forces of adhesion of the water for the soil particles produce increasingly higher [[suction]], finally up to 1500 kPa (pF = 4.2).<ref>{{cite journal |last1=Savage |first1=Michael J. |last2=Ritchie |first2=Joe T. |last3=Bland |first3=William L. |last4=Dugas |first4=William A. |lastauthoramp=yes |journal=[[Agronomy Journal]] |volume=88 |issue=4 |title=Lower limit of soil water availability |url=https://www.researchgate.net/publication/309079210 |year=1996 |pages=644–51 |doi=10.2134/agronj1996.00021962008800040024x |accessdate=12 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> At 1500 kPa suction, the soil water amount is called [[wilting point]]. At that suction the plant cannot sustain its water needs as water is still being lost from the plant by transpiration, the plant's turgidity is lost, and it wilts, although [[stoma]]tal closure may decrease transpiration and thus may retard wilting below the [[wilting point]], in particular under [[adaptation]] or [[acclimatization]] to drought.<ref>{{cite journal |last1=Al-Ani |first1=Tariq |last2=Bierhuizen |first2=Johan Frederik |lastauthoramp=yes |journal=Acta Botanica Neerlandica |volume=20 |issue=3 |title=Stomatal resistance, transpiration, and relative water content as influenced by soil moisture stress |url=http://natuurtijdschriften.nl/download?type=document&docid=539770 |year=1971 |pages=318–26 |doi=10.1111/j.1438-8677.1971.tb00715.x |accessdate=12 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> The next level, called air-dry, occurs at 100,000 kPa suction (pF = 6). Finally the oven dry condition is reached at 1,000,000 kPa suction (pF = 7). All water below wilting point is called unavailable water.{{sfn|Donahue|Miller|Shickluna|1977|pp=75–76}}

When the soil moisture content is optimal for plant growth, the water in the large and intermediate size pores can move about in the soil and be easily used by plants.<ref name="Easton"/> The amount of water remaining in a soil drained to field capacity and the amount that is available are functions of the soil type. Sandy soil will retain very little water, while clay will hold the maximum amount.<ref name="Gupta1979"/> The available water for the silt loam might be 20% whereas for the sand it might be only 6% by volume, as shown in this table.

{| class="wikitable" style="border-spacing: 5px; margin:auto;"
|+ '''Wilting point, field capacity, and available water of various soil textures (unit: % by volume)'''<ref>{{cite journal |last1=Rawls |first1=W. J. |last2=Brakensiek |first2=D. L. |last3=Saxtonn |first3=K. E.|journal=Transactions of the ASAE |volume=25 |issue=5 |title=Estimation of Soil Water Properties |url=https://www.researchgate.net/profile/RB_Brobst/post/Can_soil_bulk_density_be_calculated_or_extrapolated_from_the_values_of_known_indicators/attachment/59dc150c4cde260ad3ce4017/AS%3A547709509697536%401507595531889/download/Rawls+et+al+1982Trans+ASAE.pdf |year=1982 |pages=1316-1320 |doi=10.13031/2013.33720 |accessdate=17 March 2019 |format=[[Portable Document Format|PDF]]}}</ref>
|-
! scope="col" style="width:100px;"| Soil Texture
! scope="col" style="width:100px;"| Wilting Point
! scope="col" style="width:100px;"| Field Capacity
! scope="col" style="width:100px;"| Available water
|-
| Sand || 3.3 || 9.1 || 5.8
|-
| Sandy loam || 9.5 || 20.7 || 11.2
|-
| Loam || 11.7 || 27.0 || 15.3
|-
| Silt loam || 13.3|| 33.0 || 19.7
|-
| Clay loam || 19.7 || 31.8 || 12.1
|-
| Clay || 27.2 || 39.6 || 12.4
|}

The above are average values for the soil textures.

===Water flow===
Water moves through soil due to the force of [[gravity]], [[osmosis]] and [[capillarity]]. At zero to 33 kPa [[suction]] ([[field capacity]]), water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by the pressure of the water; this is called saturated flow. At higher suction, water movement is pulled by capillarity from wetter toward drier soil. This is caused by water's [[adhesion]] to soil solids, and is called unsaturated flow.{{sfn|Donahue|Miller|Shickluna|1977|p=85}}<ref>{{cite web |url=http://eagri.org/eagri50/AGRO103/lec03.pdf |title=Soil water movement: saturated and unsaturated flow and vapour movement, soil moisture constants and their importance in irrigation |website=[[Tamil Nadu Agricultural University]] |accessdate=19 May 2018}}</ref>

Water infiltration and movement in soil is controlled by six factors:

# Soil texture
# Soil structure. Fine-textured soils with granular structure are most favourable to infiltration of water.
# The amount of organic matter. Coarse matter is best and if on the surface helps prevent the destruction of soil structure and the creation of crusts.
# Depth of soil to impervious layers such as hardpans or bedrock
# The amount of water already in the soil
# Soil temperature. Warm soils take in water faster while frozen soils may not be able to absorb depending on the type of freezing.{{sfn|Donahue|Miller|Shickluna|1977|p=86}}

Water infiltration rates range from 0.25 cm per hour for high clay soils to 2.5 cm per hour for sand and well stabilized and aggregated soil structures.{{sfn|Donahue|Miller|Shickluna|1977|p=88}} Water flows through the ground unevenly, in the form of so-called "gravity fingers", because of the [[surface tension]] between water particles.<ref>{{cite journal|last1=Cueto-Felgueroso |first1=Luis |last2=Juanes |first2=Ruben |lastauthoramp=yes |journal=[[Physical Review Letters]] |volume=101 |issue=24 |title=Nonlocal interface dynamics and pattern formation in gravity-driven unsaturated flow through porous media |url=https://pdfs.semanticscholar.org/bcee/6f04cc7a8bc8df98d0cb48410ccb1efb6a33.pdf |year=2008 |pages=244504 |doi=10.1103/PhysRevLett.101.244504 |pmid=19113626 |accessdate=21 May 2018 |format=[[Portable Document Format|PDF]]|bibcode=2008PhRvL.101x4504C}}</ref><ref>{{cite web |url=http://soilandwater.bee.cornell.edu/research/pfweb/educators/intro/fingerflow.htm |title=Finger flow in coarse soils |website=[[Cornell University]] |accessdate=21 May 2018}}</ref>

Tree roots, whether living or dead, create preferential channels for rainwater flow through soil,<ref>{{cite journal |last1=Ghestem |first1=Murielle |last2=Sidle |first2=Roy C. |last3=Stokes |first3=Alexia |lastauthoramp=yes |journal=[[BioScience]] |volume=61 |issue=11 |title=The influence of plant root systems on subsurface flow: implications for slope stability |url=https://academic.oup.com/bioscience/article/61/11/869/223555 |year=2011 |pages=869–79 |doi=10.1525/bio.2011.61.11.6 |accessdate=21 May 2018 |format=[[Portable Document Format|PDF]]}}</ref> magnifying infiltration rates of water up to 27 times.<ref>{{cite journal |last1=Bartens |first1=Julia |last2=Day |first2=Susan D. |last3=Harris |first3=J. Roger |last4=Dove |first4=Joseph E. |last5=Wynn |first5= Theresa M. |lastauthoramp=yes |journal=[[Journal of Environmental Quality]] |volume=37 |issue=6 |title=Can urban tree roots improve infiltration through compacted subsoils for stormwater management? |url=https://www.researchgate.net/publication/23411104 |year=2008 |pages=2048–57 |doi=10.2134/jeq2008.0117 |pmid=18948457 |accessdate=21 May 2018 |format=[[Portable Document Format|PDF]]}}</ref>

[[Flooding]] temporarily increases [[soil permeability]] in [[river beds]], helping to [[groundwater recharge|recharge]] [[aquifers]].<ref>{{cite journal |last1=Zhang |first1=Guohua |last2=Feng |first2=Gary |last3=Li |first3=Xinhu |last4=Xie |first4=Congbao |last5=P |first5=Xiaoyu |lastauthoramp=yes |journal=Water |volume=9 |issue=7 |title=Flood effect on groundwater recharge on a typical silt loam soil |url=http://www.mdpi.com/2073-4441/9/7/523 |year=2017 |pages=523 |doi=10.3390/w9070523 |accessdate=21 May 2018 |format=[[Portable Document Format|PDF]]}}</ref>

Water applied to a soil is pushed by [[pressure gradients]] from the point of its application where it is [[Water content|saturated]] locally, to less saturated areas, such as the [[vadose zone]].<ref>{{cite journal |last1=Nielsen |first1=Donald R. |last2=Biggar |first2=James W. |last3=Erh |first3=Koon T. |lastauthoramp=yes |journal=Hilgardia |volume=42 |issue=7 |title=Spatial variability of field-measured soil-water properties |url=http://hilgardia.ucanr.edu/fileaccess.cfm?article=152767&p=EMWHPU |year=1973 |pages=215–59 |doi=10.3733/hilg.v42n07p215 |accessdate=9 June 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Rimon |first1=Yaara |last2=Dahan |first2=Ofer |last3=Nativ |first3=Ronit |last4=Geyer |first4=Stefan |lastauthoramp=yes |journal=[[Water Resources Research]] |volume=43 |issue=5 |title=Water percolation through the deep vadose zone and groundwater recharge: preliminary results based on a new vadose zone monitoring system |year=2007 |pages=W05402 |doi=10.1029/2006WR004855 |bibcode=2007WRR....43.5402R }}</ref> Once soil is completely wetted, any more water will move downward, or [[percolate]] out of the range of [[plant roots]], carrying with it clay, humus, nutrients, primarily cations, and various [[contaminants]], including [[pesticides]], [[pollutants]], [[viruses]] and [[bacteria]], potentially causing [[groundwater contamination]].<ref>{{Cite journal|last1=Weiss |first1=Peter T. |last2=LeFevre |first2=Greg |last3=Gulliver |first3=John S. |lastauthoramp=yes |title=Contamination of soil and groundwater due to stormwater infiltration practices: a literature review |citeseerx = 10.1.1.410.5113}}</ref><ref>{{cite journal |last1=Hagedorn |first1=Charles |last2=Hansen |first2=Debra T. |last3=Simonson |first3=Gerald H. |lastauthoramp=yes |journal=[[Journal of Environmental Quality]] |volume=7 |issue=1 |title=Survival and movement of fecal indicator bacteria in soil under conditions of saturated flow |url=https://pdfs.semanticscholar.org/cd24/0565714af7d83ab1c7f0c8640661f74d3dea.pdf |year=1978 |pages=55–59 |doi=10.2134/jeq1978.00472425000700010011x |accessdate=24 June 2018 |format=[[Portable Document Format|PDF]]}}</ref> In order of decreasing solubility, the leached nutrients are:
* Calcium
* Magnesium, Sulfur, Potassium; depending upon soil composition
* Nitrogen; usually little, unless nitrate fertiliser was applied recently
* Phosphorus; very little as its forms in soil are of low solubility.{{sfn|Donahue|Miller|Shickluna|1977|p=90}}

In the United States percolation water due to rainfall ranges from almost zero centimeters just east of the Rocky Mountains to fifty or more centimeters per day in the Appalachian Mountains and the north coast of the Gulf of Mexico.{{sfn|Donahue|Miller|Shickluna|1977|p=80}}

Water is pulled by [[capillary]] action due to the [[adhesion]] force of water to the soil solids, producing a [[suction]] [[gradient]] from wet towards drier soil<ref>{{cite journal |last1=Ng |first1=Charles W.W. |last2=Pang |first2=Wenyan |lastauthoramp=yes |journal=Journal of Geotechnical and Geoenvironmental Engineering |volume=126 |issue=2 |title=Influence of stress state on soil-water characteristics and slope stability |url=https://www.researchgate.net/publication/245293642 |year=2000 |pages=157–66 |doi=10.1061/(ASCE)1090-0241(2000)126:2(157) |accessdate=1 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> and from [[macropores]] to [[micropores]].<ref>{{cite journal |last1=Hilal |first1=Mostafa H. |last2=Anwar |first2=Nabil M. |lastauthoramp=yes |journal=Journal of American Science |volume=12 |issue=7 |title=Vital role of water flow and moisture distribution in soils and the necessity of a new out-Look and simulation modeling of soil-water relations |url=http://www.jofamericanscience.org/journals/am-sci/am120716/02_30330jas120716_6_18.pdf |year=2016 |pages=6–18 |doi=10.7537/marsjas120716.02 |accessdate=1 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> [[Richards equation]] represents the movement of water in [[Vadose zone|unsaturated]] soils.<ref>{{cite journal|author=Richards, L.A. |year=1931 |title=Capillary conduction of liquids through porous mediums |journal=Physics |volume=1 |issue=5 |pages=318–333 |doi=10.1063/1.1745010 |bibcode = 1931Physi...1..318R}}</ref> The analysis of unsaturated water flow and solute transport is available by using a readily available software such as [[Hydrus (software)|Hydrus]],<ref>{{cite web |last1=Šimůnek| first1=J. |last2=Saito | first2=H. | last3=Sakai | first3=M. | last4=van Genuchten |first4=M. Th. | year=2013 | url=https://www.researchgate.net/publication/271515313 | title=The HYDRUS-1D Software Package for Simulating the One-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media |accessdate=15 March 2019}}</ref> by giving soil hydraulic parameters of hydraulic functions ([[water retention curve|water retention function]] and unsaturated hydraulic conductivity function) and initial and boundary conditions. Preferential flow occurs along interconnected macropores, crevices, root and worm channels, which [[drainage|drain]] water under [[gravity]].<ref>{{cite journal |last=Bouma |first=Johan |journal=Geoderma |volume=3 |issue=4 |title=Soil morphology and preferential flow along macropores |url=https://www.researchgate.net/publication/223095848 |year=1981 |pages=235–50 |doi=10.1016/0378-3774(81)90009-3 |accessdate=1 July 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Luo |first1=Lifang |last2=Lin |first2=Henry |last3=Halleck |first3=Phil |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=72 |issue=4 |title=Quantifying soil structure and preferential flow in intact soil Using X-ray computed tomography |year=2008 |pages=1058–69 |doi=10.2136/sssaj2007.0179 |bibcode=2008SSASJ..72.1058L |citeseerx=10.1.1.455.2567 }}</ref>
Many models based on soil physics now allow for some representation of preferential flow as a dual continuum, dual porosity or dual permeability options, but these have generally been “bolted on” to the Richards solution without any rigorous physical underpinning.<ref>{{cite journal |last1=Beven |first1=Keith |last2=Germann |first2=Peter |lastauthoramp=yes |journal=[[Water Resources Research]] |volume=49 |issue=6 |title=Macropores and water flow in soils revisited |year=2013 |pages=3071–92 |doi=10.1002/wrcr.20156 |bibcode=2013WRR....49.3071B }}</ref>

===Water uptake by plants===
Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients. Most soil water is taken up by plants as passive [[absorption (chemistry)|absorption]] caused by the pulling force of water evaporating ([[transpiration|transpiring]]) from the long column of water ([[xylem sap]] flow) that leads from the plant's roots to its leaves, according to the [[cohesion-tension theory]].<ref>{{cite journal |last1=Aston |first1=M.J. |last2=Lawlor |first2=David W. |lastauthoramp=yes |journal=[[Journal of Experimental Botany]] |volume=30 |issue=1 |title=The relationship between transpiration, root water uptake, and leaf water potential |url=https://www.researchgate.net/publication/269624495 |year=1979 |pages=169–81 |doi=10.1093/jxb/30.1.169 |accessdate=8 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> The upward movement of water and solutes ([[hydraulic redistribution|hydraulic lift]]) is regulated in the roots by the [[endodermis]]<ref>{{cite journal |last=Powell |first=D.B.B. |journal=[[Plant, Cell and Environment]] |volume=1 |issue=1 |title=Regulation of plant water potential by membranes of the endodermis in young roots |url=https://eurekamag.com/pdf/000/000733478.pdf |year=1978 |pages=69–76 |doi=10.1111/j.1365-3040.1978.tb00749.x |accessdate=7 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> and in the plant foliage by [[stomatal conductance]],<ref>{{cite journal |last1=Irvine |first1=James |last2=Perks |first2=Michael P. |last3=Magnani |first3=Federico |last4=Grace |first4=John |lastauthoramp=yes |journal=Tree Physiology |volume=18 |issue=6 |title=The response of Pinus sylvestris to drought: stomatal control of transpiration and hydraulic conductance |url=https://academic.oup.com/treephys/article/18/6/393/1717403 |year=1998 |pages=393–402 |doi=10.1093/treephys/18.6.393 |pmid=12651364 |accessdate=8 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> and can be interrupted in root and shoot [[xylem vessels]] by [[cavitation]], also called ''xylem embolism''.<ref>{{cite journal |last1=Jackson |first1=Robert B. |last2=Sperry |first2=John S. |last3=Dawson |first3=Todd E. |lastauthoramp=yes |journal=Trends in Plant Science |volume=5 |issue=11 |title=Root water uptake and transport: using physiological processes in global predictions |url=https://pdfs.semanticscholar.org/2556/d8e629577730b17887c07784d334dd2a1751.pdf |year=2000 |pages=482–88 |doi=10.1016/S1360-1385(00)01766-0 |pmid=11077257 |accessdate=8 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> In addition, the high concentration of salts within plant roots creates an [[osmotic pressure]] gradient that pushes soil water into the roots.<ref>{{cite journal |last=Steudle |first=Ernst |journal=[[Plant and Soil]] |volume=226 |issue=1 |title=Water uptake by plant roots: an integration of views
|url=https://eurekamag.com/pdf/011/011648732.pdf |year=2000 |pages=45–56 |doi=10.1023/A:1026439226716 |accessdate=8 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> Osmotic absorption becomes more important during times of low water transpiration caused by lower temperatures (for example at night) or high humidity, and the reverse occurs under high temperature or low humidity. It is these process that cause [[guttation]] and [[wilting]], respectively.{{sfn|Donahue|Miller|Shickluna|1977|p=92}}<ref>{{cite journal |last1=Kaufmann |first1=Merrill R. |last2=Eckard |first2=Alan N. |lastauthoramp=yes |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=47 |issue=4 |title=Evaluation of water stress control with polyethylene glycols by analysis of guttation |year=1971 |pages=453–6 |doi=10.1104/pp.47.4.453 |pmid=16657642 |pmc=396708 }}</ref>

Root extension is vital for plant survival. A study of a single winter rye plant grown for four months in one cubic foot (0.0283 cubic meters) of loam soil showed that the plant developed 13,800,000 roots, a total of 620 km in length with 237 square meters in surface area; and 14 billion hair roots of 10,620 km total length and 400 square meters total area; for a total surface area of 638 square meters. The total surface area of the loam soil was estimated to be 52,000 square meters.{{sfn|Wadleigh|1957|p=46}} In other words, the roots were in contact with only 1.2% of the soil. However, root extension should be viewed as a dynamic process, allowing new roots to explore a new volume of soil each day, increasing dramatically the total volume of soil explored over a given growth period, and thus the volume of water taken up by the root system over this period.<ref>{{cite journal |last1=Kramer |first1=Paul J. |last2=Coile |first2=Theodore S. |lastauthoramp=yes |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=15 |issue=4 |title=An estimation of the volume of water made available by root extension |year=1940 |pages=743–47 |doi=10.1104/pp.15.4.743 |pmid=16653671 |pmc=437871 }}</ref> Root architecture, i.e. the spatial configuration of the root system, plays a prominent role in the adaptation of plants to soil water and nutrient availabiity, and thus in plant productivity.<ref>{{cite journal |last=Lynch |first=Jonathan |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=109 |issue=1 |title=Root architecture and plant productivity |year=1995 |pages=7–13 |doi=10.1104/pp.109.1.7 |pmid=12228579 |pmc=157559 }}</ref>

Roots must seek out water as the unsaturated flow of water in soil can move only at a rate of up to 2.5 cm per day; as a result they are constantly dying and growing as they seek out high concentrations of soil moisture.<ref>{{cite journal |last1=Comas |first1=Louise H. |last2=Eissenstat |first2=David M. |last3=Lakso |first3=Alan N. |lastauthoramp=yes |journal=[[New Phytologist]] |volume=147 |issue=1 |title=Assessing root death and root system dynamics in a study of grape canopy pruning |year=2000 |pages=171–78 |doi=10.1046/j.1469-8137.2000.00679.x }}</ref> Insufficient soil moisture, to the point of causing [[wilting]], will cause permanent damage and [[crop yield]]s will suffer. When grain [[sorghum]] was exposed to soil suction as low as 1300 kPa during the seed head emergence through bloom and seed set stages of growth, its production was reduced by 34%.{{sfn|Donahue|Miller|Shickluna|1977|p=94}}

===Consumptive use and water use efficiency===
Only a small fraction (0.1% to 1%) of the water used by a plant is held within the plant. The majority is ultimately lost via [[transpiration]], while [[evaporation]] from the soil surface is also substantial, the transpiration:evaporation ratio varying according to vegetation type and climate, peaking in [[tropical rainforests]] and dipping in [[steppes]] and [[deserts]].<ref>{{cite journal |last1=Schlesinger |first1=William H. |last2=Jasechko |first2=Scott |lastauthoramp=yes |journal=[[Agricultural and Forest Meteorology]] |volume=189/190 |title=Transpiration in the global water cycle |url=http://www.isohydro.ca/uploads/1/4/1/9/14194300/2014-schlesinger-jasechko-agformet.pdf |year=2014 |pages=115–17 |doi=10.1016/j.agrformet.2014.01.011 |accessdate=22 July 2018 |format=[[Portable Document Format|PDF]]|bibcode=2014AgFM..189..115S }}</ref> Transpiration plus evaporative soil moisture loss is called [[evapotranspiration]]. Evapotranspiration plus water held in the plant totals to consumptive use, which is nearly identical to evapotranspiration.{{sfn|Donahue|Miller|Shickluna|1977|p=94}}<ref>{{cite book |last1=Erie |first1=Leonard J. |last2=French |first2=Orrin F. |last3=Harris |first3=Karl |lastauthoramp=yes |date=1968 |title=Consumptive use of water by crops in Arizona |location=Tucson, Arizona |publisher=[[The University of Arizona]] |url=https://repository.arizona.edu/bitstream/handle/10150/607084/TB169.pdf |accessdate=15 July 2018 |format=[[Portable Document Format|PDF]]}}</ref>

The total water used in an agricultural field includes [[surface runoff]], [[drainage]] and consumptive use. The use of loose [[mulch]]es will reduce evaporative losses for a period after a field is irrigated, but in the end the total evaporative loss (plant plus soil) will approach that of an uncovered soil, while more water is immediately available for plant growth.<ref>{{cite journal |last1=Tolk |first1=Judy A. |last2=Howell |first2=Terry A. |last3=Evett |first3=Steve R. |lastauthoramp=yes |journal=Soil and Tillage Research |volume=50 |issue=2 |title=Effect of mulch, irrigation, and soil type on water use and yield of maize |url=https://pubag.nal.usda.gov/pubag/downloadPDF.xhtml?id=1896&content=PDF |year=1999 |pages=137–47 |doi=10.1016/S0167-1987(99)00011-2 |accessdate=15 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> [[Water use efficiency]] is measured by the [[transpiration ratio]], which is the ratio of the total water transpired by a plant to the dry weight of the harvested plant. Transpiration ratios for crops range from 300 to 700. For example, alfalfa may have a transpiration ratio of 500 and as a result 500&nbsp;kilograms of water will produce one&nbsp;kilogram of dry alfalfa.{{sfn|Donahue|Miller|Shickluna|1977|pp=97–99}}

==Atmosphere==
The atmosphere of soil, or [[soil gas]], is radically different from the atmosphere above. The consumption of [[oxygen]] by microbes and plant roots, and their release of [[carbon dioxide]], decrease oxygen and increase carbon dioxide concentration. Atmospheric CO<sub>2</sub> concentration is 0.04%, but in the soil [[pore space]] it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.<ref>{{cite journal |last1=Qi |first1=Jingen |last2=Marshall |first2=John D. |last3=Mattson |first3=Kim G. |lastauthoramp=yes |journal=[[New Phytologist]] |volume=128 |issue=3 |title=High soil carbon dioxide concentrations inhibit root respiration of Douglas fir |year=1994 |pages=435–42 |doi=10.1111/j.1469-8137.1994.tb02989.x }}</ref> Calcareous soils regulate CO<sub>2</sub> concentration thanks to [[carbonate]] [[Buffering agent|buffering]], contrary to acid soils in which all CO<sub>2</sub> respired accumulates in the soil pore system.<ref>{{cite journal |last1=Karberg |first1=Noah J. |last2=Pregitzer |first2=Kurt S. |last3=King |first3=John S. |last4=Friend |first4=Aaron L. |last5=Wood |first5=James R. |lastauthoramp=yes |journal=[[Oecologia]] |volume=142 |issue=2 |title=Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone |url=https://www.nrs.fs.fed.us/pubs/jrnl/2004/nc_2004_Karberg_001.pdf |year=2005 |pages=296–306 |doi=10.1007/s00442-004-1665-5 |pmid=15378342 |accessdate=26 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> At extreme levels CO<sub>2</sub> is toxic.<ref>{{cite journal |last1=Chang |first1=H.T. |last2=Loomis |first2=W.E. |lastauthoramp=yes |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=20 |issue=2 |title=Effect of carbon dioxide on absorption of water and nutrients by roots |year=1945 |pages=221–32 |doi=10.1104/pp.20.2.221 |pmid=16653979 |pmc=437214 }}</ref> This suggests a possible [[negative feedback]] control of soil CO<sub>2</sub> concentration through its inhibitory effects on root and microbial respiration (also called '[[soil respiration]]').<ref>{{cite journal |last1=McDowell |first1=Nate J. |last2=Marshall |first2=John D. |last3=Qi |first3=Jingen |last4=Mattson |first4=Kim |lastauthoramp=yes |journal=Tree Physiology |volume=19 |issue=9 |title=Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations |url=https://www.researchgate.net/publication/10842414 |year=1999 |pages=599–605 |doi=10.1093/treephys/19.9.599 |pmid=12651534 |accessdate=22 July 2018 |format=[[Portable Document Format|PDF]]}}</ref> In addition, the soil voids are saturated with water vapour, at least until the point of maximal [[hygroscopic]]ity, beyond which a [[vapour-pressure deficit]] occurs in the soil pore space.<ref name="Vannier1987"/> Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower, the [[diffusion coefficient]] decreasing with [[soil compaction]].<ref>{{cite journal |last1=Xu |first1=Xia |last2=Nieber |first2=John L. |last3=Gupta |first3=Satish C. |lastauthoramp=yes |journal=[[Soil Science Society of America Journal]] |volume=56 |issue=6 |title=Compaction effect on the gas diffusion coefficient in soils |url=https://eurekamag.com/pdf/002/002326719.pdf |year=1992 |pages=1743–50 |doi=10.2136/sssaj1992.03615995005600060014x |accessdate=29 July 2018 |format=[[Portable Document Format|PDF]]|bibcode=1992SSASJ..56.1743X }}</ref> Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including [[greenhouse gases]]) as well as water.<ref name="Smith2003">{{cite journal |last1=Smith |first1=Keith A. |last2=Ball |first2=Tom |last3=Conen |first3=Franz |last4=Dobbie |first4=Karen E. |last5=Massheder |first5=Jonathan |last6=Rey |first6=Ana |lastauthoramp=yes |journal=European Journal of Soil Science |volume=54 |issue=4 |title=Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes |url=https://pdfs.semanticscholar.org/6709/9461a22a400424440919df5b65e2fa66ace8.pdf |year=2003 |pages=779–91 |doi=10.1046/j.1351-0754.2003.0567.x |accessdate=5 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total [[pore space]] ([[porosity]]) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with [[water content]], air [[turbulence]] and [[temperature]], that determine the rate of diffusion of gases into and out of soil.{{sfn|Russell|1957|pp=35–36}}<ref name="Smith2003"/> [[Ped#Platy|Platy]] [[soil structure]] and [[soil compaction]] (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO<sub>3</sub> to the gases N<sub>2</sub>, N<sub>2</sub>O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen.<ref>{{cite journal |last1=Ruser |first1=Reiner |last2=Flessa |first2=Heiner |last3=Russow |first3=Rolf |last4=Schmidt |first4=G. |last5=Buegger |first5=Franz |last6=Munch |first6=J.C. |lastauthoramp=yes |journal=Soil Biology and Biochemistry |volume=38 |issue=2 |title=Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting |url=https://eurekamag.com/pdf/004/004425959.pdf |year=2006 |pages=263–74 |doi=10.1016/j.soilbio.2005.05.005 |accessdate=5 August 2018 |format=[[Portable Document Format|PDF]]|bibcode=1992SSASJ..56.1743X }}</ref> Aerated soil is also a net sink of [[methane]] CH<sub>4</sub><ref>{{cite journal |last1=Hartmann |first1=Adrian A. |last2=Buchmann |first2=Nina |last3=Niklaus |first3=Pascal A. |lastauthoramp=yes |journal=[[Plant and Soil]] |volume=342 |issue=1/2 |title=A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization |url=https://www.researchgate.net/publication/227202578 |year=2011 |pages=265–75 |doi=10.1007/s11104-010-0690-x |accessdate=12 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> but a net producer of methane (a strong heat-absorbing [[greenhouse gas]]) when soils are depleted of oxygen and subject to elevated temperatures.<ref>{{cite journal |last1=Moore |first1=Tim R. |last2=Dalva |first2=Moshe |lastauthoramp=yes |journal=Journal of Soil Science |volume=44 |issue=4 |title=The influence of temperature and water table position on carbon dioxide and methane emissions from laboratory columns of peatland soils |url=https://www.researchgate.net/publication/229878721 |year=1993 |pages=651–64 |doi=10.1111/j.1365-2389.1993.tb02330.x |accessdate=12 August 2018 |format=[[Portable Document Format|PDF]]}}</ref>

Soil atmosphere is also the seat of emissions of [[volatiles]] other than carbon and nitrogen oxides from various soil organisms, e.g. roots,<ref>{{cite journal |last1=Hiltpold |first1=Ivan |last2=Toepfer |first2=Stefan |last3=Kuhlmann |first3=Ulrich |last4=Turlings |first4=Ted C.J. |lastauthoramp=yes |journal=Chemoecology |volume=20 |issue=2 |title=How maize root volatiles affect the efficacy of entomopathogenic nematodes in controlling the western corn rootworm? |url=https://www.researchgate.net/publication/215470509 |year=2010 |pages=155–62 |doi=10.1007/s00049-009-0034-6 |accessdate=12 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> bacteria,<ref>{{cite journal |last1=Ryu |first1=Choong-Min |last2=Farag |first2=Mohamed A. |last3=Hu |first3=Chia-Hui |last4=Reddy |first4=Munagala S. |last5= Wei |first5= Han-Xun |last6= Paré |first6=Paul W. |last7= Kloepper |first7= Joseph W. |lastauthoramp=yes |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=100 |issue=8 |title=Bacterial volatiles promote growth in Arabidopsis |url=http://www.pnas.org/content/pnas/100/8/4927.full.pdf |year=2003 |pages=4927–32 |doi=10.1073/pnas.0730845100 |pmid=12684534 |pmc=153657 |accessdate=12 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> fungi,<ref>{{cite journal |last1=Hung |first1=Richard |last2=Lee |first2=Samantha |last3=Bennett |first3=Joan W. |lastauthoramp=yes |journal=[[Applied Microbiology and Biotechnology]] |volume=99 |issue=8 |title=Fungal volatile organic compounds and their role in ecosystems |url=https://www.researchgate.net/publication/273638498 |year=2015 |pages=3395–405 |doi=10.1007/s00253-015-6494-4 |pmid=25773975 |accessdate=12 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> animals.<ref>{{cite journal |last1=Purrington |first1=Foster Forbes |last2=Kendall |first2=Paricia A. |last3=Bater |first3=John E. |last4=Stinner |first4=Benjamin R. |lastauthoramp=yes |journal=Great Lakes Entomologist |volume=24 |issue=2 |title=Alarm pheromone in a gregarious poduromorph collembolan (Collembola: Hypogastruridae) |url=https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1732&context=tgle |year=1991 |pages=75–78 |accessdate=12 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks<ref>{{cite journal |last1=Badri |first1=Dayakar V. |last2=Weir |first2=Tiffany L. |last3=Van der Lelie |first3= Daniel |last4=Vivanco |first4=Jorge M. |lastauthoramp=yes |journal=[[Current Opinion in Biotechnology]] |volume=20 |issue=6 |title=Rhizosphere chemical dialogues: plant–microbe interactions |url=https://aglifesciences.tamu.edu/rootbiome/wp-content/uploads/sites/38/2015/06/2009-Badris-et-al-chemical-dialogues-1-s2.0-S0958166909001281-main.pdf |doi=10.1016/j.copbio.2009.09.014|pmid=19875278 |year=2009 |pages=642–50 |accessdate=19 August 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Salmon |first1=Sandrine |last2=Ponge |first2=Jean-François |lastauthoramp=yes |journal=Soil Biology and Biochemistry |volume=33 |issue=14 |title=Earthworm excreta attract soil springtails: laboratory experiments on Heteromurus nitidus (Collembola: Entomobryidae) |url=https://www.researchgate.net/publication/45724401 |doi=10.1016/S0038-0717(01)00129-8 |year=2001 |pages=1959–69 |accessdate=19 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> playing a decisive role in the stability, dynamics and evolution of soil ecosystems.<ref>{{cite journal |last1=Lambers |first1=Hans |last2=Mougel |first2=Christophe |last3=Jaillard |first3=Benoît |last4=Hinsinger |first4=Philipe |lastauthoramp=yes |journal=[[Plant and Soil]] |volume=321 |issue=1/2 |title=Plant-microbe-soil interactions in the rhizosphere: an evolutionary perspective |url=https://www.academia.edu/25517742 |doi=10.1007/s11104-009-0042-x |year=2009 |pages=83–115 |accessdate=19 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.<ref>{{cite journal |last1=Peñuelas |first1=Josep |last2=Asensio |first2=Dolores |last3=Tholl |first3=Dorothea |last4=Wenke |first4=Katrin |last5=Rosenkranz |first5=Maaria |last6=Piechulla |first6=Birgit |last7=Schnitzler |first7=Jörg-Petter |lastauthoramp=yes |journal=[[Plant, Cell and Environment]] |volume=37 |issue=8 |title=Biogenic volatile emissions from the soil |year=2014 |pages=1866–91 |doi=10.1111/pce.12340 |pmid=24689847 }}</ref>

We humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,<ref>{{cite journal |last1=Buzuleciu |first1=Samuel A. |last2=Crane |first2=Derek P. |last3=Parker |first3=Scott L. |lastauthoramp=yes |journal=[[Herpetological Conservation and Biology]] |volume=11 |issue=3 |title=Scent of disinterred soil as an olfactory cue used by raccoons to locate nests of diamond-backed terrapins (Malaclemys terrapin) |url=https://www.researchgate.net/publication/311722263 |year=2016 |pages=539–51 |accessdate=19 August 2018 |format=[[Portable Document Format|PDF]]}}</ref> a bulk property attributed in a [[reductionist]] manner to particular biochemical compounds such as [[petrichor]] or [[geosmin]].

==Composition of the solid phase (soil matrix)==

Soil particles can be classified by their chemical composition ([[mineralogy]]) as well as their size. The particle size distribution of a soil, its [[soil texture|texture]], determines many of the properties of that soil, in particular [[hydraulic conductivity]] and [[water potential]],<ref>{{cite journal |last1=Saxton |first1=Keith E. |last2=Rawls |first2=Walter J. |lastauthoramp=yes |journal=Soil Science Society of America Journal |volume=70 |issue=5 |title=Soil water characteristic estimates by texture and organic matter for hydrologic solutions |url=https://pdfs.semanticscholar.org/5e63/c886c4f68af5e5c242c006d2d882f0a65bfe.pdf |year=2006 |pages=1569–78 |doi=10.2136/sssaj2005.0117 |accessdate=2 September 2018 |format=[[Portable Document Format|PDF]]|citeseerx=10.1.1.452.9733 }}</ref> but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.<ref>{{cite web |last=College of Tropical Agriculture and Human Resources |title=Soil Mineralogy |url=https://www.ctahr.hawaii.edu/mauisoil/a_factor_mineralogy.aspx |website=cms.ctahr.hawaii.edu/ |publisher=University of Hawai‘i at Mānoa |accessdate=2 September 2018}}</ref>

===Gravel, sand and silt===
[[Gravel]], [[sand]] and [[silt]] are the larger [[Soil texture#Soil separates|soil particles]], and their mineralogy is often inherited from the [[parent material]] of the soil, but may include products of [[weathering]] (such as [[concretions]] of [[calcium carbonate]] or [[iron oxide]]), or residues of plant and animal life (such as silica [[phytoliths]]).<ref name=Russell1973>{{cite book |last=Russell |first=E. Walter |title=Soil conditions and plant growth |date=1973 |publisher=Longman |location=London |isbn=978-0-582-44048-7 |pages=67–70 |edition=10th}}</ref><ref>{{cite journal |last1=Mercader |first1=Julio |last2=Bennett |first2=Tim |last3=Esselmont |first3=Chris |last4=Simpson |first4=Steven |last5=Walde |first5= Dale |lastauthoramp=yes |journal=[[Quaternary Research]] |volume=75 |issue=1 |title=Soil phytoliths from miombo woodlands in Mozambique |url=https://www.academia.edu/3269735 |year=2011 |pages=138–50 |doi=10.1016/j.yqres.2010.09.008 |accessdate=9 September 2018 |format=[[Portable Document Format|PDF]]}}</ref> [[Quartz]] is the most common mineral in the sand or silt fraction as it is resistant to [[chemical weathering]], except under hot climate;<ref>{{cite journal |last1=Sleep |first1=Norman H. |last2=Hessler |first2=Angela M. |lastauthoramp=yes |journal=[[Earth and Planetary Science Letters]] |volume=241 |issue=3–4 |title=Weathering of quartz as an Archean climatic indicator |url=https://geosci.uchicago.edu/~archer/deep_earth_readings/sleep.2006.archean_weat.pdf |year=2006 |pages=594–602 |doi=10.1016/j.epsl.2005.11.020 |accessdate=9 September 2018 |format=[[Portable Document Format|PDF]]}}</ref> other common minerals are [[feldspar]]s, [[micas]] and [[ferromagnesian]] minerals such as [[pyroxenes]], [[amphiboles]] and [[olivines]], which are dissolved or transformed in clay under the combined influence of physico-chemical and biological processes.<ref name=Russell1973 /><ref>{{cite journal |last1=Banfield |first1=Jillian F. |last2=Barker |first2=William W. |last3=Welch |first3=Susan A. |last4=Taunton |first4=Anne |lastauthoramp=yes |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=96 |issue=7 |title=Biological impact on mineral dissolution: application of the lichen model to understanding mineral weathering in the rhizosphere |url=http://www.pnas.org/content/pnas/96/7/3404.full.pdf |year=1999 |pages=3404–11 |doi=10.1073/pnas.96.7.3404 |accessdate=9 September 2018 |format=[[Portable Document Format|PDF]]}}</ref>

===Mineral colloids; soil clays===
{{Further|Clay minerals}}
Due to its high [[specific surface area]] and its unbalanced negative [[electric charges]], [[clay]] is the most active mineral component of soil.<ref>{{cite journal |last1=Santamarina |first1=J. Carlos |last2=Klein |first2=Katherine A. |last3=Wang |first3=Yu-Hsing |last4=Prencke |first4=E. |lastauthoramp=yes |year=2002 |title=Specific surface: determination and relevance |journal=[[Canadian Geotechnical Journal]] |volume=39 |issue=1 |pages=233–41 |url=https://egel.kaust.edu.sa/Documents/Papers/Santamarina_2002aaa.pdf |doi=10.1139/t01-077 |accessdate=30 September 2018 |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |last1=Tombácz |first1=Etelka |last2=Szekeres |first2=Márta |lastauthoramp=yes |year=2006 |title=Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite |journal=Applied Clay Science |volume=34 |issue=1–4 |pages=105–24 |url=https://www.academia.edu/11482380 |doi=10.1016/j.clay.2006.05.009 |accessdate=30 September 2018 |format=[[Portable Document Format|PDF]]}}</ref> It is a colloidal and most often a crystalline material.<ref>{{cite journal |last=Brown |first=George |year=1984 |title=Crystal structures of clay minerals and related phyllosilicates |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=311 |issue=1517 |pages=221–40 |url=https://www.researchgate.net/publication/243687416 |doi=10.1098/rsta.1984.0025 |accessdate=30 September 2018 |format=[[Portable Document Format|PDF]]}}</ref> In soils, clay is a soil textural class and is defined in a physical sense as any mineral particle less than {{convert|2|µm|in|abbr=on|sigfig=1}} in effective diameter. Many soil minerals, such as gypsum, carbonates, or quartz, are small enough to be classified as clay based on their physical size, but chemically they do not afford the same utility as do mineralogically-defined [[clay minerals]].<ref>{{cite book |last=Hillier |first=Stephen |date=1978 |chapter=Clay mineralogy |doi=10.1007/3-540-31079-7_47 |title=Encyclopedia of sediments and Sedimentary rocks |editor-last1=Middleton |editor-first1=Gerard V. |editor-last2=Church |editor-first2=Michael J. |editor-last3=Coniglio |editor-first3=Mario |editor-last4=Hardie |editor-first4=Lawrence A. |editor-last5=Longstaffe |editor-first5=Frederick J. |publisher=[[Springer Science+Business Media B.V.]] |location=Dordrecht, The Netherlands |pages=139–42 |url=https://www.researchgate.net/publication/303201730 |accessdate=30 September 2018 |format=[[Portable Document Format|PDF]]}}</ref> Chemically, [[clay minerals]] are a range of [[phyllosilicate]] minerals with certain reactive properties.{{sfn|Donahue|Miller|Shickluna|1977|pp=101–02}}

Before the advent of [[X-ray diffraction]] clay was thought to be very small particles of [[quartz]], [[feldspar]], [[mica]], [[hornblende]] or [[augite]], but it is now known to be (with the exception of mica-based clays) a precipitate with a mineralogical composition that is dependent on but different from its parent materials and is classed as a secondary mineral.<ref>{{cite web |last1=Bergaya |first1=Faïza |last2=Beneke |first2=Klaus |last3=Lagaly |first3=Gerhard |title=History and perspectives of clay science |url=http://www.uni-kiel.de/anorg/lagaly/group/klausSchiver/clayhistory.pdf |publisher=[[University of Kiel]] |accessdate=20 October 2018 |format=[[Portable Document Format|PDF]]}}</ref> The type of clay that is formed is a function of the parent material and the composition of the minerals in solution.<ref>{{cite journal |last=Wilson |first=M. Jeff |year=1999 |title=The origin and formation of clay minerals in soils: past, present and future perspectives |journal=Clay Minerals |volume=34 |issue=1 |pages=7–25 |url=http://www.minersoc.org/pages/Archive-CM/Volume_34/34-1-7.pdf |doi=10.1180/000985599545957 |accessdate=20 October 2018 |format=[[Portable Document Format|PDF]] |archive-url=https://web.archive.org/web/20180329061907/http://www.minersoc.org/pages/Archive-CM/Volume_34/34-1-7.pdf |archive-date=29 March 2018 |dead-url=yes |df=dmy-all }}</ref> Clay minerals continue to be formed as long as the soil exists.{{sfn|Simonson|1957|p=19}} Mica-based clays result from a modification of the primary mica mineral in such a way that it behaves and is classed as a clay.<ref>{{cite journal |last=Churchman |first=G. Jock |year=1980 |title=Clay minerals formed from micas and chlorites in some New Zealand soils |journal=Clay Minerals |volume=15 |issue=1 |pages=59–76 |url=https://www.researchgate.net/publication/249852539 |doi=10.1180/claymin.1980.015.1.05 |accessdate=20 October 2018 |format=[[Portable Document Format|PDF]]}}</ref> Most clays are crystalline, but some clays or some parts of clay minerals are amorphous.<ref>{{cite journal |last1=Wada |first1=Koji |last2=Greenland |first2=Dennis J. |year=1970 |title=Selective dissolution and differential infrared spectroscopy for characterization of 'amorphous' constituents in soil clays |journal=Clay Minerals |volume=8 |issue=3 |pages=241–54 |url=http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.624.1439&rep=rep1&type=pdf |doi=10.1180/claymin.1970.008.3.02 |accessdate=20 October 2018 |format=[[Portable Document Format|PDF]]}}</ref> The clays of a soil are a mixture of the various types of clay, but one type predominates.{{sfn|Donahue|Miller|Shickluna|1977|p=102}}

Typically there are four main groups of clay minerals: [[kaolinite]], [[montmorillonite]]-[[smectite]], [[illite]], and [[chlorite]].<ref>{{cite web |title=The clay mineral group |url=http://www.galleries.com/Clays_Group |publisher=Amethyst Galleries, Inc. |accessdate=28 October 2018 |format=[[Portable Document Format|PDF]]}}</ref> Most clays are crystalline and most are made up of three or four planes of oxygen held together by planes of aluminium and silicon by way of ionic bonds that together form a single layer of clay. The spatial arrangement of the oxygen atoms determines clay's structure.<ref>{{cite book |last=Schulze |first=Darrell G. |date=2005 |chapter=Clay minerals |doi=10.1016/b0-12-348530-4/00189-2 |title=Encyclopedia of soils in the environment |editor-last=Hillel |editor-first=Daniel |publisher=Academic Press |location=Amsterdam |pages=246–54 |url=http://www.geoinfo.amu.edu.pl/geoinf/m/GLEB/1b%20Clay%20minerals_EncSoilEnv_SCHULZE%2005.pdf |accessdate=28 October 2018 |format=[[Portable Document Format|PDF]]}}</ref> Half of the weight of clay is oxygen, but on a volume basis oxygen is ninety percent.{{sfn|Russell|1957|p=33}} The layers of clay are sometimes held together through [[hydrogen bonds]], sodium or potassium bridges and as a result will swell less in the presence of water.<ref>{{cite journal |last1=Tambach |first1=Tim J. |last2=Bolhuis |first2=Peter G. |last3=Hensen |first3=Emiel J.M. |last4=Smit |first4=Berend |year=2006 |title=Hysteresis in clay swelling induced by hydrogen bonding: accurate prediction of swelling states |journal=[[Langmuir (journal)|Langmuir]] |volume=22 |issue=3 |pages=1223–34 |url=https://pdfs.semanticscholar.org/8012/819c1e06adc056ea770fae7f68adca09e61f.pdf |doi=10.1021/la051367q |accessdate=3 November 2018 |format=[[Portable Document Format|PDF]]}}</ref> Clays such as [[montmorillonite]] have layers that are loosely attached and will swell greatly when water intervenes between the layers.{{sfn|Donahue|Miller|Shickluna|1977|pp=102–07}}

In a wider sense clays can be classified as:

# Layer Crystalline '''alumino-silica clays''': [[montmorillonite]], [[illite]], [[vermiculite]], [[Chlorite group|chlorite]], [[kaolinite]].
# Crystalline Chain '''carbonate and sulfate minerals''': [[calcite]] (CaCO<sub>3</sub>), [[dolomite]] (CaMg(CO<sub>3</sub>)<sub>2</sub>) and [[gypsum]] (CaSO<sub>4</sub>·2H2O).
# '''Amorphous clays''': young mixtures of [[silica]] (SiO<sub>2</sub>-OH) and [[alumina]] (Al(OH)<sub>3</sub>) which have not had time to form regular crystals.
# '''Sesquioxide clays''': old, highly leached clays which result in oxides of [[iron]], [[aluminium]] and [[titanium]].{{sfn|Donahue|Miller|Shickluna|1977|pp=101–07}}

====Alumino-silica clays====
'''Alumino-silica clays''' or [[aluminosilicate]] clays are characterised by their regular [[crystalline]] or quasi-crystalline structure.<ref>{{cite journal |last1=Aylmore |first1=L.A. Graham |last2=Quirk |first2=James P. |lastauthoramp=yes |year=1971 |title=Domains and quasicrystalline regions in clay systems |journal=[[Soil Science Society of America Journal]] |volume=35 |issue=4 |pages=652–54 |url=https://www.researchgate.net/publication/285159912 |doi=10.2136/sssaj1971.03615995003500040046x |accessdate=18 November 2018 |format=[[Portable Document Format|PDF]]}}</ref> [[Oxygen]] in ionic bonds with [[silicon]] forms a [[tetrahedral]] coordination (silicon at the center) which in turn forms sheets of [[silica]]. Two sheets of silica are bonded together by a plane of [[aluminium]] which forms an [[octahedral]] coordination, called [[alumina]], with the oxygens of the silica sheet above and that below it.<ref name="Barton2002">{{cite book |last1=Barton |first1=Christopher D. |last2=Karathanasis |first2=Anastasios D. |date=2002 |chapter=Clay minerals |title=Encyclopedia of Soil Science |editor-last=Lal |editor-first=Rattan |publisher=[[Marcel Dekker]] |location=New York |pages=187–92 |url=https://www.srs.fs.usda.gov/pubs/ja/ja_barton002.pdf |accessdate=3 November 2018 |format=[[Portable Document Format|PDF]]}}</ref> [[Hydroxyl]] ions (OH<sup>−</sup>) sometimes substitute for oxygen. During the clay formation process, Al<sup>3+</sup> may substitute for Si<sup>4+</sup> in the silica layer, and as much as one fourth of the aluminium Al<sup>3+</sup> may be substituted by Zn<sup>2+</sup>, Mg<sup>2+</sup> or Fe<sup>2+</sup> in the alumina layer. The substitution of lower-[[Valence (chemistry)|valence]] [[cations]] for higher-valence cations ([[Isomorphism (crystallography)|isomorphous]] substitution) gives clay a local negative [[Electric charge|charge]] on an oxygen atom<ref name="Barton2002"/> that attracts and holds water and positively charged soil cations, some of which are of value for [[plant growth]].<ref>{{cite book |last1=Schoonheydt |first1=Robert A. |last2=Johnston |first2=Cliff T. |date=2011 |chapter=The surface properties of clay minerals |title=Layered mineral structures and their application in advanced technologies |editor-last1=Brigatti |editor-first1=Maria Franca |editor-last2=Mottana |editor-first2=Annibale |publisher=[[Mineralogical Society of Great Britain & Ireland]] |location=Twickenham, UK |pages=337–73 |url=https://www.researchgate.net/publication/280884094 |accessdate=2 December 2018 |format=[[Portable Document Format|PDF]]}}</ref> Isomorphous substitution occurs during the clay's formation and does not change with time.{{sfn|Donahue|Miller|Shickluna|1977|p=107}}{{sfn|Simonson|1957|pp=20–21}}
* '''Montmorillonite''' clay is made of four planes of oxygen with two silicon and one central aluminium plane intervening. The alumino-silicate montmorillonite clay is thus said to have a 2:1 ratio of silicon to aluminium, in short it is called a 2:1 clay mineral.<ref>{{cite journal |last=Lagaly |first=Gerhard |year=1979 |title=The "layer charge" of regular interstratified 2:1 clay minerals |journal=Clays and Clay Minerals |volume=27 |issue=1 |pages=1–10 |url=http://www.clays.org/journal/archive/volume%2027/27-1-1.pdf |doi=10.1346/CCMN.1979.0270101 |accessdate=2 December 2018 |format=[[Portable Document Format|PDF]]}}</ref> The seven planes together form a single crystal of montmorillonite. The crystals are weakly held together and water may intervene, causing the clay to swell up to ten times its dry volume.<ref>{{cite journal |last1=Eirish |first1=M. V. |last2=Tret'yakova |first2=L. I. |year=1970 |title=The role of sorptive layers in the formation and change of the crystal structure of montmorillonite |journal=Clay Minerals |volume=8 |issue=3 |pages=255–66 |url=http://www.minersoc.org/pages/Archive-CM/Volume_8/8-3-255.pdf |doi=10.1180/claymin.1970.008.3.03 |accessdate=2 December 2018 |format=[[Portable Document Format|PDF]] |archive-url=https://web.archive.org/web/20180719032202/http://www.minersoc.org/pages/Archive-CM/Volume_8/8-3-255.pdf |archive-date=19 July 2018 |dead-url=yes |df=dmy-all }}</ref> It occurs in soils which have had little leaching, hence it is found in arid regions, although it may also occur in humid climates, depending on its mineralogical origin.<ref>{{cite journal |last1=Tardy |first1=Yves |last2=Bocquier |first2=Gérard |last3=Paquet |first3=Hélène |last4=Millot |first4=Georges |year=1973 |title=Formation of clay from granite and its distribution in relation to climate and topography |journal=Geoderma |volume=10 |issue=4 |pages=271–84 |url=https://eurekamag.com/pdf/000/000097672.pdf |doi=10.1016/0016-7061(73)90002-5 |accessdate=15 December 2018 |format=[[Portable Document Format|PDF]]}}</ref> As the crystals are not bonded face to face, the entire surface is exposed and available for surface reactions, hence it has a high [[cation exchange capacity]] (CEC).{{sfn|Donahue|Miller|Shickluna|1977|p=108}}{{sfn|Russell|1957|pp=33–34}}{{sfn|Coleman|Mehlich|1957|p=74}}
* '''Illite''' is a 2:1 clay similar in structure to montmorillonite but has potassium bridges between the faces of the clay crystals and the degree of swelling depends on the degree of weathering of potassium-[[feldspar]].<ref>{{cite book |last1=Meunier |first1=Alain |last2=Velde |first2=Bruce |date=2004 |chapter=The geology of illite |title=Illite: origins, evolution and metamorphism |publisher=[[Springer Science+Business Media|Springer]] |location=Berlin|pages=63–143 |url=https://archive.org/details/springer_10.1007-978-3-662-07850-1 |accessdate=15 December 2018 |format=[[Portable Document Format|PDF]]}}</ref> The active surface area is reduced due to the potassium bonds. Illite originates from the modification of [[mica]], a primary mineral. It is often found together with montmorillonite and its primary minerals. It has moderate CEC.{{sfn|Donahue|Miller|Shickluna|1977|pp=108–10}}{{sfn|Russell|1957|pp=33–34}}{{sfn|Dean|1957|p=82}}{{sfn|Allison|1957|p=90}}{{sfn|Reitemeier|1957|p=103}}
* '''Vermiculite''' is a mica-based clay similar to illite, but the crystals of clay are held together more loosely by hydrated magnesium and it will swell, but not as much as does montmorillonite.<ref>{{cite journal |last1=Norrish |first1=Keith |last2=Rausell-Colom |first2=José Antonio |year=1961 |title=Low-angle X-ray diffraction studies of the swelling of montmorillonite and vermiculite |journal=Clays and Clay Minerals |volume=10 |issue=1 |pages=123–49 |url=http://www.clays.org/journal/archive/volume%2010/10-1-123.pdf |doi=10.1346/CCMN.1961.0100112 |accessdate=16 December 2018 |format=[[Portable Document Format|PDF]]}}</ref> It has very high CEC.{{sfn|Donahue|Miller|Shickluna|1977|p=110}}{{sfn|Coleman|Mehlich|1957|p=73}}{{sfn|Allison|1957|p=90}}{{sfn|Reitemeier|1957|p=103}}
* '''Chlorite''' is similar to vermiculite, but the loose bonding by occasional hydrated magnesium, as in vermiculite, is replaced by a hydrated magnesium sheet, that firmly bonds the planes above and below it. It has two planes of silicon, one of aluminium and one of magnesium; hence it is a 2:2 clay.<ref>{{cite book |last1=Moore |first1=Duane M. |last2=Reynolds |first2=Robert C. Jr |date=1997 |title=X-ray diffraction and the identification and analysis of clay minerals |publisher=[[Oxford University Press]] |location=Oxford|url=http://www.labpku.com/UploadFiles/2014-01/admin/2014011016073967283.pdf |accessdate=16 December 2018 |format=[[Portable Document Format|PDF]]}}</ref> Chlorite does not swell and it has low CEC.{{sfn|Donahue|Miller|Shickluna|1977|p=110}}{{sfn|Holmes|Brown|1957|p=112}}
* '''Kaolinite''' is very common, highly weathered clay, and more common than montmorillonite in acid soils.<ref>{{cite journal |last1=Karathanasis |first1=Anastasios D. |last2=Hajek |first2=Benjamin F. |year=1983 |title=Transformation of smectite to kaolinite in naturally acid soil systems: structural and thermodynamic considerations |journal=[[Soil Science Society of America Journal]] |volume=47 |issue=1 |pages=158–63 |url=https://dl.sciencesocieties.org/publications/sssaj/abstracts/47/1/SS0470010158 |doi=10.2136/sssaj1983.03615995004700010031x}}</ref> It has one silica and one alumina plane per crystal; hence it is a 1:1 type clay. One plane of silica of montmorillonite is dissolved and is replaced with hydroxyls, which produces strong hydrogen bonds to the oxygen in the next crystal of clay.<ref>{{cite journal |last1=Tombácz |first1=Etelka |last2=Szekeres |first2= Márta |year=2006 |title=Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite |journal=Applied Clay Science |volume=34 |issue=1–4 |pages=105–24 |url=https://www.academia.edu/11482380 |doi=10.1016/j.clay.2006.05.009 |accessdate=16 February 2019 |format=[[Portable Document Format|PDF]]}}</ref> As a result, kaolinite does not swell in water and has a low specific surface area, and as almost no isomorphous substitution has occurred it has a low CEC.<ref>{{cite journal |last1=Coles |first1=Cynthia A. |last2=Yong |first2=Raymond N. |year=2002 |title=Aspects of kaolinite characterization and retention of Pb and Cd |journal=Applied Clay Science |volume=22 |issue=1–2 |pages=39–45 |url=https://pdfs.semanticscholar.org/0a67/90d14853df562568cd6ceaa17689cf08a55d.pdf |doi=10.1016/S0169-1317(02)00110-2 |accessdate=24 February 2019 |format=[[Portable Document Format|PDF]]}}</ref> Where rainfall is high, acid soils selectively leach more silica than alumina from the original clays, leaving kaolinite.<ref>{{cite journal |last1=Fisher |first1=G. Burch |last2=Ryan |first2=Peter C. |year=2006 |title=The smectite-to-disordered kaolinite transition in a tropical soil chronosequence, Pacific coast, Costa Rica |journal=Clays and Clay Minerals |volume=54 |issue=5 |pages=571–86 |url=https://www.researchgate.net/publication/240744358 |doi=10.1346/CCMN.2006.0540504 |accessdate=24 February 2019 |format=[[Portable Document Format|PDF]]}}</ref> Even heavier weathering results in sesquioxide clays.{{sfn|Donahue|Miller|Shickluna|1977|p=111}}{{sfn|Russell|1957|p=33}}{{sfn|Coleman|Mehlich|1957|p=74}}{{sfn|Dean|1957|p=82}}{{sfn|Olsen|Fried|1957|p=96}}{{sfn|Reitemeier|1957|p=101}}

====Crystalline chain clays====
The carbonate and sulfate clay minerals are much more soluble and hence are found primarily in desert soils where leaching is less active.<ref>{{cite journal |last1=Hamdi-Aïssa |first1=Belhadj |last2=Vallès |first2=Vincent |last3=Aventurier |first3=Alain |last4=Ribolzi |first4=Olivier |year=2004 |title=Soils and brine geochemistry and mineralogy of hyperarid Desert Playa, Ouargla Basin, Algerian Sahara |journal=Arid Land Research and Management |volume=18 |issue=2 |pages=103–26 |url=https://www.researchgate.net/publication/233230446 |doi=10.1080/1532480490279656 |accessdate=24 February 2019 |format=[[Portable Document Format|PDF]]}}</ref>

====Amorphous clays====
'''Amorphous clays''' are young, and commonly found in recent volcanic ash deposits such as [[tephra]].<ref>{{cite journal |last1=Shoji |first1=Sadao |last2=Saigusa |first2=Masahiko |year=1977 |title=Amorphous clay materials of Towada Ando soils |journal=Soil Science and Plant Nutrition |volume=23 |issue=4 |pages=437–55 |url=https://www.tandfonline.com/doi/pdf/10.1080/00380768.1977.10433063 |doi=10.1080/00380768.1977.10433063 |accessdate=3 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> They are mixtures of alumina and silica which have not formed the ordered crystal shape of alumino-silica clays which time would provide. The majority of their negative charges originates from hydroxyl ions, which can gain or lose a hydrogen ion (H<sup>+</sup>) in response to soil pH, in such way was as to buffer the soil pH. They may have either a negative charge provided by the attached hydroxyl ion (OH<sup>−</sup>), which can attract a cation, or lose the hydrogen of the hydroxyl to solution and display a positive charge which can attract anions. As a result, they may display either high CEC in an acid soil solution, or high anion exchange capacity in a basic soil solution.{{sfn|Donahue|Miller|Shickluna|1977|p=111}}

====Sesquioxide clays====
[[File:San Joaquin soil profile.png|thumb|upright|silica-sesquioxide]]

'''[[Sesquioxide]] clays''' are a product of heavy rainfall that has leached most of the silica from alumino-silica clay, leaving the less soluble oxides iron hematite (Fe<sub>2</sub>O<sub>3</sub>), iron hydroxide (Fe(OH)<sub>3</sub>), aluminium hydroxide gibbsite (Al(OH)<sub>3</sub>), hydrated manganese birnessite (MnO<sub>2</sub>), as can be observed in most [[lateritic]] [[weathering]] profiles of tropical soils.<ref>{{cite journal |last1=Tardy |first1=Yves |last2=Nahon |first2=Daniel |year=1985 |title=Geochemistry of laterites, stability of Al-goethite, Al-hematite, and Fe3+-kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion formation |journal=[[American Journal of Science]] |volume=285 |issue=10 |pages=865–903 |url=http://earth.geology.yale.edu/~ajs/1985/10.1985.01.Tardy.pdf |doi=10.2475/ajs.285.10.865 |accessdate=10 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> It takes hundreds of thousands of years of leaching to create sesquioxide clays.<ref>{{cite journal |last1=Nieuwenhuyse |first1=André |last2=Verburg |first2=Paul S.J. |last3=Jongmans |first3=Antoine G. |year=2000 |title=Mineralogy of a soil chronosequence on andesitic lava in humid tropical Costa Rica |journal=Geoderma |volume=98 |issue=1–2 |pages=61–82 |url=https://eurekamag.com/pdf/003/003499878.pdf |doi=10.1016/S0016-7061(00)00052-5 |accessdate=10 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> ''Sesqui'' is Latin for "one and one-half": there are three parts oxygen to two parts iron or aluminium; hence the ratio is one and one-half (not true for all). They are hydrated and act as either amorphous or crystalline. They are not sticky and do not swell, and soils high in them behave much like sand and can rapidly pass water. They are able to hold large quantities of phosphates, a [[sorption|sorptive]] process which can at lest partly inhibited in the presence of decomposed ([[humus|humified]]) organic matter.<ref>{{cite journal |last1=Hunt |first1=James F. |last2=Ohno |first2=Tsutomu |last3=He |first3=Zhongqi |last4=Honeycutt |first4=C. Wayne |last5=Dail |first5=D. Bryan |year=2007 |title=Inhibition of phosphorus sorption to goethite, gibbsite, and kaolin by fresh and decomposed organic matter |journal=Biology and Fertility of Soils |volume=44 |issue=2 |pages=277–88 |url=https://naldc.nal.usda.gov/download/35758/PDF |doi=10.1007/s00374-007-0202-1 |accessdate=10 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> Sesquioxides have low CEC but these variable-charge minerals are able to hold anions as well as cations.<ref>{{cite journal |last1=Shamshuddin |first1=Jusop |last2=Anda |first2=Markus |year=2008 |title=Charge properties of soils in Malaysia dominated by kaolinite, gibbsite, goethite and hematite |journal=Bulletin of the Geological Society of Malaysia |volume=54 |pages=27–31 |url=https://gsm.org.my/products/702001-100489-PDF.pdf |doi=10.7186/bgsm54200805 |accessdate=10 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> Such soils range from yellow to red in colour. Such clays tend to hold phosphorus so tightly that it is unavailable for absorption by plants.{{sfn|Donahue|Miller|Shickluna|1977|pp=103–12}}{{sfn|Simonson|1957|pp=18, 21–24, 29}}{{sfn|Russell|1957|pp=32, 35}}

===Organic colloids===
[[Humus]] is one of the two final stages of [[decomposition]] of organic matter. It remains in the soil as the organic component of the soil matrix while the other stage, [[carbon dioxide]], is freely liberated in the [[atmosphere]] or reacts with [[calcium]] to form the soluble [[calcium bicarbonate]]. While humus may linger for a thousand years,<ref>{{cite book |last1=Paul |first1=Eldor A. |last2=Campbell |first2=Colin A. |last3=Rennie |first3=David A. |last4=McCallum |first4=Kenneth J. |lastauthoramp=yes |date=1964 |chapter=Investigations of the dynamics of soil humus utilizing carbon dating techniques |title=Transactions of the 8th International Congress of Soil Science, Bucharest, Romania, 1964 |publisher=Publishing House of the Academy of the Socialist Republic of Romania |location=Bucharest, Romania |pages=201–08 |url=http://www.nrel.colostate.edu/assets/nrel_files/labs/paul-lab/docs/NREL_Paul_Paul_8th_ICSS.pdf |accessdate=16 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> on the larger scale of the age of the mineral soil components, it is temporary, being finally released as CO<sub>2</sub>. It is composed of the very stable [[lignin]]s (30%) and complex [[sugars]] (polyuronides, 30%), [[proteins]] (30%), [[waxes]], and [[fat]]s that are resistant to breakdown by microbes and can form [[metal complexes|complexes with metals]], facilitating their downward migration ([[podzolization]]).<ref>{{cite journal |last1=Bin |first1=Gao |last2=Cao |first2=Xinde |last3=Dong |first3=Yan |last4=Luo |first4=Yongming |last5=Ma |first5=Lena Q. |lastauthoramp=yes |year=2011 |title=Colloid deposition and release in soils and their association with heavy metals |journal=Critical Reviews in Environmental Science and Technology |volume=41 |issue=4 |pages=336–72 |url=https://pdfs.semanticscholar.org/337c/26d44a43150e9d5eca46e4ba80a4849aff1b.pdf?_ga=2.265737423.1754694067.1553360310-127282468.1551019891 |doi=10.1080/10643380902871464 |accessdate=24 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> However, although originating for its main part from dead plant organs (wood, bark, foliage, roots), a large part of humus comes from organic compounds excreted by soil organisms (roots, microbes, animals) and from their decomposition upon death.<ref>{{cite journal |last1=Six |first1=Johan |last2=Frey |first2=Serita D. |last3=Thiet |first3=Rachel K. |last4=Batten |first4=Katherine M. |lastauthoramp=yes |year=2006 |title=Bacterial and fungal contributions to carbon sequestration in agroecosystems |journal=[[Soil Science Society of America Journal]] |volume=70 |issue=2 |pages=555–69 |url=https://pdfs.semanticscholar.org/65a5/f3923273bab7658b7b4a0775163c767595d4.pdf |doi=10.2136/sssaj2004.0347 |accessdate=16 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the [[Cation-exchange capacity|CEC]] of humus is many times greater than that of clay.{{sfn|Donahue|Miller|Shickluna|1977|p=112}}{{sfn|Russell|1957|p=35}}{{sfn|Allaway|1957|p=69}}

Humus plays a major role in the regulation of [[Carbon dioxide in Earth's atmosphere|atmospheric carbon]], through [[carbon sequestration]] in the soil profile, more especially in deeper horizons with reduced [[biological activity]].<ref>{{cite journal |last1=Thornton |first1=Peter E. |last2=Doney |first2=Scott C. |last3=Lindsay |first3=Konkel |last4=Moore |first4=J. Keith |last5=Mahowald |first5=Natalie |last6=Randerson |first6=James T. |last7=Fung |first7=Inez |last8=Lamarque |first8=Jean-François |last9=Feddema |first9=Johannes J. |last10=Lee |first10=Y. Hanna |lastauthoramp=yes |year=2009 |title=Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model |journal=[[Biogeosciences]] |volume=6 |issue=10 |pages=2099–120 |url=https://www.biogeosciences.net/6/2099/2009/bg-6-2099-2009.pdf |doi=10.5194/bg-6-2099-2009 |accessdate=23 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> Stocking and destocking of soil carbon are under strong climate influence.<ref>{{cite journal |last1=Morgan |first1=Jack A. |last2=Follett |first2=Ronald F. |last3=Allen Jr |first3=Leon Hartwell |last4=Del Grosso |first4= Stephen |last5=Derner |first5=Justin D. |last6=Dijkstra |first6=Feike |last7=Franzluebbers |first7=Alan |last8=Fry |first8=Robert |last9=Paustian |first9=Keith |last10=Schoeneberger |first10=Michele M. |lastauthoramp=yes |year=2010 |title=Carbon sequestration in agricultural lands of the United States |journal=[[Journal of Soil and Water Conservation]] |volume=65 |issue=1 |pages=6A–13A |url=https://www.srs.fs.fed.us/pubs/ja/2010/ja_2010_morgan_001.pdf |doi=10.2489/jswc.65.1.6A |accessdate=24 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> They are normally balanced through an equilibrium between production and [[Mineralization (soil science)|mineralization]] of organic matter, but the balance is in favour of destocking under present-day [[climate warming]],<ref>{{cite journal |last1=Parton |first1=Willam J. |last2=Scurlock |first2=Jonathan M. O. |last3=Ojima |first3=Dennis S. |last4=Schimel |first4=David |last5=Hall |first5=David O. |last6=The SCOPEGRAM Group |lastauthoramp=yes |year=1995 |title=Impact of climate change on grassland production and soil carbon worldwide |journal=[[Global Change Biology]] |volume=1 |issue=1 |pages=13–22 |url=https://www.researchgate.net/publication/233714480 |doi=10.1111/j.1365-2486.1995.tb00002.x |accessdate=24 March 2019 |format=[[Portable Document Format|PDF]]}}</ref> and more especially in [[permafrost]].<ref>{{cite journal |last1=Schuur |first1=Edward A. G. |last2=Vogel |first2=Jason G. |last3=Crummer |first3=Kathryn G. |last4=Lee |first4=Hanna |last5=Sickman |first5=James O. |last6=Osterkamp |first6=T. E. |lastauthoramp=yes |year=2009 |title=The effect of permafrost thaw on old carbon release and net carbon exchange from tundra |journal=[[Nature (journal)|Nature]] |volume=459 |pages=556–59 |url=https://www.academia.edu/18296573 |doi=10.1038/nature08031 |accessdate=24 March 2019 |format=[[Portable Document Format|PDF]]}}</ref>

===Carbon and terra preta===
In the extreme environment of high temperatures and the leaching caused by the heavy rain of tropical rain forests, the clay and organic colloids are largely destroyed. The heavy rains wash the alumino-silicate clays from the soil leaving only sesquioxide clays of low CEC. The high temperatures and humidity allow bacteria and fungi to virtually dissolve any organic matter on the rain-forest floor overnight and much of the nutrients are volatilized or leached from the soil and lost. However, carbon in the form of charcoal is far more stable than soil colloids and is capable of performing many of the functions of the soil colloids of sub-tropical soils. Soil containing substantial quantities of charcoal, of an anthropogenic origin, is called [[terra preta]]. Research into terra preta is still young but is promising. Fallow periods "on the Amazonian Dark Earths can be as short as 6 months, whereas fallow periods on [[oxisol]]s are usually 8 to 10 years long"<ref>{{cite web|last=Lehmann|first=J.|title=Terra Preta de Indio|url=http://www.css.cornell.edu/faculty/lehmann/research/terra%20preta/terrapretamain.html|publisher=University of Cornell, Dept. of Crop and Soil Sciences|accessdate=30 March 2013|deadurl=no|archiveurl=https://web.archive.org/web/20130424061552/http://www.css.cornell.edu/faculty/lehmann/research/terra%20preta/terrapretamain.html|archivedate=24 April 2013|df=dmy-all}}</ref>


==Chemistry==
==Chemistry==
{{for|the [[academic discipline]]|Soil chemistry}}
{{for|the academic discipline|Soil chemistry}}


The chemistry of a soil determines its ability to supply available [[Plant nutrition|plant nutrients]] and affects its physical properties and the health of its microbial population. In addition, a soil's chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic [[colloids]] that determines soil's chemical properties. "A colloid is a small, insoluble, nondiffusible particle larger than a molecule but small enough to remain suspended in a fluid medium without settling. Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays." The very high [[specific surface area]] of colloids and their net charges, gives soil its ability to hold and release ions. Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. [[Cation-exchange capacity]] (CEC) is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100&nbsp;grams of soil (or centimoles of positive charge per kilogram of soil; cmol<sub>c</sub>/kg). Similarly, positively charged sites on colloids can attract and release anions in the soil giving the soil [[cation-exchange capacity|anion exchange capacity]] (AEC).
The chemistry of a soil determines its ability to supply available [[Plant nutrition|plant nutrients]] and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines its [[corrosivity]], stability, and ability to [[Sorption|absorb]] [[pollutants]] and to filter water. It is the [[surface chemistry]] of mineral and organic [[colloids]] that determines soil's chemical properties.<ref>{{cite book |last=Sposito |first=Garrison |date=1984 |title=The surface chemistry of soils |publisher=[[Oxford University Press]] |location=New York |url=https://epdf.pub/the-surface-chemistry-of-soils.html |access-date=15 January 2023}}</ref> A colloid is a small, insoluble particle ranging in size from 1 [[nanometer]] to 1 [[micrometre|micrometer]], thus small enough to remain suspended by [[Brownian motion]] in a fluid medium without settling.<ref>{{cite web |last=Wynot |first=Christopher |title=Theory of diffusion in colloidal suspensions |url=http://www.owlnet.rice.edu/~ceng402/proj02/cwynot/402project.htm |access-date=15 January 2023}}</ref> Most soils contain organic colloidal particles called [[humus]] as well as the inorganic colloidal particles of [[clays]]. The very high [[specific surface area]] of colloids and their net [[electrical charge]]s give soil its ability to hold and release [[ions]]. Negatively charged sites on colloids attract and release [[cations]] in what is referred to as [[cation exchange]]. [[Cation-exchange capacity]] is the amount of exchangeable [[cations]] per unit weight of dry soil and is expressed in terms of [[milliequivalents]] of [[positively charged]] ions per 100&nbsp;grams of soil (or centimoles of positive charge per kilogram of soil; [[Cation-exchange capacity|cmol<sub>c</sub>/kg]]). Similarly, positively charged sites on colloids can attract and release [[anions]] in the soil, giving the soil anion exchange capacity.


===Cation and anion exchange===
===Cation and anion exchange===
{{Further|Cation-exchange capacity}}
{{Further|Cation-exchange capacity}}
The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.
The cation exchange, that takes place between colloids and soil water, [[Buffer solution|buffers]] (moderates) soil pH, alters soil structure, and purifies [[Percolation|percolating]] water by adsorbing cations of all types, both useful and harmful.


The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.{{sfn|Donahue|Miller|Shickluna|1977|p=103–06}}
The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.{{sfn|Donahue|Miller|Shickluna|1977|p=103–106}}


# Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure. Substitutions in the outermost layers are more effective than for the innermost layers, as the charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
# Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.<ref name="PMID10097044">{{cite journal |last1=Sposito |first1= Garrison |last2=Skipper |first2=Neal T. |last3=Sutton |first3=Rebecca |last4=Park |first4=Sung-Ho |last5=Soper |first5=Alan K. |last6=Greathouse |first6=Jeffery A. |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=96 |issue=7 |title=Surface geochemistry of the clay minerals |year=1999 |pages=3358–3364 |doi=10.1073/pnas.96.7.3358 |pmid=10097044 |bibcode=1999PNAS...96.3358S |pmc=34275 |doi-access=free}}</ref> Substitutions in the outermost layers are more effective than for the innermost layers, as the [[electric charge]] strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
# Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.<ref>{{cite journal |last1=Bickmore |first1=Barry R. |last2=Rosso |first2=Kevin M. |last3=Nagy |first3=Kathryn L. |last4=Cygan |first4=Randall T. |last5=Tadanier |first5=Christopher J. |year=2003 |title=Ab initio determination of edge surface structures for dioctahedral 2:1 phyllosilicates: implications for acid-base reactivity |journal=Clays and Clay Minerals |volume=51 |issue=4 |pages=359–371 |url=http://randallcygan.com/wp-content/uploads/2017/06/Bickmore2003CCM.pdf |doi=10.1346/CCMN.2003.0510401 |access-date=15 January 2023 |bibcode=2003CCM....51..359B |s2cid=97428106}}</ref>
# Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.
# Hydroxyls may substitute for oxygens of the silica layers. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge.
# [[Hydroxyl]]s may substitute for oxygens of the silica layers, a process called [[hydroxylation]]. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).<ref>{{cite journal |last1=Rajamathi |first1=Michael |last2=Thomas |first2=Grace S. |last3=Kamath |first3=P. Vishnu |year=2001 |title=The many ways of making anionic clays |journal=[[Journal of Chemical Sciences]] |volume=113 |issue=5–6 |pages=671–680 |doi=10.1007/BF02708799 |s2cid=97507578 |url=https://www.academia.edu/56207482 |access-date=15 January 2023}}</ref>
# Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.<ref>{{cite journal |last1=Moayedi |first1=Hossein |last2=Kazemian |first2=Sina |year= 2012 |title=Zeta potentials of suspended humus in multivalent cationic saline solution and its effect on electro-osomosis behavior |journal=Journal of Dispersion Science and Technology |volume=34 |issue=2 |pages=283–294 |url=https://www.academia.edu/10587240 |doi=10.1080/01932691.2011.646601 |s2cid= 94333872 |access-date=15 January 2023}}</ref>
# Hydrogens of humus hydroxyl groups may be ionised into solution, leaving an oxygen with a negative charge.


Cations held to the negatively charged colloids resist being washed downward by water and out of reach of plants' roots, thereby preserving the fertility of soils in areas of moderate rainfall and low temperatures.
Cations held to the negatively charged colloids resist being washed downward by water and are out of reach of plant roots, thereby preserving the [[soil fertility]] in areas of moderate rainfall and low temperatures.<ref>{{cite web |last=Pettit |first=Robert E. |title=Organic matter, humus, humate, humic acid, fulvic acid and humin: their importance in soil fertility and plant health |url=http://www.harvestgrow.com/.pdf%20web%20site/Humates%20General%20Info.pdf |access-date=15 January 2023}}</ref><ref>{{cite journal |last1=Diamond |first1=Sidney |last2=Kinter |first2=Earl B. |year=1965 |title=Mechanisms of soil-lime stabilization: an interpretive review |journal=Highway Research Record |volume=92 |pages=83–102 |url=http://onlinepubs.trb.org/onlinepubs/hrr/1965/92/92-006.pdf |access-date=15 January 2023}}</ref>


There is a hierarchy in the process of cation exchange on colloids, as they differ in the strength of adsorption by the colloid and hence their ability to replace one another. If present in equal amounts in the soil water solution:
There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another ([[ion exchange]]). If present in equal amounts in the soil water solution:


Al<sup>3+</sup> replaces H<sup>+</sup> replaces Ca<sup>2+</sup> replaces Mg<sup>2+</sup> replaces K<sup>+</sup> same as NH<sup>4+</sup> replaces Na<sup>+</sup>
Al<sup>3+</sup> replaces H<sup>+</sup> replaces Ca<sup>2+</sup> replaces Mg<sup>2+</sup> replaces K<sup>+</sup> same as {{chem|NH|4|+}} replaces Na<sup>+</sup><ref>{{cite journal |last=Woodruff |first=Clarence M. |year=1955 |title=The energies of replacement of calcium by potassium in soils |journal=[[Soil Science Society of America Journal]] |volume=19 |issue=2 |pages=167–171 |doi=10.2136/sssaj1955.03615995001900020014x |url=https://www.ipipotash.org/uploads/pdf/review/30_1956_1.pdf |bibcode=1955SSASJ..19..167W |access-date=15 January 2023}}</ref>


If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called mass action. This is largely what occurs with the addition of fertiliser.
If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called [[law of mass action]]. This is largely what occurs with the addition of cationic [[Fertilizer|fertilisers]] ([[potash]], [[Lime (material)|lime]]).<ref>{{cite journal |last=Fronæus |first=Sture |year=1953 |title=On the application of the mass action law to cation exchange equilibria |journal=[[Acta Chemica Scandinavica]] |volume=7 |pages=469–480 |doi=10.3891/acta.chem.scand.07-0469 |doi-access=free}}</ref>


As the soil solution becomes more acidic (low pH, and an abundance of H<sup>+</sup>), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy those sites. A low pH may cause hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxyl groups on the surface of soil colloids creates what is described as pH-dependent charges. Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.<ref name="CEC"/> Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile. Plants are able to excrete H<sup>+</sup> into the soil and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.
As the soil solution becomes more acidic (low [[pH]], meaning an abundance of H<sup>+</sup>), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites ([[protonation]]). A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This [[Ionization|ionisation]] of [[hydroxy group]]s on the surface of soil colloids creates what is described as pH-dependent surface charges.<ref>{{cite journal |last1=Bolland |first1=Mike D. A. |last2=Posner |first2=Alan M. |last3=Quirk |first3=James P. |year=1980 |title=pH-independent and pH-dependent surface charges on kaolinite |journal=Clays and Clay Minerals |volume=28 |issue=6 |pages=412–418 |doi=10.1346/CCMN.1980.0280602 |bibcode=1980CCM....28..412B |s2cid=12462516 |url=https://www.researchgate.net/publication/237294635 |access-date=15 January 2023|doi-access=free }}</ref> Unlike permanent charges developed by [[Isomorphous replacement|isomorphous substitution]], pH-dependent charges are variable and increase with increasing pH.<ref name="CEC">{{cite web |last=Chakraborty |first=Meghna |url=http://www.soilminerals.com/Cation_Exchange_Simplified.htm |title=What is cation exchange capacity in soils? |date=8 August 2022 |access-date=15 January 2023}}</ref> Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.<ref>{{cite journal |last1=Silber |first1=Avner |last2=Levkovitch |first2=Irit |last3= Graber |first3=Ellen R. |year=2010 |title=pH-dependent mineral release and surface properties of cornstraw biochar: agronomic implications |journal=[[Environmental Science and Technology]] |volume=44 |issue=24 |pages=9318–23 |url=https://www.academia.edu/24532141 |doi=10.1021/es101283d |pmid=21090742 |access-date=15 January 2023 |bibcode=2010EnST...44.9318S}}</ref> Plants are able to excrete H<sup>+</sup> into the soil through the synthesis of [[organic acid]]s and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.<ref>{{cite journal |last1=Dakora |first1=Felix D. |last2=Phillips |first2=Donald D. |year=2002 |title=Root exudates as mediators of mineral acquisition in low-nutrient environments |journal=[[Plant and Soil]] |volume=245 |pages=35–47 |url=https://www.researchgate.net/publication/225265745 |doi=10.1023/A:1020809400075 |s2cid=3330737 |access-date=15 January 2023 |archive-url= https://web.archive.org/web/20190819123707/http://www.plantstress.com/articles/min_deficiency_i/root_exudates.pdf |archive-date=19 August 2019 |url-status=live}}</ref>


====Cation exchange capacity (CEC)====
====Cation exchange capacity (CEC)====
Cation exchange capacity should be thought of as the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. CEC is the amount of exchangeable hydrogen cation (H<sup>+</sup>) that will combine with 100&nbsp;grams dry weight of soil and whose measure is one milliequivalents per 100&nbsp;grams of soil (1&nbsp;meq/100&nbsp;g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100&nbsp;grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40/2) x 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100&nbsp;grams of dry soil or 20&nbsp;meq/100&nbsp;g.{{sfn|Donahue|Miller|Shickluna|1977|p=114}} The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.
[[Cation exchange capacity]] is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution.<ref>{{cite journal |last=Brown |first=John C. |year=1978 |title=Mechanism of iron uptake by plants |journal=[[Plant, Cell & Environment|Plant, Cell and Environment]] |volume=1 |issue=4 |pages=249–257 |doi=10.1111/j.1365-3040.1978.tb02037.x |url=https://booksc.me/book/9318043/764ac6 |access-date=29 January 2023 }}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> CEC is the amount of exchangeable hydrogen cation (H<sup>+</sup>) that will combine with 100&nbsp;grams dry weight of soil and whose measure is one milliequivalents per 100&nbsp;grams of soil (1&nbsp;meq/100&nbsp;g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100&nbsp;grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to {{nowrap|(40 ÷ 2) × 1 milliequivalent}} = 20 milliequivalents of hydrogen ion per 100&nbsp;grams of dry soil or 20&nbsp;meq/100&nbsp;g.{{sfn|Donahue|Miller|Shickluna|1977|p=114}} The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.


Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (such as [[tropical rainforest]]s), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.<ref>{{cite journal |last1=Singh |first1=Jamuna Sharan |last2=Raghubanshi |first2=Akhilesh Singh |last3=Singh |first3=Raj S. |last4=Srivastava |first4=S. C. |year=1989 |title=Microbial biomass acts as a source of plant nutrient in dry tropical forest and savanna |journal=[[Nature (journal)|Nature]] |volume=338 |issue=6215 |pages=499–500 |url=https://www.researchgate.net/publication/236941524 |doi=10.1038/338499a0 |access-date=29 January 2023 |bibcode=1989Natur.338..499S |s2cid=4301023}}</ref> Live plant roots also have some CEC, linked to their specific surface area.<ref>{{cite journal |last1=Szatanik-Kloc |first1=Alicja |last2=Szerement |first2=Justyna |last3=Józefaciuk |first3=Grzegorz |year=2017 |title=The role of cell walls and pectins in cation exchange and surface area of plant roots |journal=[[Journal of Plant Physiology]] |volume=215 |pages=85–90 |url=https://booksc.me/book/65260543/5df35d |doi=10.1016/j.jplph.2017.05.017 |pmid=28600926 |bibcode=2017JPPhy.215...85S |access-date=29 January 2023 }}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates, due to leaching and decomposition respectively, explains the relative sterility of tropical soils. Live plant roots also have some CEC.


{| class="wikitable" style="border-spacing: 5px; margin:auto;"
{| class="wikitable" style="border-spacing: 5px; margin:auto;"
|+ '''Cation exchange capacity for soils; soil textures; soil colloids'''{{sfn|Donahue|Miller|Shickluna|1977|pp=115–16}}
|+ Cation exchange capacity for soils; soil textures; soil colloids{{sfn|Donahue|Miller|Shickluna|1977|pp=115–116}}
|-
|-
! scope="col" style="width:200px;"| Soil
! scope="col" style="width:200px;"| Soil
! scope="col" style="width:100px;"| State
! scope="col" style="width:100px;"| State
! scope="col" style="width:100px;"| CEC meq/100 g
! scope="col" style="width:100px;"| CEC meq/100&nbsp;g
|-
|-
| Charlotte fine sand ||Florida|| 1.0
| Charlotte fine sand ||Florida|| 1.0
|-
|-
| Ruston fine sandy loam ||Texas|| 1.9
| Ruston fine sandy loam ||Texas|| 1.9
|-
|-
| Glouchester loam ||New Jersey || 11.9
| Glouchester loam ||New Jersey || 11.9
|-
|-
| Grundy silt loam || Illinois || 26.3
| Grundy silt loam || Illinois || 26.3
Line 612: Line 159:
| Davie mucky fine sand || Florida || 100.8
| Davie mucky fine sand || Florida || 100.8
|-
|-
| Sands || ------ || 1 - 5
| Sands || {{n/a}} || 1–5
|-
|-
| Fine sandy loams || ------ || 5-10
| Fine sandy loams || {{n/a}} || 5–10
|-
|-
| Loams and silt loams || ----- || 5-15
| Loams and silt loams || {{n/a}} || 5–15
|-
|-
| Clay loams || ----- || 15-30
| Clay loams || {{n/a}} || 15–30
|-
|-
| Clays || ----- || over 30
| Clays || {{n/a}} || over 30
|-
|-
| Sesquioxides || ----- || 0-3
| Sesquioxides || {{n/a}} || 0–3
|-
|-
| Kaolinite || ----- || 3-15
| Kaolinite || {{n/a}} || 3–15
|-
|-
| Illite || ----- || 25-40
| Illite || {{n/a}} || 25–40
|-
|-
| Montmorillonite || ----- || 60-100
| Montmorillonite || {{n/a}} || 60–100
|-
|-
| Vermiculite (similar to illite) || ----- || 80-150
| Vermiculite (similar to illite) || {{n/a}} || 80–150
|-
|-
| Humus || ----- || 100-300
| Humus || {{n/a}} || 100–300
|}
|}


====Anion exchange capacity (AEC)====
====Anion exchange capacity (AEC)====
Anion exchange capacity is the soil's ability to remove anions (such as [[nitrate]], [[phosphate]]) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution.<ref name="Hinsinger 2001 173–195">{{cite journal |last= Hinsinger |first=Philippe |year=2001 |title=Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review |journal=[[Plant and Soil]] |volume=237 |issue=2 |pages=173–95 |doi=10.1023/A:1013351617532 |s2cid=8562338 |url=https://www.researchgate.net/publication/225852665 |access-date=29 January 2023}}</ref> Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC,<ref>{{cite report |last1=Gu |first1=Baohua |last2=Schulz |first2=Robert K. |title=Anion retention in soil: possible application to reduce migration of buried technetium and iodine, a review |year=1991 |doi=10.2172/5980032 |s2cid=91359494 |url=https://www.osti.gov/servlets/purl/5980032 |access-date=29 January 2023}}</ref> followed by the iron oxides.<ref>{{cite journal |last1=Lawrinenko |first1=Michael |last2=Jing |first2=Dapeng |last3=Banik |first3=Chumki |last4=Laird |first4=David A. |year=2017 |title=Aluminum and iron biomass pretreatment impacts on biochar anion exchange capacity |journal=[[Carbon (journal)|Carbon]] |volume=118 |pages=422–30 |doi=10.1016/j.carbon.2017.03.056 |bibcode=2017Carbo.118..422L |url=https://www.academia.edu/90757446 |access-date=29 January 2023}}</ref> Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils.<ref>{{cite journal |last1=Sollins |first1=Phillip |last2=Robertson |first2=G. Philip |last3=Uehara |first3=Goro |year=1988 |title=Nutrient mobility in variable- and permanent-charge soils |journal=Biogeochemistry |volume=6 |issue=3 |pages=181–99 |url=https://lter.kbs.msu.edu/docs/robertson/Sollins_et_al._1988_Biogeochemistry.pdf |doi=10.1007/BF02182995 |bibcode=1988Biogc...6..181S |s2cid=4505438 |access-date=29 January 2023}}</ref> Phosphates tend to be held at anion exchange sites.<ref>{{cite journal |last=Sanders |first=W. M. H. |year=1964 |title=Extraction of soil phosphate by anion-exchange membrane |journal=New Zealand Journal of Agricultural Research |volume=7 |issue=3 |pages=427–31 |doi=10.1080/00288233.1964.10416423 |bibcode=1964NZJAR...7..427S |doi-access=free}}</ref>
Anion exchange capacity should be thought of as the soil's ability to remove anions from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution. Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC, followed by the iron oxides. Levels of AEC are much lower than for CEC. Phosphates tend to be held at anion exchange sites.


Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH<sup>−</sup>) for other anions. The order reflecting the strength of anion adhesion is as follows:
Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH<sup>−</sup>) for other anions.<ref name="Hinsinger 2001 173–195"/> The order reflecting the strength of anion adhesion is as follows:


:H<sub>2</sub>PO<sub>4</sub><sup></sup> replaces SO<sub>4</sub><sup>2−</sup> replaces NO<sub>3</sub><sup></sup> replaces Cl<sup>−</sup>
:{{chem|H|2|PO|4|}} replaces {{chem|SO|4|2−}} replaces {{chem|NO|3|}} replaces Cl<sup>−</sup>


The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100&nbsp;g dry soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=115–16}} As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).
The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100&nbsp;g dry soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=115–116}} As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).<ref>{{cite journal |last1=Lawrinenko |first1=Mike |last2=Laird |first2=David A. |year=2015 |title=Anion exchange capacity of biochar |journal=[[Green Chemistry (journal)|Green Chemistry]] |volume=17 |issue=9 |pages=4628–36 |doi=10.1039/C5GC00828J |s2cid=52972476 |url=https://www.researchgate.net/publication/280973853 |access-date=29 January 2023}}</ref>


===Reactivity (pH)===
===Reactivity (pH)===
{{Main|Soil pH|Soil pH#Effect of soil pH on plant growth}}
{{Main|Soil pH|Soil pH#Effect of soil pH on plant growth}}
Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydrogen ion concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.
Soil reactivity is expressed in terms of pH and is a measure of the [[acid]]ity or [[Base (chemistry)|alkalinity]] of the soil. More precisely, it is a measure of [[hydronium]] concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.<ref>{{cite web |last=Robertson |first=Bryan |title=pH requirements of freshwater aquatic life |url=https://www.waterboards.ca.gov/centralvalley/water_issues/basin_plans/ph_turbidity/ph_turbidity_04phreq.pdf |access-date=6 June 2021 |archive-date=8 May 2021 |archive-url=https://web.archive.org/web/20210508070517/https://www.waterboards.ca.gov/centralvalley/water_issues/basin_plans/ph_turbidity/ph_turbidity_04phreq.pdf |url-status=dead }}</ref>


At 25&nbsp;°C an aqueous solution that has a pH of 3.5 has 10<sup>−3.5</sup> [[mole (unit)|moles]] H<sup>+</sup> (hydrogen ions) per litre of solution (and also 10<sup>−10.5</sup> mole/litre OH<sup>−</sup>). A pH of 7, defined as neutral, has 10<sup>−7</sup> moles hydrogen ions per litre of solution and also 10<sup>−7</sup> moles of OH<sup>−</sup> per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10<sup>−9.5</sup> moles hydrogen ions per litre of solution (and also 10<sup>−2.5</sup> mole per litre OH<sup>−</sup>). A pH of 3.5 has one million times more hydrogen ions per litre than a solution with pH of 9.5 (9.5–3.5 = 6 or 10<sup>6</sup>) and is more acidic.<ref>{{cite book|last=Chang|first=Raymond|title=Chemistry|year=1984|publisher=Random House, Inc.|isbn=978-0-394-32983-3|page=424}}</ref>
At 25&nbsp;°C an aqueous solution that has a pH of 3.5 has 10<sup>−3.5</sup> [[mole (unit)|moles]] H<sub>3</sub>O<sup>+</sup> (hydronium ions) per litre of solution (and also 10<sup>−10.5</sup> moles per litre OH<sup>−</sup>). A pH of 7, defined as neutral, has 10<sup>−7</sup> moles of hydronium ions per litre of solution and also 10<sup>−7</sup> moles of OH<sup>−</sup> per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10<sup>−9.5</sup> moles hydronium ions per litre of solution (and also 10<sup>−2.5</sup> moles per litre OH<sup>−</sup>). A pH of 3.5 has one million times more hydronium ions per litre than a solution with pH of 9.5 ({{nowrap|9.5 3.5 {{=}} 6}} or 10<sup>6</sup>) and is more acidic.<ref>{{cite book |editor-last=Chang |editor-first=Raymond |title=Chemistry |date=2010 |edition=12th |url=https://www.academia.edu/44394574 |publisher=[[McGraw-Hill]] |location=New York, New York |isbn=9780078021510 |page=666 |access-date=6 June 2021}}</ref>


The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of [[aluminium]] and [[manganese]].<ref>{{cite journal |last1=Singleton |first1=Peter L. |last2=Edmeades |first2=Doug C. |last3=Smart |first3=R. E. |last4=Wheeler |first4=David M. |year=2001 |title=The many ways of making anionic clays |journal=[[Journal of Chemical Sciences]] |volume=113 |issue=5–6 |pages=671–680 |doi=10.1007/BF02708799 |s2cid=97507578 |doi-access=free}}</ref> As a result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH,<ref>{{cite book |last1=Läuchli |first1=André |last2=Grattan |first2=Steve R. |date=2012 |chapter=Soil pH extremes |title=Plant stress physiology |edition=1st |editor-first=Sergey |editor-last=Shabala |publisher=[[CAB International]] |location=Wallingford, United Kingdom |pages=194–209 |isbn=978-1845939953 |chapter-url=https://www.researchgate.net/publication/269112359 |doi=10.1079/9781845939953.0194 |access-date=13 June 2021}}</ref> although most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH&nbsp;6.5 and organic soils of pH&nbsp;5.5.{{sfn|Donahue|Miller|Shickluna|1977|pp=116–117}} Given that at low pH toxic metals (e.g. cadmium, zinc, lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both made more available to organisms,<ref>{{cite journal |last1=Calmano |first1=Wolfgang |last2=Hong |first2=Jihua |last3=Förstner |first3=Ulrich |year=1993 |title=Binding and mobilization of heavy metals in contaminated sediments affected by pH and redox potential |journal=[[Water Science and Technology]] |volume=28 |issue=8–9 |pages=223–235 |url=https://www.researchgate.net/publication/234056281 |doi=10.2166/wst.1993.0622 |access-date=13 June 2021}}</ref><ref>{{cite journal |last1=Ren |first1=Xiaoya |last2=Zeng |first2=Guangming |last3=Tang |first3=Lin |last4=Wang |first4=Jingjing |last5=Wan |first5=Jia |last6=Liu |first6=Yani |last7=Yu |first7=Jiangfang |last8=Yi |first8=Huan |last9=Ye |first9=Shujing |last10=Deng |first10=Rui |year=2018 |title=Sorption, transport and biodegradation: an insight into bioavailability of persistent organic pollutants in soil |journal=[[Science of the Total Environment]] |volume=610–611 |pages=1154–1163 |url=http://ee.hnu.edu.cn/__local/E/E3/44/F76DCA19501AE153573A22D4C29_17709BE2_110161.pdf |doi=10.1016/j.scitotenv.2017.08.089 |pmid=28847136 |access-date=13 June 2021 |bibcode=2018ScTEn.610.1154R}}</ref> it has been suggested that plants, animals and microbes commonly living in acid soils are [[pre-adapted]] to every kind of pollution, whether of natural or human origin.<ref>{{cite journal |last=Ponge |first=Jean-François |year=2003 |title=Humus forms in terrestrial ecosystems: a framework to biodiversity |journal=[[Soil Biology and Biochemistry]] |volume=35 |issue=7 |pages=935–945 |url=https://www.academia.edu/20508983 |doi=10.1016/S0038-0717(03)00149-4 |bibcode=2003SBiBi..35..935P |access-date=13 June 2021 |citeseerx=10.1.1.467.4937 |s2cid=44160220}}</ref>
The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese. Plants which need calcium need moderate alkalinity, but most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH&nbsp;6.5 and organic soils of pH&nbsp;5.5.{{sfn|Donahue|Miller|Shickluna|1977|pp=116–17}}


In high rainfall areas, soils tend to acidity as the basic cations are forced off the soil colloids by the mass action of hydrogen ions from the rain as those attach to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil sterile. Once the colloids are saturated with H<sup>+</sup>, the addition of any more hydrogen ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity. In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil. In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH&nbsp;10. Beyond a pH of 9, plant growth is reduced. High pH results in low micro-nutrient mobility, but water-soluble chelates of those nutrients can correct the deficit. Sodium can be reduced by the addition of [[gypsum]] (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.{{sfn|Donahue|Miller|Shickluna|1977|pp=116–19}}
In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual [[Acid rain|rain acidity]] against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in [[tropical rainforests]].<ref>{{cite journal |last=Fujii |first=Kazumichi |year=2003 |title=Soil acidification and adaptations of plants and microorganisms in Bornean tropical forests |journal=Ecological Research |volume=29 |issue=3 |pages=371–381 |doi=10.1007/s11284-014-1144-3 |doi-access=free}}</ref> Once the colloids are saturated with H<sub>3</sub>O<sup>+</sup>, the addition of any more hydronium ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity.<ref>{{cite journal |last1=Kauppi |first1=Pekka |last2=Kämäri |first2=Juha |last3=Posch |first3=Maximilian |last4=Kauppi |first4=Lea |year=1986 |title=Acidification of forest soils: model development and application for analyzing impacts of acidic deposition in Europe |journal=[[Ecological Modelling]] |volume=33 |issue=2–4 |pages=231–253 |url=http://pure.iiasa.ac.at/id/eprint/2766/1/RR-87-05.pdf |doi=10.1016/0304-3800(86)90042-6 |bibcode=1986EcMod..33..231K |access-date=13 June 2021}}</ref> In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil.<ref>{{cite journal |last=Andriesse |first=Jacobus Pieter |year=1969 |title=A study of the environment and characteristics of tropical podzols in Sarawak (East-Malaysia) |journal=Geoderma |volume=2 |issue=3 |pages=201–227 |url=https://coek.info/pdf-a-study-of-the-environment-and-characteristics-of-tropical-podzols-in-sarawak-ea.html |doi=10.1016/0016-7061(69)90038-X |access-date=13 June 2021 |bibcode=1969Geode...2..201A}}</ref> In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH&nbsp;10.<ref>{{cite journal |last=Rengasamy |first=Pichu |year=2006 |title=World salinization with emphasis on Australia |journal=[[Journal of Experimental Botany]] |volume=57 |issue=5 |pages=1017–1023 |doi=10.1093/jxb/erj108 |pmid=16510516 |doi-access=free}}</ref> Beyond a pH of 9, plant growth is reduced.<ref>{{cite journal |last1=Arnon |first1=Daniel I. |last2=Johnson |first2=Clarence M. |year=1942 |title=Influence of hydrogen ion concentration on the growth of higher plants under controlled conditions |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=17 |issue=4 |pages=525–539 |doi=10.1104/pp.17.4.525 |pmid=16653803 |pmc=438054}}</ref> High pH results in low [[micro-nutrient]] mobility, but water-soluble [[chelates]] of those nutrients can correct the deficit.<ref>{{cite journal |last1=Chaney |first1=Rufus L. |last2=Brown |first2=John C. |last3=Tiffin |first3=Lee O. |year=1972 |title=Obligatory reduction of ferric chelates in iron uptake by soybeans |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=50 |issue=2 |pages=208–213 |doi=10.1104/pp.50.2.208 |pmid=16658143 |pmc=366111}}</ref> Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.{{sfn|Donahue|Miller|Shickluna|1977|pp=116–119}}<ref>{{cite journal |last1=Ahmad |first1=Sagheer |last2=Ghafoor |first2=Abdul |last3=Qadir |first3=Manzoor |last4=Aziz |first4=M. Abbas |year=2006 |title=Amelioration of a calcareous saline-sodic soil by gypsum application and different crop rotations |journal=International Journal of Agriculture and Biology |volume=8 |issue=2 |pages=142–46 |url=https://www.researchgate.net/publication/228966353 |access-date=13 June 2021}}</ref>


====Base saturation percentage====
==== Base saturation percentage ====
There are acid-forming cations (hydrogen and aluminium) and there are base-forming cations. The fraction of the base-forming cations that occupy positions on the soil colloids is called the base saturation percentage. If a soil has a CEC of 20 meq and 5 meq are aluminium and hydrogen cations (acid-forming), the remainder of positions on the colloids (20-5 = 15 meq) are assumed occupied by base-forming cations, so that the percentage base saturation is 15/20 x 100% = 75% (the compliment 25% is assumed acid-forming cations). When the soil pH is 7 (neutral), base saturation is 100&nbsp;percent and there are no hydrogen ions stored on the colloids. Base saturation is almost in direct proportion to pH (increases with increasing pH). It is of use in calculating the amount of lime needed to neutralise an acid soil. The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids not just those in the soil water solution. The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.{{sfn|Donahue|Miller|Shickluna|1977|pp=119–20}}
There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called [[base saturation]]. If a soil has a CEC of 20&nbsp;meq and 5&nbsp;meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids ({{nowrap|1=205 = 15 meq}}) are assumed occupied by base-forming cations, so that the base saturation is {{nowrap|1=15 ÷ 20 × 100% = 75%}} (the compliment 25% is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH).<ref>{{cite journal |last1=McFee |first1=William W. |last2=Kelly |first2=J. Michael |last3=Beck |first3=Robert H. |year=1977 |title=Acid precipitation effects on soil pH and base saturation of exchange sites |journal=[[Water, Air, & Soil Pollution|Water, Air, and Soil Pollution]] |volume=7 |issue=3 |pages=4014–08 |doi=10.1007/BF00284134 |bibcode=1977WASP....7..401M |doi-access=free}}</ref> It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity).<ref>{{cite journal |last1=Farina |first1=Martin Patrick W. |last2=Sumner |first2=Malcolm E. |last3=Plank |first3=C. Owen |last4=Letzsch |first4=W. Stephen |year=1980 |title=Exchangeable aluminum and pH as indicators of lime requirement for corn |journal=[[Soil Science Society of America Journal]] |volume=44 |issue=5 |pages=1036–1041 |url=https://www.researchgate.net/publication/250123873 |access-date=20 June 2021 |doi=10.2136/sssaj1980.03615995004400050033x |bibcode=1980SSASJ..44.1036F}}</ref> The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.{{sfn|Donahue|Miller|Shickluna|1977|pp=119–120}}


===Buffering===
====Buffering====
{{Further|Soil conditioner}}
{{Further|Soil conditioner}}
The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, though soils high in colloids (whether mineral or organic) have high [[buffering capacity]].<ref>{{cite journal |last1=Sposito |first1=Garrison |last2=Skipper |first2=Neal T. |last3=Sutton |first3=Rebecca |last4=Park |first4=Sun-Ho |last5=Soper |first5=Alan K. |last6=Greathouse |first6=Jeffery A. |year=1999 |title=Surface geochemistry of the clay minerals |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=96 |issue=7 |pages=3358–3364 |doi=10.1073/pnas.96.7.3358 |pmid=10097044 |pmc=34275 |bibcode=1999PNAS...96.3358S |doi-access=free}}</ref> Buffering occurs by cation exchange and [[Neutralization (chemistry)|neutralisation]]. However, colloids are not the only regulators of soil pH. The role of [[carbonates]] should be underlined, too.<ref>{{cite web |last=Sparks |first=Donald L. |title=Acidic and basic soils: buffering |url=https://lawr.ucdavis.edu/classes/ssc102/Section8.pdf |publisher=[[University of California, Davis]], Department of Land, Air, and Water Resources |location=Davis, California |access-date=20 June 2021}}</ref> More generally, according to pH levels, several buffer systems take precedence over each other, from [[calcium carbonate]] [[buffer range]] to iron buffer range.<ref>{{cite book |last=Ulrich |first=Bernhard |title=Effects of Accumulation of Air Pollutants in Forest Ecosystems |chapter=Soil Acidity and its Relations to Acid Deposition |date=1983 |chapter-url=https://rd.springer.com/content/pdf/10.1007%2F978-94-009-6983-4_10.pdf |pages=127–146 |edition=1st |editor-last1=Ulrich |editor-first1=Bernhard |editor-last2=Pankrath |editor-first2=Jürgen |publisher=[[D. Reidel Publishing Company]] |location=Dordrecht, The Netherlands |isbn=978-94-009-6985-8 |doi=10.1007/978-94-009-6983-4_10 |access-date=21 June 2021}}</ref>
The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, while soils high in colloids have high buffering capacity. Buffering occurs by cation exchange and neutralisation.


The addition of a small amount highly basic aqueous ammonia to a soil will cause the ammonium to displace hydrogen ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.
The addition of a small amount of highly basic aqueous ammonia to a soil will cause the [[ammonium]] to displace hydronium ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.


The addition of a small amount of [[liming (soil)|lime]], Ca(OH)<sub>2</sub>, will displace hydrogen ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO<sub>2</sub> and water, with little permanent change in soil pH.
The addition of a small amount of [[liming (soil)|lime]], Ca(OH)<sub>2</sub>, will displace hydronium ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO<sub>2</sub> and water, with little permanent change in soil pH.


The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=120–21}}
The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=120–121}}


==Nutrients==
=== Redox ===
{{main|Redox#Redox_reactions_in_soils}}
{{Main|Plant nutrition|Soil pH#Effect of soil pH on plant growth}}
{{See also|Table of standard reduction potentials for half-reactions important in biochemistry}}
Sixteen elements or nutrients are essential for plant growth and reproduction. They are [[carbon]] '''C''', [[hydrogen]] '''H''', [[oxygen]] '''O''', [[nitrogen]] '''N''', [[phosphorus]] '''P''', [[potassium]] '''K''', [[sulfur]] '''S''', [[calcium]] '''Ca''', [[magnesium]] '''Mg''', [[iron]] '''Fe''', [[boron]] '''B''', [[manganese]] '''Mn''', [[copper]] '''Cu''', [[zinc]] '''Zn''', [[molybdenum]] '''Mo''', [[nickel]] '''Ni''' and [[chlorine]] '''Cl'''.{{sfn|Dean|1957|p=80}}{{sfn|Russel|1957|pp=123–25}}<ref name = BradyWeil>{{cite book | title = The nature and properties of soils | year = 2008 | edition = 14th | last1 = Brady | first1 = Nyle C. | last2 = Weil | first2 = Ray R. | publisher = Pearson | location = Upper Saddle River }}</ref> Nutrients required for plants to complete their life cycle are considered '''essential nutrients'''. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered '''non-essential'''. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through [[nitrogen fixation]],<ref name = BradyWeil/> the nutrients derive originally from the mineral component of the soil.


Soil chemical reactions involve some combination of proton and electron transfer. Oxidation occurs if there is a loss of electrons in the transfer process while reduction occurs if there is a gain of electrons. [[Reduction potential]] is measured in volts or millivolts. Soil microbial communities develop along [[electron transport chain]]s, forming electrically conductive [[Geobacter#Biofilm conductivity|biofilms]], and developing networks of [[bacterial nanowires]].
Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an ionic form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they [[weathering|weather]] too slowly to support rapid plant growth. For example, The application of finely ground minerals, [[feldspar]] and [[apatite]], to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.{{sfn|Dean|1957|pp=80–81}}


Redox factors in soil development, where formation of [[redoximorphic features|redoximorphic color features]] provides critical information for soil interpretation. Understanding the [[Redox gradient#Terrestrial Environments|redox gradient]] is important to managing carbon sequestration, bioremediation, [[Pedosphere#Redox conditions in wetland soils|wetland delineation]], and [[soil-based microbial fuel cell]]s.
The nutrients adsorbed onto the surfaces of clay colloids and [[soil organic matter]] provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of [[soil organic matter]] by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.<ref name="Roy2006Chapter4">{{cite book|url=http://www.fao.org/fileadmin/templates/soilbiodiversity/Downloadable_files/fpnb16.pdf|title=Plant nutrition for food security: a guide for integrated nutrient management|last1=Roy|first1=R.N.|last2=Finck|first2=A.|last3=Blair|first3=G.J.|last4=Tandon|first4=H.L.S.|date=2006|publisher=Food and Agriculture Organization of the United Nations|year=|isbn=978-92-5-105490-1|location=Rome|pages=43–90|chapter=Chapter 4: Soil fertility and crop production|accessdate=}}</ref>


==Nutrients==
Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals. All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.{{sfn|Donahue|Miller|Shickluna|1977|pp=123–31}}<ref name="Roy2006Chapter4" />
{| class="wikitable sortable floatright"

|+ Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake{{sfn|Donahue|Miller|Shickluna|1977|p=125}}
{| class="wikitable sortable" style="border-spacing: 2px; margin:auto;"
|+ '''Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake'''{{sfn|Donahue|Miller|Shickluna|1977|p=125}}
|-
|-
! Element !! Symbol !! Ion or molecule
! Element !! Symbol !! Ion or molecule
Line 684: Line 230:
| Carbon || C || CO<sub>2</sub> (mostly through leaves)
| Carbon || C || CO<sub>2</sub> (mostly through leaves)
|-
|-
| Hydrogen || H || H<sup>+</sup>, HOH (water)
| Hydrogen || H || H<sup>+</sup>, H<sub>2</sub>O (water)
|-
|-
| Oxygen || O || O<sup>2−</sup>, OH<sup> −</sup>, CO<sub>3</sub><sup>2−</sup>, SO<sub>4</sub><sup>2−</sup>, CO<sub>2</sub>
| Oxygen || O || O<sup>2−</sup>, OH<sup>−</sup>, {{chem|CO|3|2−}}, {{chem|SO|4|2−}}, CO<sub>2</sub>
|-
|-
| Phosphorus || P || H<sub>2</sub>PO<sub>4</sub><sup> </sup>, HPO<sub>4</sub><sup>2−</sup> (phosphates)
| Phosphorus || P || {{chem|H|2|PO|4|}}, {{chem|HPO|4|2−}} (phosphates)
|-
|-
| Potassium || K || K<sup>+</sup>
| Potassium || K || K<sup>+</sup>
|-
|-
| Nitrogen || N || NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup> </sup> (ammonium, nitrate)
| Nitrogen || N || {{chem|NH|4|+}}, {{chem|NO|3|}} (ammonium, nitrate)
|-
|-
| Sulfur || S || SO<sub>4</sub><sup>2−</sup>
| Sulfur || S || {{chem|SO|4|2−}}
|-
|-
| Calcium || Ca || Ca<sup>2+</sup>
| Calcium || Ca || Ca<sup>2+</sup>
Line 702: Line 248:
| Magnesium || Mg || Mg<sup>2+</sup>
| Magnesium || Mg || Mg<sup>2+</sup>
|-
|-
| Boron || B || H<sub>3</sub>BO<sub>3</sub>, H<sub>2</sub>BO<sub>3</sub><sup> </sup>, B(OH)<sub>4</sub><sup> </sup>
| Boron || B || H<sub>3</sub>BO<sub>3</sub>, {{chem|H|2|BO|3|}}, {{chem|B(OH)|4|}}
|-
|-
| Manganese || Mn || Mn<sup>2+</sup>
| Manganese || Mn || Mn<sup>2+</sup>
Line 710: Line 256:
| Zinc || Zn || Zn<sup>2+</sup>
| Zinc || Zn || Zn<sup>2+</sup>
|-
|-
| Molybdenum || Mo || MoO<sub>4</sub><sup>2−</sup> (molybdate)
| Molybdenum || Mo || {{chem|MoO|4|2−}} (molybdate)
|-
|-
| Chlorine || Cl || Cl<sup> −</sup> (chloride)
| Chlorine || Cl || Cl<sup>−</sup> (chloride)
|}
|}
{{Main|Plant nutrients in soil|Plant nutrition|Soil pH#Effect of soil pH on plant growth}}
Seventeen elements or nutrients are essential for plant growth and reproduction. They are [[carbon]] (C), [[hydrogen]] (H), [[oxygen]] (O), [[nitrogen]] (N), [[phosphorus]] (P), [[potassium]] (K), [[sulfur]] (S), [[calcium]] (Ca), [[magnesium]] (Mg), [[iron]] (Fe), [[boron]] (B), manganese (Mn), [[copper]] (Cu), [[zinc]] (Zn), [[molybdenum]] (Mo), [[nickel]] (Ni) and [[chlorine]] (Cl).{{sfn|Dean|1957|p=80}}{{sfn|Russel|1957|pp=123–125}}<ref name=BradyWeil>{{cite book |title=The nature and properties of soils |year=2016 |edition=15th |last1=Weil |first1=Ray R. |last2=Brady |first2=Nyle C. |publisher=[[Pearson Education|Pearson]] |location=Upper Saddle River, New Jersey |url=https://fr.z-library.se/book/6018037/51b2d6 |access-date=10 December 2023 |isbn=978-0133254488 |archive-date=10 December 2023 |archive-url=https://web.archive.org/web/20231210092639/https://fr.z-library.se/book/6018037/51b2d6 |url-status=dead }}</ref> Nutrients required for plants to complete their life cycle are considered [[essential nutrients]]. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation,<ref name = BradyWeil/> the nutrients derive originally from the mineral component of the soil. The [[Law of the Minimum]] expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant.<ref>{{cite journal |last1=Van der Ploeg |first1=Rienk R. |last2=Böhm |first2=Wolfgang |last3=Kirkham |first3=Mary Beth |year=1999 |title=On the origin of the theory of mineral nutrition of plants and the Law of the Minimum |journal=[[Soil Science Society of America Journal]] |volume=63 |issue=5 |pages=1055–1062 |doi=10.2136/sssaj1999.6351055x |citeseerx=10.1.1.475.7392 |bibcode=1999SSASJ..63.1055V |doi-access=free }}</ref> A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.<ref>{{cite journal |last1=Knecht |first1=Magnus F. |last2=Göransson |first2=Anders |year=2004 |title=Terrestrial plants require nutrients in similar proportions |journal=Tree Physiology |volume=24 |issue=4 |pages=447–460 |doi=10.1093/treephys/24.4.447 |pmid=14757584 |doi-access=free }}</ref>


Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an [[Ionic compound|ionic]] form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within [[Primary mineral|primary]] and [[Secondary mineral|secondary minerals]], they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals, [[feldspar]] and [[apatite]], to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.{{sfn|Dean|1957|pp=80–81}}
===Uptake processes===
Nutrients in the soil are taken up by the plant through its roots. To be taken up by a plant, a nutrient element must be located near the root surface; however, the supply of nutrients in contact with the root is rapidly depleted. There are three basic mechanisms whereby nutrient ions dissolved in the soil solution are brought into contact with plant roots:


The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.<ref name="Roy2006Chapter4">{{cite book |chapter-url=https://www.fao.org/fileadmin/templates/soilbiodiversity/Downloadable_files/fpnb16.pdf |title=Plant nutrition for food security: a guide for integrated nutrient management |last1=Roy |first1=R. N. |last2=Finck |first2=Arnold |last3=Blair |first3=Graeme J. |last4=Tandon |first4=Hari Lal Singh |publisher=[[Food and Agriculture Organization of the United Nations]] |year=2006 |isbn=978-92-5-105490-1|location=Rome, Italy |pages=43–90 |chapter=Soil fertility and crop production |access-date=17 December 2023 }}</ref>
# Mass flow of water
# Diffusion within water
# Interception by root growth


Gram for gram, the capacity of [[humus]] to hold nutrients and water is far greater than that of clay minerals, most of the soil [[Cation-exchange capacity|cation exchange capacity]] arising from charged [[carboxylic]] groups on organic matter.<ref>{{cite journal |last1=Parfitt |first1=Roger L. |last2=Giltrap |first2=Donna J. |last3=Whitton |first3=Joe S. |year=1995 |title=Contribution of organic matter and clay minerals to the cation exchange capacity of soil |journal=Communications in Soil Science and Plant Analysis |volume=26 |issue=9–10 |pages=1343–55 |url=https://www.researchgate.net/publication/249073571 |doi=10.1080/00103629509369376 |bibcode=1995CSSPA..26.1343P |access-date=17 December 2023 }}</ref> However, despite the great capacity of humus to retain water once water-soaked, its high [[hydrophobicity]] decreases its [[wettability]] once dry.<ref>{{cite journal |last1=Hajnos |first1=Mieczyslaw |last2=Jozefaciuk |first2=Grzegorz |last3=Sokołowska |first3=Zofia |last4=Greiffenhagen |first4=Andreas |last5=Wessolek |first5=Gerd |year=2003 |title=Water storage, surface, and structural properties of sandy forest humus horizons |journal=Journal of Plant Nutrition and Soil Science |volume=166 |issue=5 |pages=625–34 |url=https://www.researchgate.net/publication/229970348 |doi=10.1002/jpln.200321161 |bibcode=2003JPNSS.166..625H |access-date=17 December 2023 }}</ref> All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.{{sfn|Donahue|Miller|Shickluna|1977|pp=123–131}}<ref name="Roy2006Chapter4"/>
All three mechanisms operate simultaneously, but one mechanism or another may be most important for a particular nutrient. For example, in the case of calcium, which is generally plentiful in the soil solution, mass flow alone can usually bring sufficient amounts to the root surface. However, in the case of phosphorus, diffusion is needed to supplement mass flow. For the most part, nutrient ions must travel some distance in the soil solution to reach the root surface. This movement can take place by mass flow, as when dissolved nutrients are carried along with the soil water flowing toward a root that is actively drawing water from the soil. In this type of movement, the nutrient ions are somewhat analogous to leaves floating down a stream. In addition, nutrient ions continually move by diffusion from areas of greater concentration toward the nutrient-depleted areas of lower concentration around the root surface. That process is due to random motion of molecules. By this means, plants can continue to take up nutrients even at night, when water is only slowly absorbed into the roots as transpiration has almost stopped. Finally, root interception comes into play as roots continually grow into new, undepleted soil.

{| class="wikitable" style="border-spacing: 5px; margin:auto;"
|+ '''Estimated relative importance of mass flow, diffusion and root interception as mechanisms in supplying plant nutrients to corn plant roots in soils'''{{sfn|Donahue|Miller|Shickluna|1977|p=126}}
|-
! scope="col" style="width:100px;" rowspan="2"| Nutrient
! colspan="3"| Approximate percentage supplied by:
|-
! scope="col" style="width:100px;"| Mass flow
! scope="col" style="width:100px;"| Root interception
! scope="col" style="width:100px;"| Diffusion
|-
| Nitrogen || 98.8 || 1.2 || 0
|-
| Phosphorus || 6.3 || 2.8 || 90.9
|-
| Potassium || 20.0 || 2.3 || 77.7
|-
| Calcium || 71.4 || 28.6 || 0
|-
| Sulfur || 95.0 || 5.0 || 0
|-
| Molybdenum || 95.2 || 4.8 || 0
|}

In the above table, phosphorus and potassium nutrients move more by diffusion than they do by mass flow in the soil water solution, as they are rapidly taken up by the roots creating a concentration of almost zero near the roots (the plants cannot transpire enough water to draw more of those nutrients near the roots). The very steep concentration gradient is of greater influence in the movement of those ions than is the movement of those by mass flow.<ref>
{{cite web
|url = http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec22/Lec22.html
|title = Lecture 22
|publisher = Northern Arizona University
|accessdate = 22 March 2013
|deadurl = no
|archiveurl = https://web.archive.org/web/20130514090308/http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec22/Lec22.html
|archivedate = 14 May 2013
|df = dmy-all
}}</ref> The movement by mass flow requires the transpiration of water from the plant causing water and solution ions to also move toward the roots. Movement by root interception is slowest as the plants must extend their roots.

Plants move ions out of their roots in an effort to move nutrients in from the soil. Hydrogen H<sup>+</sup> is exchanged for other cations, and carbonate (HCO<sub>3</sub><sup>−</sup>) and hydroxide (OH<sup>−</sup>) anions are exchanged for nutrient anions. As plant roots remove nutrients from the soil water solution, they are replenished as other ions move off of clay and humus (by [[ion exchange]] or [[desorption]]), are added from the [[weathering]] of soil minerals, and are released by the [[Soil organic matter#Decomposition|decomposition of soil organic matter]]. Plants derive a large proportion of their anion nutrients from decomposing organic matter, which typically holds about 95&nbsp;percent of the soil nitrogen, 5 to 60&nbsp;percent of the soil phosphorus and about 80&nbsp;percent of the soil sulfur. Where crops are produced, the replenishment of nutrients in the soil must usually be augmented by the addition of fertilizer or organic matter.{{sfn|Donahue|Miller|Shickluna|1977|p=126}}

Because nutrient uptake is an active metabolic process, conditions that inhibit root metabolism may also inhibit nutrient uptake. Examples of such conditions include [[Waterlogging (agriculture)|waterlogging]] or [[soil compaction]] resulting in poor soil aeration, excessively high or low soil temperatures, and above-ground conditions that result in low translocation of sugars to plant roots.{{sfn|Donahue|Miller|Shickluna|1977|pp=123–28}}

===Carbon===
[[File:SRS2000 soil respiration system.jpg|thumb|Measuring soil respiration in the field using an SRS2000 system.]]
Plants obtain their carbon from atmospheric carbon dioxide. About 45% of a plant's dry mass is carbon; plant residues typically have a carbon to nitrogen ratio (C/N) of between 13:1 and 100:1. As the soil organic material is digested by arthropods and micro-organisms, the C/N decreases as the carbonaceous material is metabolized and carbon dioxide (CO<sub>2</sub>) is released as a byproduct which then finds its way out of the soil and into the atmosphere. The nitrogen is sequestered in the bodies of the living matter of those decomposing organisms and so it builds up in the soil. Normal CO<sub>2</sub> concentration in the atmosphere is 0.03%, this can be the factor limiting plant growth. In a field of maize on a still day during high light conditions in the growing season, the CO<sub>2</sub> concentration drops very low, but under such conditions the crop could use up to 20 times the normal concentration. The respiration of CO<sub>2</sub> by soil micro-organisms decomposing soil organic matter contributes an important amount of CO<sub>2</sub> to the photosynthesising plants. Within the soil, CO<sub>2</sub> concentration is 10 to 100 times that of atmospheric levels but may rise to toxic levels if the soil porosity is low or if diffusion is impeded by flooding.{{sfn|Wadleigh|1957|p=41}}{{sfn|Dean|1957|p=80}}{{sfn|Broadbent|1957|p=153}}

===Nitrogen===
{{Further|Nitrogen cycle}}
[[File:SoilNitrogen.jpg|thumb|upright=0.9|{{center|Generalization of percent soil nitrogen by soil order}}]]
Nitrogen is the most critical element obtained by plants from the soil and [[nitrogen deficiency]] often limits plant growth.{{sfn|Donahue|Miller|Shickluna|1977|p=128}} Plants can use the nitrogen as either the ammonium cation (NH<sub>4</sub><sup>+</sup>) or the anion nitrate (NO<sub>3</sub><sup>−</sup>). Usually, most of the nitrogen in soil is bound within organic compounds that make up the soil organic matter, and must be [[Mineralization (soil science)|mineralized]] to the ammonium or nitrate form before it can be taken up by most plants. The total nitrogen content depends largely on the soil organic matter content, which in turn depends on the climate, vegetation, topography, age and soil management. Soil nitrogen typically decreases by 0.2 to 0.3% for every temperature increase by 10&nbsp;°C. Usually, grassland soils contain more soil nitrogen than forest soils. Cultivation decreases soil nitrogen by exposing soil organic matter to decomposition by microorganisms, and soils under no-tillage maintain more soil nitrogen than tilled soils.

Some micro-organisms are able to metabolise organic matter and release ammonium in a process called ''mineralisation''. Others take free ammonium and oxidise it to nitrate. Nitrogen-fixing bacteria are capable of metabolising N<sub>2</sub> into the form of ammonia in a process called [[nitrogen fixation]]. Both ammonium and nitrate can be ''immobilized'' by their incorporation into the microbes' living cells, where it is temporarily sequestered in the form of amino acids and protein. Nitrate may also be lost from the soil when bacteria metabolise it to the gases N<sub>2</sub> and N<sub>2</sub>O. The loss of gaseous forms of nitrogen to the atmosphere due to microbial action is called ''denitrification''. Nitrogen may also be ''leached'' from the soil if it is in the form of nitrate or lost to the atmosphere as ammonia due to a chemical reaction of ammonium with alkaline soil by way of a process called ''volatilisation''. Ammonium may also be sequestered in clay by ''fixation''. A small amount of nitrogen is added to soil by rainfall.<ref name="Roy2006Chapter4" />{{sfn|Allison|1957|pp=85–94}}{{sfn|Broadbent|1957|pp=152–55}}{{sfn|Donahue|Miller|Shickluna|1977|pp=128–31}}

====Gains====
In the process of [[mineralization (soil)|mineralisation]], microbes feed on organic matter, releasing ammonia (NH<sub>3</sub>), ammonium (NH<sub>4</sub><sup>+</sup>) and other nutrients. As long as the carbon to nitrogen ratio (C/N) of fresh residues in the soil is above 30:1, nitrogen will be in short supply and other bacteria will feed on the ammonium and incorporate its nitrogen into their cells in the [[immobilization (soil science)|immobilization]] process. In that form the nitrogen is said to be ''immobilised''. Later, when such bacteria die, they too are ''mineralised'' and some of the nitrogen is released as ammonium and nitrate. If the C/N is less than 15, ammonia is freed to the soil, where it may be used by bacteria which [[redox|oxidise]] it to nitrate ([[nitrification]]). Bacteria may on average add {{convert|25|lb}} nitrogen per acre, and in an unfertilised field, this is the most important source of usable nitrogen. In a soil with 5% organic matter perhaps 2 to 5% of that is released to the soil by such decomposition. It occurs fastest in warm, moist, well aerated soil. The mineralisation of 3% of the organic material of a soil that is 4% organic matter overall, would release {{convert|120|lb}} of nitrogen as ammonium per acre.{{sfn|Donahue|Miller|Shickluna|1977|pp=129–30}}

{| class="wikitable sortable" style="border-spacing: 10px; margin:auto;"
|+ '''Carbon/Nitrogen Ratio of Various Organic Materials'''{{sfn|Donahue|Miller|Shickluna|1977|p=145}}
|-
! scope="col" | Organic Material
! scope="col" | C:N Ratio
|-
| Alfalfa || 13
|-
| Bacteria || 4
|-
| Clover, green sweet || 16
|-
| Clover, mature sweet || 23
|-
| Fungi || 9
|-
| Forest litter || 30
|-
| Humus in warm cultivated soils || 11
|-
| Legume-grass hay || 25
|-
| Legumes (alfalfa or clover), mature || 20
|-
| Manure, cow || 18
|-
| Manure, horse || 16–45
|-
| Manure, human || 10
|-
| Oat straw || 80
|-
| Straw, cornstalks || 90
|-
| Sawdust || 250
|}

In [[nitrogen fixation]], [[rhizobium]] bacteria convert N<sub>2</sub> to ammonia (NH<sub>3</sub>). [[Rhizobia]] share a [[Symbiosis|symbiotic relationship]] with host plants, since rhizobia supply the host with nitrogen and the host provides rhizobia with nutrients and a safe environment. It is estimated that such symbiotic bacteria in the [[root nodule]]s of [[legume]]s add 45 to 250&nbsp;pounds of nitrogen per acre per year, which may be sufficient for the crop. Other, free-living nitrogen-fixing bacteria and [[blue-green algae]] live independently in the soil and release nitrate when their dead bodies are converted by way of mineralisation.{{sfn|Donahue|Miller|Shickluna|1977|pp=128–29}}

Some amount of usable nitrogen is fixed by [[lightning]] as nitric oxide (NO) and nitrogen dioxide (NO<sub>2</sub><sup>−</sup>). Nitrogen dioxide is soluble in water to form [[nitric acid]] (HNO<sub>3</sub>) solution of H<sup>+</sup> and NO<sub>3</sub><sup>−</sup>. Ammonia, NH<sub>3</sub>, previously released from the soil or from combustion, may fall with precipitation as nitric acid at a rate of about five pounds nitrogen per acre per year.{{sfn|Allison|1957|p=87}}

====Sequestration====
When bacteria feed on soluble forms of nitrogen (ammonium and nitrate), they temporarily sequester that nitrogen in their bodies in a process called ''immobilisation''. At a later time when those bacteria die, their nitrogen may be released as ammonium by the processes of mineralisation.

Protein material is easily broken down, but the rate of its decomposition is slowed by its attachment to the crystalline structure of clay and when trapped between the clay layers. The layers are small enough that bacteria cannot enter. Some organisms can exude extracellular enzymes that can act on the sequestered proteins. However, those enzymes too may be trapped on the clay crystals.

Ammonium fixation occurs when ammonium pushes [[potassium]] ions from between the layers of clay such as [[illite]] or [[montmorillonite]]. Only a small fraction of soil nitrogen is held this way.{{sfn|Allison|1957|p=90}}

====Losses====
Usable nitrogen may be lost from soils when it is in the form of nitrate, as it is easily [[Leaching (chemistry)|leached]]. Further losses of nitrogen occur by denitrification, the process whereby soil bacteria convert nitrate (NO<sub>3</sub><sup>−</sup>) to nitrogen gas, N<sub>2</sub> or N<sub>2</sub>O. This occurs when poor [[soil aeration]] limits free oxygen, forcing bacteria to use the oxygen in nitrate for their respiratory process. Denitrification increases when oxidisable organic material is available and when soils are warm and slightly acidic. Denitrification may vary throughout a soil as the aeration varies from place to place. Denitrification may cause the loss of 10 to 20&nbsp;percent of the available nitrates within a day and when conditions are favourable to that process, losses of up to 60&nbsp;percent of nitrate applied as fertiliser may occur.{{sfn|Donahue|Miller|Shickluna|1977|p=130}}

''Ammonium volatilisation'' occurs when ammonium reacts chemically with an alkaline soil, converting NH<sub>4</sub><sup>+</sup> to NH<sub>3</sub>. The application of ammonium fertiliser to such a field can result in volatilisation losses of as much as 30 percent.{{sfn|Donahue|Miller|Shickluna|1977|p=131}}

===Phosphorus===
After nitrogen, phosphorus is probably the element most likely to be deficient in soils. The soil mineral [[apatite]] is the most common mineral source of phosphorus. While there is on average 1000&nbsp;lb of phosphorus per acre in the soil, it is generally in the form of phosphates with low solubility. Total phosphorus is about 0.1&nbsp;percent by weight of the soil, but only one percent of that is available. Of the part available, more than half comes from the mineralisation of organic matter. Agricultural fields may need to be fertilised to make up for the phosphorus that has been removed in the crop.{{sfn|Olsen|Fried|1957|p=96}}

When phosphorus does form solubilised ions of H<sub>2</sub>PO<sub>4</sub><sup>−</sup>, they rapidly form insoluble phosphates of calcium or hydrous oxides of iron and aluminum. Phosphorus is largely immobile in the soil and is not leached but actually builds up in the surface layer if not cropped. The application of soluble fertilisers to soils may result in zinc deficiencies as zinc phosphates form. Conversely, the application of zinc to soils may immobilise phosphorus again as zinc phosphate. Lack of phosphorus may interfere with the normal opening of the plant leaf stomata, resulting in plant temperatures 10 percent higher than normal. Phosphorus is most available when soil pH is 6.5 in mineral soils and 5.5 in organic soils.{{sfn|Donahue|Miller|Shickluna|1977|p=131}}

===Potassium===
The amount of potassium in a soil may be as much as 80,000&nbsp;lb per acre-foot, of which only 150&nbsp;lb is available for plant growth. Common mineral sources of potassium are the mica [[biotite]] and potassium feldspar, KAlSi<sub>3</sub>O<sub>8</sub>. When solubilised, half will be held as exchangeable cations on clay while the other half is in the soil water solution. Potassium fixation often occurs when soils dry and the potassium is bonded between layers of illite clay. Under certain conditions, dependent on the soil texture, intensity of drying, and initial amount of exchangeable potassium, the fixed percentage may be as much as 90&nbsp;percent within ten minutes. Potassium may be leached from soils low in clay.{{sfn|Donahue|Miller|Shickluna|1977|pp=134–35}}{{sfn|Reitemeier|1957|pp=101–04}}

===Calcium===
Calcium is one percent by weight of soils and is generally available but may be low as it is soluble and can be leached. It is thus low in sandy and heavily leached soil or strongly acidic mineral soil. Calcium is supplied to the plant in the form of exchangeable ions and moderately soluble minerals. Calcium is more available on the soil colloids than is potassium because the common mineral calcite, CaCO<sub>3</sub>, is more soluble than potassium-bearing minerals.{{sfn|Donahue|Miller|Shickluna|1977|pp=135–36}}

===Magnesium===
Magnesium is one of the dominant exchangeable cations in most soils (as are calcium and potassium). Primary minerals that weather to release magnesium include [[hornblende]], [[biotite]] and [[vermiculite]]. Soil magnesium concentrations are generally sufficient for optimal plant growth, but highly weathered and sandy soils may be magnesium deficient due to leaching by heavy precipitation.<ref name="Roy2006Chapter4" />{{sfn|Donahue|Miller|Shickluna|1977|p=136}}

===Sulfur===
Most sulfur is made available to plants, like phosphorus, by its release from decomposing organic matter.{{sfn|Donahue|Miller|Shickluna|1977|p=136}} Deficiencies may exist in some soils (especially sandy soils) and if cropped, sulfur needs to be added. The application of large quantities of nitrogen to fields that have marginal amounts of sulfur may cause sulfur deficiency in the rapidly growing plants by the plant's growth outpacing the supply of sulfur. A 15-ton crop of onions uses up to 19&nbsp;lb of sulfur and 4&nbsp;tons of alfalfa uses 15&nbsp;lb per acre. Sulfur abundance varies with depth. In a sample of soils in Ohio, United States, the sulfur abundance varied with depths, 0-6&nbsp;inches, 6-12&nbsp;inches, 12-18&nbsp;inches, 18-24&nbsp;inches in the amounts: 1056, 830, 686, 528&nbsp;lb per acre respectively.{{sfn|Jordan|Reisenauer|1957|p=107}}

===Micronutrients===
The micronutrients essential in plant life, in their order of importance, include [[iron]],{{sfn|Holmes|Brown|1957|pp=111}} [[manganese]],{{sfn|Sherman|1957|p=135}} [[zinc]],{{sfn|Seatz|Jurinak|1957|p=115}} [[copper]],{{sfn|Reuther|1957|p=128}} [[boron]],{{sfn|Russel|1957|p=121}} [[chlorine]]{{sfn|Stout|Johnson|1957|p=146}} and [[molybdenum]].{{sfn|Stout|Johnson|1957|p=141}} The term refers to plants' needs, not to their abundance in soil. They are required in very small amounts but are essential to plant health in that most are required parts of some enzyme system which speeds up plants' metabolisms. They are generally available in the mineral component of the soil, but the heavy application of phosphates can cause a deficiency in zinc and iron by the formation of insoluble zinc and iron phosphates. Iron deficiency may also result from excessive amounts of heavy metals or calcium minerals (lime) in the soil. Excess amounts of soluble boron, molybdenum and chloride are toxic.{{sfn|Donahue|Miller|Shickluna|1977|pp=136–37}}{{sfn|Stout|Johnson|1957|p=107}}

===Non-essential nutrients===
Nutrients which enhance the health but whose deficiency does not stop the life cycle of plants include: [[cobalt]], [[strontium]], [[vanadium]], [[silicon]] and [[nickel]].<ref>{{cite web|last1=Faria Pereira|first1=B.F.|title=Nutrients and Nonessential Elements in Soil after 11 Years of Wastewater Irrigation|url=https://dl.sciencesocieties.org/publications/jeq/abstracts/41/3/920|website=ACSESS Digital Library|accessdate=17 January 2018}}</ref> As their importance are evaluated they may be added to the list of essential plant nutrients.


==Soil organic matter==
==Soil organic matter==
{{main|Soil organic matter}}
{{main|Soil organic matter}}{{Overly detailed|section|details=details could be moved into main article|date=April 2021}}
Soil organic matter is made up of [[organic compounds]] and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900&nbsp;lb of earthworms, 2400&nbsp;lb of fungi, 1500&nbsp;lb of bacteria, 133&nbsp;lb of protozoa and 890&nbsp;lb of arthropods and algae.<ref>{{Cite journal|author=Pimentel, D.|display-authors=etal|year=1995|title=Environmental and economic costs of soil erosion and conservation benefits|journal=[[Science (journal)|Science]]|volume=267|pages=1117–22|bibcode = 1995Sci...267.1117P |doi = 10.1126/science.267.5201.1117|issue=24|pmid=17789193}}</ref>
The organic material in soil is made up of [[organic compounds]] and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900&nbsp;lb of earthworms, 2400&nbsp;lb of fungi, 1500&nbsp;lb of bacteria, 133&nbsp;lb of protozoa and 890&nbsp;lb of arthropods and algae.<ref>{{cite journal |last1=Pimentel |first1=David |last2=Harvey |first2=Celia |last3=Resosudarmo |first3=Pradnja |last4=Sinclair |first4=K. |last5=Kurz |first5=D. |last6=McNair |first6=M. |last7=Crist |first7=S. |last8=Shpritz |first8=L. |last9=Fitton |first9=L. |last10=Saffouri |first10=R. |last11=Blair |first11=R. |year=1995 |title=Environmental and economic costs of soil erosion and conservation benefits |journal=[[Science (journal)|Science]] |volume=267 |issue=5201 |pages=1117–23 |url=https://www.academia.edu/9512072 |doi=10.1126/science.267.5201.1117 |pmid=17789193 |bibcode=1995Sci...267.1117P |s2cid=11936877 |access-date=4 July 2021 |archive-url=https://web.archive.org/web/20161213065558/http://www.rachel.org/files/document/Environmental_and_Economic_Costs_of_Soil_Erosi.pdf |archive-date=13 December 2016 |url-status=live}}</ref>

A small part of the organic matter consists of the living cells such as bacteria, molds, and actinomycetes that work to break down the dead organic matter. Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil.


A few percent of the soil organic matter, with small [[residence time]], consists of the microbial [[biomass]] and [[metabolites]] of bacteria, molds, and actinomycetes that work to break down the dead organic matter.<ref>{{cite journal |last1=Schnürer |first1=Johan |last2=Clarholm |first2=Marianne |last3=Rosswall |first3=Thomas |year=1985 |title=Microbial biomass and activity in an agricultural soil with different organic matter contents |journal=[[Soil Biology and Biochemistry]] |volume=17 |issue=5 |pages=611–618 |url=https://www.academia.edu/20647751 |doi=10.1016/0038-0717(85)90036-7 |bibcode=1985SBiBi..17..611S |access-date=4 July 2021}}</ref><ref>{{cite journal |last=Sparling |first=Graham P. |year=1992 |title=Ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter |journal=[[Australian Journal of Soil Research]] |volume=30 |issue=2 |pages=195–207 |url=https://www.researchgate.net/publication/248884528 |doi=10.1071/SR9920195 |access-date=4 July 2021}}</ref> Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. However, in the same time soil microbes contribute to [[carbon sequestration]] in the topsoil through the formation of stable humus.<ref>{{cite journal |last1=Varadachari |first1=Chandrika |last2=Ghosh |first2=Kunal |year=1984 |title=On humus formation |journal=[[Plant and Soil]] |volume=77 |issue=2 |pages=305–313 |doi=10.1007/BF02182933 |bibcode=1984PlSoi..77..305V |s2cid=45102095 |doi-access=free}}</ref> In the aim to sequester more carbon in the soil for alleviating the [[greenhouse effect]] it would be more efficient in the long-term to stimulate [[humification]] than to decrease litter [[decomposition]].<ref>{{cite journal |last=Prescott |first=Cindy E. |year=2010 |title=Litter decomposition: what controls it and how can we alter it to sequester more carbon in forest soils? |journal=Biogeochemistry |volume=101 |issue=1 |pages=133–q49 |doi=10.1007/s10533-010-9439-0 |bibcode=2010Biogc.101..133P |s2cid=93834812 |doi-access=free}}</ref>
Chemically, organic matter is classed as follows:


The main part of soil organic matter is a complex assemblage of small organic molecules, collectively called humus or [[humic]] substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete.<ref>{{cite journal |last1=Lehmann |first1=Johannes |last2=Kleber |first2=Markus |year=2015 |title=The contentious nature of soil organic matter |journal=[[Nature (journal)|Nature]] |volume=528 |issue=7580 |pages=60–68 |url=http://www.css.cornell.edu/faculty/lehmann/publ/Nature%20528,%2060-68,%202015%20Lehmann.pdf |doi=10.1038/nature16069 |pmid=26595271 |bibcode=2015Natur.528...60L |s2cid=205246638 |access-date=4 July 2021}}</ref> Other studies showed that the classical notion of molecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct from polysaccharides, lignins and proteins.<ref name="Piccolo2002">{{cite journal |last=Piccolo |first=Alessandro |year=2002 |title=The supramolecular structure of humic substances: a novel understanding of humus chemistry and implications in soil science |journal=Advances in Agronomy |volume=75 |pages=57–134 |url=https://www.researchgate.net/publication/222526145 |doi=10.1016/S0065-2113(02)75003-7 |isbn=9780120007936 |access-date=4 July 2021}}</ref>
# Polysaccharides
## cellulose
## hemicellulose
## starch
## pectin
# Lignins
# Proteins


Most living things in soils, including plants, insects, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition which rate is dependent on the temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by amoebas, which in turn are fed upon by nematodes and arthropods. Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as [[peat]] ([[histosols]]), are infertile.<ref name=Foth1984>{{Cite book | last = Foth | first = Henry D. | year = 1984 | title = Fundamentals of soil science | page = 151 | isbn = 978-0-471-88926-7 | publisher = Wiley | location = New York}}</ref> In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.
Most living things in soils, including plants, animals, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition, the rate of which is dependent on the temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by [[protozoa]], which in turn are fed upon by [[nematodes]], [[annelids]] and [[arthropod]]s, themselves able to consume and transform raw or humified organic matter. This has been called the [[soil food web]], through which all organic matter is processed as in a [[digestive system]].<ref>{{cite journal |last=Scheu |first=Stefan |year=2002 |title=The soil food web: structure and perspectives |journal=European Journal of Soil Biology |volume=38 |issue=1 |pages=11–20 |url=https://www.researchgate.net/publication/263041521 |doi=10.1016/S1164-5563(01)01117-7 |bibcode=2002EJSB...38...11S |access-date=4 July 2021}}</ref> Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as [[peat]] ([[histosols]]), are infertile.<ref name=Foth1984>{{Cite book |last=Foth |first=Henry D. |year=1984 |title=Fundamentals of soil science |edition=8th |page=139 |url=http://base.dnsgb.com.ua/files/book/Agriculture/Soil/Fundamentals-of-Soil-Science.pdf |isbn=978-0471522799 |publisher=Wiley |location=New York, New York |access-date=4 July 2021 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112034423/http://base.dnsgb.com.ua/files/book/Agriculture/Soil/Fundamentals-of-Soil-Science.pdf |url-status=dead }}</ref> In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.


In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker A horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor (O horizon) and thin A horizon.
In [[grassland]], much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker [[A horizon]] with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor ([[O horizon]]) and thin A horizon.<ref name="Ponge2003">{{cite journal |last=Ponge |first=Jean-François |year=2003 |title=Humus forms in terrestrial ecosystems: a framework to biodiversity |journal=[[Soil Biology and Biochemistry]] |volume=35 |issue=7 |pages=935–945 |doi=10.1016/S0038-0717(03)00149-4 |bibcode=2003SBiBi..35..935P |url=https://www.academia.edu/45579598 |url-status=live |archive-url=https://web.archive.org/web/20160129153903/https://www.researchgate.net/publication/222567430 |archive-date=29 January 2016 |df=dmy-all |citeseerx=10.1.1.467.4937 |s2cid=44160220}}</ref>


===Humus===
===Humus===
[[Humus]] refers to organic matter that has been decomposed by soil flora and fauna to the point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to [[soil health]] and plant growth. Humus also hold bits of undecomposed organic matter which feed arthropods and worms which further improve the soil. The end product, humus, is soluble in water and forms a weak acid that can attack silicate minerals.<ref>{{cite book|last=Gilluly, Waters, Woodford|title=Principles of Geology|year=1975|edition=4th|publisher=W.H. Freeman|location=|isbn=978-0-7167-0269-6|page=216}}</ref> Humus is a colloid with a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.
Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to [[soil health]] and plant growth.<ref>{{cite web |url=http://www.harvestgrow.com/.pdf%20web%20site/Humates%20General%20Info.pdf |last=Pettit |first=Robert E. |title=Organic matter, humus, humate, humic acid, fulvic acid and humin: their importance in soil fertility and plant health |access-date=11 July 2021}}</ref> Humus also feeds arthropods, [[termite]]s and [[earthworm]]s which further improve the soil.<ref>{{cite journal |last1=Ji |first1=Rong |last2=Kappler |first2=Andreas |last3=Brune |first3=Andreas |year=2000 |title=Transformation and mineralization of synthetic <sup>14</sup>C-labeled humic model compounds by soil-feeding termites |journal=[[Soil Biology and Biochemistry]] |volume=32 |issue=8–9 |pages=1281–1291 |doi=10.1016/S0038-0717(00)00046-8 |citeseerx=10.1.1.476.9400 }}</ref> The end product, humus, is suspended in [[colloidal]] form in the soil solution and forms a [[weak acid]] that can attack silicate minerals by [[Chelation|chelating]] their iron and aluminum atoms.<ref>{{cite book |last1=Drever |first1=James I. |last2=Vance |first2=George F. |title=Organic Acids in Geological Processes |chapter=Role of Soil Organic Acids in Mineral Weathering Processes |year=1994 |doi=10.1007/978-3-642-78356-2_6 |editor-last1=Pittman |editor-first1=Edward D. |editor-last2=Lewan |editor-first2=Michael D. |publisher=[[Springer Science+Business Media|Springer]] |location=Berlin, Germany |pages=138–161 |isbn=978-3-642-78356-2 |chapter-url=https://link.springer.com/content/pdf/10.1007%2F978-3-642-78356-2_6.pdf |access-date=11 July 2021}}</ref> Humus has a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.<ref name="Piccolo1996">{{cite book |last=Piccolo |first=Alessandro |year=1996 |chapter=Humus and soil conservation |doi=10.1016/B978-044481516-3/50006-2 |title=Humic substances in terrestrial ecosystems |editor-first=Alessandro |editor-last=Piccolo |publisher= [[Elsevier]] |location=Amsterdam, the Netherlands |pages=225–264 |isbn=978-0-444-81516-3 |chapter-url=https://www.researchgate.net/publication/281451183 |access-date=11 July 2021}}</ref>


[[Humic acid]]s and [[fulvic acid]]s, which begin as raw organic matter, are important constituents of humus. After the death of plants and animals, microbes begin to feed on the residues, resulting finally in the formation of humus. With decomposition, there is a reduction of water-soluble constituents, [[cellulose]] and [[hemicellulose]], and nutrients such as nitrogen, phosphorus, and sulfur. As the residues break down, only stable molecules made of aromatic carbon rings, oxygen and hydrogen remain in the form of [[humin]], [[lignin]] and lignin complexes collectively called humus. While the structure of humus has few nutrients, it is able to attract and hold cation and anion nutrients by weak bonds that can be released into the soil solution in response to changes in soil pH.
[[Humic acid]]s and [[fulvic acid]]s, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus.<ref>{{cite journal |last1=Varadachari |first1=Chandrika |last2=Ghosh |first2=Kunal |year=1984 |title=On humus formation |journal=[[Plant and Soil]] |volume=77 |issue=2 |pages=305–313 |url=https://www.researchgate.net/publication/225528442 |doi=10.1007/BF02182933 |bibcode=1984PlSoi..77..305V |s2cid=45102095 |access-date=11 July 2021}}</ref> As the residues break down, only molecules made of [[aliphatic compound|aliphatic]] and [[aromatic hydrocarbon|aromatic]] hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus.<ref name="Piccolo2002"/> Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure.<ref name="Piccolo1996"/> Although the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.<ref>{{cite journal |last1=Mendonça |first1=Eduardo S. |last2=Rowell |first2=David L. |year=1996 |title=Mineral and organic fractions of two oxisols and their influence on effective cation-exchange capacity |journal=[[Soil Science Society of America Journal]] |volume=60 |issue=6 |pages=1888–1892 |url=https://www.researchgate.net/publication/250128642 |doi=10.2136/sssaj1996.03615995006000060038x |bibcode=1996SSASJ..60.1888M |access-date=11 July 2021}}</ref>


Lignin is resistant to breakdown and accumulates within the soil. It also reacts with amino acids, which further increases its resistance to decomposition, including enzymatic decomposition by microbes. [[Fat]]s and [[wax]]es from plant matter have some resistance to decomposition and persist in soils for a while. Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay. Proteins normally decompose readily, but when bound to clay particles, they become more resistant to decomposition. Clay particles also absorb the enzymes exuded by microbes which would normally break down proteins. The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years. High soil [[tannin]] ([[polyphenol]]) content can cause nitrogen to be sequestered in proteins or cause nitrogen immobilisation.<ref name=Verkaik2006>{{cite journal |last=Verkaik |first=Eric |last2=Jongkind |first2=Anne G. |last3=Berendse |first3=Frank |date=2006 |title=Short-term and long-term effects of tannins on nitrogen mineralization and litter decomposition in kauri (Agathis australis (D. Don) Lindl.) forests |journal=Plant and Soil |volume=287 |pages=337–45 |doi=10.1007/s11104-006-9081-8 |url=http://edepot.wur.nl/25615 |format=Submitted manuscript }}</ref><ref name=Fierer2001>{{Cite journal |last=Fierer |first=N. |last2=Schimel |first2=Joshua P. |last3=Cates |first3=Rex G. |last4=Zou |first4=Jiping |title=Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils |journal=Soil Biology and Biochemistry |date=2001 |volume=33 |pages=1827–39 |doi=10.1016/S0038-0717(01)00111-0 |issue=12–13}}</ref>
[[Lignin]] is resistant to breakdown and accumulates within the soil. It also reacts with [[proteins]],<ref>{{cite journal |last1=Heck |first1=Tobias |last2=Faccio |first2=Greta |last3=Richter |first3=Michael |last4=Thöny-Meyer |first4=Linda |year=2013 |title=Enzyme-catalyzed protein crosslinking |journal=[[Applied Microbiology and Biotechnology]] |volume=97 |issue=2 |pages=461–475 |url=https://www.researchgate.net/publication/233769618 |doi=10.1007/s00253-012-4569-z |pmid=23179622 |pmc=3546294 |access-date=11 July 2021}}</ref> which further increases its resistance to decomposition, including enzymatic decomposition by microbes.<ref>{{cite journal |last1=Lynch |first1=D. L. |last2=Lynch |first2=C. C. |year=1958 |title=Resistance of protein–lignin complexes, lignins and humic acids to microbial attack |journal=[[Nature (journal)|Nature]] |volume=181 |issue=4621 |pages=1478–1479 |url=https://www.nature.com/articles/1811478a0.pdf |doi=10.1038/1811478a0 |pmid=13552710 |bibcode=1958Natur.181.1478L |s2cid=4193782 |access-date=11 July 2021}}</ref> [[Fat]]s and [[wax]]es from plant matter have still more resistance to decomposition and persist in soils for thousand years, hence their use as tracers of past vegetation in buried soil layers.<ref>{{cite journal |last1=Dawson |first1=Lorna A. |last2=Hillier |first2=Stephen |year=2010 |title=Measurement of soil characteristics for forensic applications |journal=[[Surface and Interface Analysis]] |volume=42 |issue=5 |pages=363–377 |url=https://people.ok.ubc.ca/robrien/soil%20characteristics.pdf |doi=10.1002/sia.3315 |s2cid=54213404 |access-date=18 July 2021 |archive-date=8 May 2021 |archive-url=https://web.archive.org/web/20210508065204/https://people.ok.ubc.ca/robrien/soil%20characteristics.pdf |url-status=dead }}</ref> Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay.<ref>{{cite journal |last1=Manjaiah |first1=K.M. |last2=Kumar |first2=Sarvendra |last3=Sachdev |first3=M. S. |last4=Sachdev |first4=P. |last5=Datta |first5=S. C. |year=2010 |title=Study of clay–organic complexes |journal=[[Current Science]] |volume=98 |issue=7 |pages=915–921 |url=https://www.researchgate.net/publication/228867334 |access-date=18 July 2021}}</ref> Proteins normally decompose readily, to the exception of [[scleroproteins]], but when bound to clay particles they become more resistant to decomposition.<ref>{{cite journal |last=Theng |first=Benny K.G. |year=1982 |title=Clay-polymer interactions: summary and perspectives |journal=Clays and Clay Minerals |volume=30 |issue=1 |pages=1–10 |doi=10.1346/CCMN.1982.0300101 |bibcode=1982CCM....30....1T |citeseerx=10.1.1.608.2942 |s2cid=98176725 }}</ref> As for other proteins clay particles absorb the enzymes exuded by microbes, decreasing [[enzyme activity]] while protecting [[extracellular enzymes]] from degradation.<ref>{{cite journal |last1=Tietjen |first1=Todd |last2=Wetzel |first2=Robert G. |year=2003 |title=Extracellular enzyme-clay mineral complexes: enzyme adsorption, alteration of enzyme activity, and protection from photodegradation |journal=Aquatic Ecology |volume=37 |issue=4 |pages=331–339 |doi=10.1023/B:AECO.0000007044.52801.6b |bibcode=2003AqEco..37..331T |s2cid=6930871 |url=http://www.vliz.be/imisdocs/publications/54440.pdf |access-date=18 July 2021}}</ref> The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years.<ref>{{cite journal |last1=Tahir |first1=Shermeen |last2=Marschner |first2=Petra |year=2017 |title=Clay addition to sandy soil: influence of clay type and size on nutrient availability in sandy soils amended with residues differing in C/N ratio |journal=[[Pedosphere]] |volume=27 |issue=2 |pages=293–305 |url=https://www.researchgate.net/publication/314221508 |doi=10.1016/S1002-0160(17)60317-5 |bibcode=2017Pedos..27..293T |access-date=18 July 2021}}</ref> A study showed increased soil fertility following the addition of mature compost to a clay soil.<ref>{{cite journal |last1=Melero |first1=Sebastiana |last2=Madejón |first2=Engracia |last3=Ruiz |first3=Juan Carlos |last4=Herencia |first4=Juan Francisco |year=2007 |title=Chemical and biochemical properties of a clay soil under dryland agriculture system as affected by organic fertilization |journal=European Journal of Agronomy |volume=26 |issue=3 |pages=327–334 |url=https://coek.info/pdf-chemical-and-biochemical-properties-of-a-clay-soil-under-dryland-agriculture-sys.html |doi=10.1016/j.eja.2006.11.004 |bibcode=2007EuJAg..26..327M |access-date=18 July 2021}}</ref> High soil [[tannin]] content can cause nitrogen to be sequestered as resistant tannin-protein complexes.<ref>{{cite journal |last1=Joanisse |first1=Gilles D. |last2=Bradley |first2=Robert L. |last3=Preston |first3=Caroline M. |last4=Bending |first4=Gary D. |title=Sequestration of soil nitrogen as tannin–protein complexes may improve the competitive ability of sheep laurel (Kalmia angustifolia) relative to black spruce (Picea mariana) |journal=[[New Phytologist]] |year=2009 |volume=181 |pages=187–198 |doi=10.1111/j.1469-8137.2008.02622.x |issue=1 |pmid=18811620 |doi-access=free}}</ref><ref name=Fierer2001>{{cite journal |last1=Fierer |first1=Noah |last2=Schimel |first2=Joshua P. |last3=Cates |first3=Rex G. |last4=Zou |first4=Jiping |title=Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils |journal=[[Soil Biology and Biochemistry]] |year=2001 |volume=33 |pages=1827–1839 |doi=10.1016/S0038-0717(01)00111-0 |issue=12–13 |bibcode=2001SBiBi..33.1827F |url=https://www.academia.edu/12814037 |access-date=18 July 2021}}</ref>


Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present. Soils with humus can vary in nitrogen content but typically have 3 to 6&nbsp;percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting [[Fertile soil|soil fertility]].<ref name="Foth1984"/> Humus also absorbs water, and expands and shrinks between dry and wet states, increasing soil [[porosity]]. Humus is less stable than the soil's mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminshes without the addition of new organic matter. However, humus may persist over centuries if not millennia.
Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present.<ref name="Ponge2003"/> Soils with humus can vary in nitrogen content but typically have 3 to 6&nbsp;percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting [[Fertile soil|soil fertility]].<ref name="Foth1984"/> Humus also absorbs water, and expands and shrinks between dry and wet states to a higher extent than clay, increasing soil porosity.<ref>{{cite journal |last1=Peng |first1=Xinhua |last2=Horn |first2=Rainer |title=Anisotropic shrinkage and swelling of some organic and inorganic soils |journal=European Journal of Soil Science |year=2007 |volume=58 |issue=1 |pages=98–107 |doi=10.1111/j.1365-2389.2006.00808.x |bibcode=2007EuJSS..58...98P |doi-access=free}}</ref> Humus is less stable than the soil's mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminishes without the addition of new organic matter. However, humus in its most stable forms may persist over centuries if not millennia.<ref>{{cite journal |last1=Wang |first1=Yang |last2=Amundson |first2=Ronald |last3=Trumbmore |first3=Susan |title=Radiocarbon dating of soil organic matter |journal=[[Quaternary Research]] |year=1996 |volume=45 |issue=3 |pages=282–288 |doi=10.1006/qres.1996.0029 |bibcode=1996QuRes..45..282W |s2cid=73640995 |url=https://escholarship.org/content/qt6b14h4bv/qt6b14h4bv.pdf |access-date=18 July 2021}}</ref> [[Charcoal]] is a source of highly stable humus, called [[black carbon]],<ref>{{cite journal |last1=Brodowski |first1=Sonja |last2=Amelung |first2=Wulf |last3=Haumaier |first3=Ludwig |last4=Zech |first4=Wolfgang |title=Black carbon contribution to stable humus in German arable soils |journal=Geoderma |year=2007 |volume=139 |issue=1–2 |pages=220–228 |doi=10.1016/j.geoderma.2007.02.004 |bibcode=2007Geode.139..220B |url=https://www.academia.edu/33858429 |access-date=18 July 2021}}</ref> which had been used traditionally to improve the fertility of nutrient-poor tropical soils. This very ancient practice, as ascertained in the genesis of [[Amazonian dark earths]], has been renewed and became popular under the name of [[biochar]]. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.<ref>{{cite journal |last1=Criscuoli |first1=Irene |last2=Alberti |first2=Giorgio |last3=Baronti |first3=Silvia |last4=Favilli |first4=Filippo |last5=Martinez |first5=Cristina |last6=Calzolari |first6=Costanza |last7=Pusceddu |first7=Emanuela |last8=Rumpel |first8=Cornelia |last9=Viola |first9=Roberto |last10=Miglietta |first10=Franco |title=Carbon sequestration and fertility after centennial time scale incorporation of charcoal into soil |journal=[[PLOS ONE]] |year=2014 |volume=9 |issue=3 |pages=e91114 |doi=10.1371/journal.pone.0091114 |pmc=3948733 |pmid=24614647|bibcode=2014PLoSO...991114C |doi-access=free}}</ref>


===Climatological influence===
===Climatological influence===
The production, accumulation and degradation of organic matter are greatly dependent on climate. Temperature, soil moisture and [[topography]] are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where [[decomposer]] activity is impeded by low temperature<ref name="Wagai2008">{{cite journal |last=Wagai |first=Rota |last2=Mayer |first2=Lawrence M. |last3=Kitayama |first3=Kanehiro |last4=Knicker |first4=Heike |date=2008 |title=Climate and parent material controls on organic matter storage in surface soils: A three-pool, density-separation approach |journal=Geoderma |volume=147 |issue=1–2 |pages=23–33 |doi=10.1016/j.geoderma.2008.07.010|bibcode=2008Geode.147...23W |url=https://digital.csic.es/bitstream/10261/82461/1/Acceso%20restringido%20Digital%20CSIC.pdf |format=Submitted manuscript |hdl=10261/82461 }}</ref> or excess moisture which results in anaerobic conditions.<ref name="Minayeva2008">{{cite journal |last=Minayeva |first=T. Yu. |last2=Trofimov |first2=S. Ya. |last3=Chichagova |first3=O.A. |last4=Dorofeyeva |first4=E.I. |last5=Sirin |first5=A.A. |last6=Glushkov |first6=I.V. |last7=Mikhailov |first7=N.D. |last8=Kromer |first8=B. |date=2008 |title=Carbon accumulation in soils of forest and bog ecosystems of southern Valdai in the Holocene |journal=Biology Bulletin |volume=35 |pages=524–32 |doi=10.1134/S1062359008050142 |issue=5}}</ref> Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients; forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter to maintain their productivity.<ref name=Sanchez1976>{{cite book|last1=Sanchez|first1=Pedro A.|title=Properties and management of soils in the tropics|date=1976|publisher=Wiley|location=New York|isbn=978-0-471-75200-4}}</ref> Excessive slope may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.
The production, accumulation and degradation of organic matter are greatly dependent on climate. For example, when a [[Thaw (weather)|thawing]] event occurs, the flux of [[soil gas]]es with atmospheric gases is significantly influenced.<ref>{{cite journal |last1=Kim |first1=Dong Jim |last2=Vargas |first2=Rodrigo |last3=Bond-Lamberty |first3=Ben |last4=Turetsky |first4=Merritt R. |title=Effects of soil rewetting and thawing on soil gas fluxes: a review of current literature and suggestions for future research |journal=[[Biogeosciences]] |year=2012 |volume=9 |issue=7 |pages=2459–2483 |doi=10.5194/bg-9-2459-2012 |bibcode=2012BGeo....9.2459K |url=https://www.researchgate.net/publication/307827983 |access-date=3 October 2021|doi-access=free }}</ref> Temperature, soil moisture and [[topography]] are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where [[decomposer]] activity is impeded by low temperature<ref name="Wagai2008">{{cite journal |last1=Wagai |first1=Rota |last2=Mayer |first2=Lawrence M. |last3=Kitayama |first3=Kanehiro |last4=Knicker |first4=Heike |year=2008 |title=Climate and parent material controls on organic matter storage in surface soils: a three-pool, density-separation approach |journal=Geoderma |volume=147 |issue=1–2 |pages=23–33 |doi=10.1016/j.geoderma.2008.07.010 |bibcode=2008Geode.147...23W |url=https://www.academia.edu/20165844 |access-date=25 July 2021 |hdl=10261/82461 |hdl-access=free}}</ref> or excess moisture which results in anaerobic conditions.<ref name="Minayeva2008">{{cite journal |last1=Minayeva |first1=Tatiana Y. |last2=Trofimov |first2=Sergey Ya. |last3=Chichagova |first3=Olga A. |last4=Dorofeyeva |first4=E. I. |last5=Sirin |first5=Andrey A. |last6=Glushkov |first6=Igor V. |last7=Mikhailov |first7=N. D. |last8=Kromer |first8=Bernd |year=2008 |title=Carbon accumulation in soils of forest and bog ecosystems of southern Valdai in the Holocene |journal=Biology Bulletin |volume=35 |issue=5 |pages=524–532 |doi=10.1134/S1062359008050142 |bibcode=2008BioBu..35..524M |s2cid=40927739 |url=https://www.researchgate.net/publication/225229436 |access-date=25 July 2021}}</ref> Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients. Forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter by the living plant and microbial biomass to maintain their productivity, a process which is disturbed by human activities.<ref>{{cite journal |last1=Vitousek |first1=Peter M. |last2=Sanford |first2=Robert L. |title=Nutrient cycling in moist tropical forest |journal=[[Annual Review of Ecology and Systematics]] |year=1986 |volume=17 |pages=137–167 |doi=10.1146/annurev.es.17.110186.001033 |s2cid=55212899 |url=https://www.researchgate.net/publication/234150505 |access-date=25 July 2021}}</ref> Excessive slope, in particular in the presence of cultivation for the sake of agriculture, may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.<ref>{{cite journal |last1=Rumpel |first1=Cornelia |last2=Chaplot |first2=Vincent |last3=Planchon |first3=Olivier |last4=Bernadou |first4=J. |last5=Valentin |first5=Christian |last6=Mariotti |first6=André |title=Preferential erosion of black carbon on steep slopes with slash and burn agriculture |journal=Catena |year=2006 |volume=65 |issue=1 |pages=30–40 |url=https://www.academia.edu/14788543 |doi=10.1016/j.catena.2005.09.005 |bibcode=2006Caten..65...30R |access-date=25 July 2021}}</ref>


===Plant residue===
=== Plant residue ===
{{Pie chart
{{Pie chart
|caption = Typical types and percentages of plant residue components
|caption = Typical types and percentages of plant residue components
Line 902: Line 310:
}}
}}


[[Cellulose]] and [[hemicellulose]] undergo fast decomposition by fungi and bacteria, with a half-life of 12–18 days in a temperate climate.<ref name=Paul1997>{{cite book|last1=Paul|first1=E.A.|title=Soil organic matter in temperate agroecosystems : long-term experiments in North America|date=1997|publisher=CRC Press|location=Boca Raton|isbn=978-0-8493-2802-2|page=80}}</ref> [[Wood-decay fungus|Brown rot fungi]] can decompose the cellulose and hemicellulose, leaving the [[lignin]] and [[Phenols|phenolic compounds]] behind. [[Starch]], which is an energy storage system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists of polymers composed of 500 to 600 units with a highly branched, amorphous structure. Lignin undergoes very slow decomposition, mainly by [[white rot]] fungi and [[actinomycetes]]; its half-life under temperate conditions is about six months.<ref name=Paul1997 />
[[Cellulose]] and [[hemicellulose]] undergo fast decomposition by fungi and bacteria, with a half-life of 12–18&nbsp;days in a temperate climate.<ref name=Paul1997>{{cite book|last1=Paul |first1=Eldor A. |last2=Paustian |first2=Keith H. |last3=Elliott |first3=E. T. |last4=Cole |first4=C. Vernon |title=Soil organic matter in temperate agroecosystems: long-term experiments in North America |date=1997 |publisher=[[CRC Press]] |location=Boca Raton, Florida |isbn=978-0-8493-2802-2 |page=80}}</ref> [[Wood-decay fungus|Brown rot fungi]] can decompose the cellulose and hemicellulose, leaving the lignin and [[Phenols|phenolic compounds]] behind. [[Starch]], which is an [[energy storage]] system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists of [[polymers]] composed of 500 to 600 units with a highly branched, amorphous structure, linked to cellulose, hemicellulose and [[pectin]] in [[plant cell walls]]. Lignin undergoes very slow decomposition, mainly by [[white rot]] fungi and [[actinomycetes]]; its half-life under temperate conditions is about six months.<ref name=Paul1997 />


==Horizons==
==Horizons==
{{Main|Soil horizon}}
{{Main|Soil horizon}}


A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a soil horizon. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions.<ref>{{cite web |url=https://soilsofcanada.ca/soil-formation/horizons.php |title=Horizons |website=Soils of Canada |access-date=1 August 2021 |archive-url=https://web.archive.org/web/20190922153041/https://soilsofcanada.ca/soil-formation/horizons.php |archive-date=22 September 2019 |url-status=live}}</ref> No soil profile has all the major horizons. Some, called [[entisols]], may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimed [[mining waste]] deposits,<ref>{{cite journal |last1=Frouz |first1=Jan |last2=Prach |first2=Karel |last3=Pizl |first3=Václav |last4=Háněl |first4=Ladislav |last5=Starý |first5=Josef |last6=Tajovský |first6=Karel |last7=Materna |first7=Jan |last8=Balík |first8=Vladimír |last9=Kalčík |first9=Jiří |last10=Řehounková |first10=Klára |year=2008 |title=Interactions between soil development, vegetation and soil fauna during spontaneous succession in post mining sites |journal=European Journal of Soil Biology |volume=44 |issue=1 |pages=109–121 |url=https://www.researchgate.net/publication/223699609 |doi=10.1016/j.ejsobi.2007.09.002 |bibcode=2008EJSB...44..109F |access-date=1 August 2021}}</ref> [[moraines]],<ref>{{cite journal |last1=Kabala |first1=Cezary |last2=Zapart |first2=Justyna |year=2012 |title=Initial soil development and carbon accumulation on moraines of the rapidly retreating Werenskiold Glacier, SW Spitsbergen, Svalbard archipelago |journal=Geoderma |volume=175–176 |pages=9–20 |url=https://www.academia.edu/31221217 |doi=10.1016/j.geoderma.2012.01.025 |bibcode=2012Geode.175....9K |access-date=1 August 2021}}</ref> [[volcanic cones]]<ref>{{cite journal |last1=Ugolini |first1=Fiorenzo C. |last2=Dahlgren |first2=Randy A. |year=2002 |title=Soil development in volcanic ash |journal=Global Environmental Research |volume=6 |issue=2 |pages=69–81 |url=http://www.airies.or.jp/attach.php/6a6f75726e616c5f30362d32656e67/save/0/0/06_2-09.pdf |access-date=1 August 2021}}</ref> [[sand dunes]] or [[alluvial terrace]]s.<ref>{{cite journal |last=Huggett |first=Richard J. |year=1998 |title=Soil chronosequences, soil development, and soil evolution: a critical review |journal=Catena |volume=32 |issue=3 |pages=155–172 |url=https://www.academia.edu/2116704 |doi=10.1016/S0341-8162(98)00053-8 |bibcode=1998Caten..32..155H |access-date=1 August 2021}}</ref> Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage.<ref>{{cite journal |last1=De Alba |first1=Saturnio |last2=Lindstrom |first2=Michael |last3=Schumacher |first3=Thomas E. |last4=Malo |first4=Douglas D. |year=2004 |title=Soil landscape evolution due to soil redistribution by tillage: a new conceptual model of soil catena evolution in agricultural landscapes |journal=Catena |volume=58 |issue=1 |pages=77–100 |url=https://www.academia.edu/22300477 |doi=10.1016/j.catena.2003.12.004 |bibcode=2004Caten..58...77D |access-date=1 August 2021}}</ref> The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died.<ref>{{cite journal |last1=Phillips |first1=Jonathan D. |last2=Marion |first2=Daniel A. |year=2004 |title=Pedological memory in forest soil development |journal=[[Forest Ecology and Management]] |volume=188 |issue=1 |pages=363–380 |url=https://www.srs.fs.usda.gov/pubs/ja/ja_phillips004.pdf |doi=10.1016/j.foreco.2003.08.007 |bibcode=2004ForEM.188..363P |access-date=1 August 2021}}</ref> By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like in [[sediment]] layers. Sampling [[pollen]], [[testate amoebae]] and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change, [[land use]] change) which occurred in the course of soil formation.<ref>{{cite journal |last1=Mitchell |first1=Edward A.D. |last2=Van der Knaap |first2=Willem O. |last3=Van Leeuwen |first3=Jacqueline F.N. |last4=Buttler |first4=Alexandre |last5=Warner |first5=Barry G. |last6=Gobat |first6=Jean-Michel |year=2001 |title=The palaeoecological history of the Praz-Rodet bog (Swiss Jura) based on pollen, plant macrofossils and testate amoebae(Protozoa) |journal=[[The Holocene]] |volume=11 |issue=1 |pages=65–80 |url=https://www.academia.edu/31915005 |doi=10.1191/095968301671777798 |bibcode=2001Holoc..11...65M |s2cid=131032169 |access-date=1 August 2021}}</ref> Soil horizons can be dated by several methods such as [[radiocarbon]], using pieces of charcoal provided they are of enough size to escape [[pedoturbation]] by earthworm activity and other mechanical disturbances.<ref>{{cite journal |last=Carcaillet |first=Christopher |year=2001 |title=Soil particles reworking evidences by AMS <sup>14</sup>C dating of charcoal |journal=[[Comptes Rendus de l'Académie des Sciences, Série IIA]] |volume=332 |issue=1 |pages=21–28 |url=https://www.researchgate.net/publication/238379602 |doi=10.1016/S1251-8050(00)01485-3 |bibcode=2001CRASE.332...21C |access-date=1 August 2021}}</ref> Fossil soil horizons from [[paleosols]] can be found within [[sedimentary rock]] sequences, allowing the study of past environments.<ref>{{cite journal |last=Retallack |first=Gregory J. |year=1991 |title=Untangling the effects of burial alteration and ancient soil formation |journal=[[Annual Review of Earth and Planetary Sciences]] |volume=19 |issue=1 |pages=183–206 |doi=10.1146/annurev.ea.19.050191.001151 |bibcode=1991AREPS..19..183R |url=https://www.researchgate.net/publication/234148901 |access-date=1 August 2021}}</ref>
A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a [[soil horizon]]. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers<ref name=Retallack1990>{{Cite book| last = Retallack | first = G.J.| year = 1990| title = Soils of the past : an introduction to paleopedology| page = 32| url = https://books.google.com/?id=YVkVAAAAIAAJ&pg=PA32&dq=Soil+horizons| isbn = 978-0-04-445757-2| publisher = Unwin Hyman| location = Boston}}</ref> which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions.<ref name=Buol1990>{{Cite book| last = Buol | first = S.W.| year = 1990| title = Soil genesis and classification| page = 36| doi = 10.1081/E-ESS| url = https://books.google.com/?id=QM0kfIGYMjcC&printsec=frontcover&dq=Soil| isbn = 978-0-8138-2873-2| location = Ames, Iowa| publisher = Iowa State University Press}}</ref> No soil profile has all the major horizons. Some may have only one horizon.


The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils.<ref>{{cite journal |last1=Bakker |first1=Martha M. |last2=Govers |first2=Gerard |last3=Jones |first3=Robert A. |last4=Rounsevell |first4=Mark D.A. |year=2007 |title=The effect of soil erosion on Europe's crop yields |journal=Ecosystems |volume=10 |issue=7 |pages=1209–1219 |doi=10.1007/s10021-007-9090-3 |bibcode=2007Ecosy..10.1209B |doi-access=free}}</ref> The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts ([[leaf litter]]) or are directly produced belowground for subterranean plant organs (root litter), and then release [[dissolved organic matter]].<ref>{{cite journal |last1=Uselman |first1=Shauna M. |last2=Qualls |first2=Robert G. |last3=Lilienfein |first3=Juliane |year=2007 |title=Contribution of root vs. leaf litter to dissolved organic carbon leaching through soil |journal=[[Soil Science Society of America Journal]] |volume=71 |issue=5 |pages=1555–1563 |url=https://www.academia.edu/34475958 |doi=10.2136/sssaj2006.0386 |bibcode=2007SSASJ..71.1555U |access-date=8 August 2021}}</ref> The remaining surficial organic layer, called the [[forest floor|O horizon]], produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live.<ref>{{cite journal |last1=Schulz |first1=Stefanie |last2=Brankatschk |first2=Robert |last3=Dümig |first3=Alexander |last4=Kögel-Knabner |first4=Ingrid |last5=Schloter |first5=Michae |last6=Zeyer |first6=Josef |year=2013 |title=The role of microorganisms at different stages of ecosystem development for soil formation |journal=[[Biogeosciences]] |volume=10 |issue=6 |pages=3983–3996 |doi=10.5194/bg-10-3983-2013 |bibcode=2013BGeo...10.3983S |doi-access=free}}</ref> After sufficient time, humus moves downward and is deposited in a distinctive organic-mineral surface layer called the A horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called pedoturbation. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil<ref>{{cite journal |last1=Gillet |first1=Servane |last2=Ponge |first2=Jean-François |year=2002 |title=Humus forms and metal pollution in soil |journal=European Journal of Soil Science |volume=53 |issue=4 |pages=529–539 |url=https://www.academia.edu/45705588 |doi=10.1046/j.1365-2389.2002.00479.x |bibcode=2002EuJSS..53..529G |s2cid=94900982 |access-date=8 August 2021}}</ref> and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.<ref>{{cite journal |last1=Bardy |first1=Marion |last2=Fritsch |first2=Emmanuel |last3=Derenne |first3=Sylvie |last4=Allard |first4=Thierry |last5=do Nascimento |first5=Nadia Régina |last6=Bueno |first6=Guilherme |year=2008 |title=Micromorphology and spectroscopic characteristics of organic matter in waterlogged podzols of the upper Amazon basin |journal=Geoderma |volume=145 |issue=3 |pages=222–230 |doi=10.1016/j.geoderma.2008.03.008 |bibcode=2008Geode.145..222B |citeseerx=10.1.1.455.4179 }}</ref>
The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth. That growth often results in the accumulation of organic residues. The accumulated organic layer called the [[forest floor|O horizon]] produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live. After sufficient time, humus moves downward and is deposited in a distinctive organic surface layer called the A horizon.


==Classification==
==Classification==
{{main|Soil classification}}
{{main|Soil classification}}


Soil is classified into categories in order to understand relationships between different soils and to determine the suitability of a soil for a particular use. One of the first classification systems was developed by Russian scientist [[Dokuchaev]] around 1880. It was modified a number of times by American and European researchers, and developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on [[soil morphology]] instead of parental materials and soil-forming factors. Since then it has undergone further modifications. The [[World Reference Base for Soil Resources]] (WRB)<ref name=IUSS2014>{{cite book|last1=IUSS Working Group WRB|title=World Reference Base for Soil Resources 2014. International soil classification system for naming soils and creating legends for soil maps|date=2014|publisher=FAO|location=Rome|isbn=978-92-5-108370-3|edition=3rd|url=http://www.fao.org/3rd/a-i3794e.pdf|accessdate=29 August 2014}}{{dead link|date=December 2017 |bot=KAP03 |fix-attempted=yes }}</ref> aims to establish an international reference base for soil classification.
One of the first soil classification systems was developed by Russian scientist [[Vasily Dokuchaev]] around 1880.<ref>{{cite web |url=https://fr.scribd.com/doc/206859253/Russian-Chernozem |title=Russian Chernozem |last=Dokuchaev |first=Vasily Vasilyevich |publisher=Israel Program for Scientific Translations |location=Jerusalem, Israel |year=1967 |access-date=15 August 2021}}</ref> It was modified a number of times by American and European researchers and was developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on [[soil morphology]] instead of parental materials and soil-forming factors. Since then, it has undergone further modifications. The [[World Reference Base for Soil Resources]]<ref name=WRB>{{Cite web|url = https://www3.ls.tum.de/boku/?id=1419|title = World Reference Base for Soil Resources, 4th edition|author=IUSS Working Group WRB|year = 2022|publisher = IUSS, Vienna}}</ref> aims to establish an international reference base for soil classification.


===Systems===
==Uses==
Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants. The types of soil and available moisture determine the species of plants that can be cultivated. [[Agricultural soil science]] was the primeval domain of soil knowledge, long time before the advent of [[pedology]] in the 19th century. However, as demonstrated by [[aeroponics]], [[aquaponics]] and [[hydroponics]], soil material is not an absolute essential for agriculture, and soilless cropping systems have been claimed as the future of agriculture for an endless growing mankind.<ref>{{cite journal |last1=Sambo |first1=Paolo |last2=Nicoletto |first2=Carlo |last3=Giro |first3=Andrea |last4=Pii |first4=Youry |last5=Valentinuzzi |first5=Fabio |last6=Mimmo |first6=Tanja |last7=Lugli |first7=Paolo |last8=Orzes |first8=Guido |last9=Mazzetto |first9=Fabrizio |last10=Astolfi |first10=Stefania |last11=Terzano |first11=Roberto |last12=Cesco |first12=Stefano |year=2019 |title=Hydroponic solutions for soilless production systems: issues and opportunities in a smart agriculture perspective |journal=[[Frontiers in Plant Science]] |volume=10 |issue=123 |page=923 |doi=10.3389/fpls.2019.00923 |pmid=31396245 |pmc=6668597 |doi-access=free}}</ref>
{{main|Soil classification#Systems}}


Soil material is also a critical component in mining, construction and landscape development industries.<ref>{{cite book |title=Soils for landscape development: selection, specification and validation |last1=Leake |first1=Simon |last2=Haege |first2=Elke |publisher=[[CSIRO Publishing]] |location=Clayton, Victoria, Australia |year=2014 |isbn=978-0643109650}}</ref> Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in [[surface mining]], [[road building]] and [[dam]] construction. [[Earth sheltering]] is the architectural practice of using soil for external [[thermal mass]] against building walls. Many [[building material]]s are soil based. Loss of soil through urbanization is growing at a high rate in many areas and can be critical for the maintenance of [[subsistence agriculture]].<ref>{{cite journal |last1=Pan |first1=Xian-Zhang |last2=Zhao |first2=Qi-Guo |year=2007 |title=Measurement of urbanization process and the paddy soil loss in Yixing city, China between 1949 and 2000 |journal=Catena |volume=69 |issue=1 |pages=65–73 |doi=10.1016/j.catena.2006.04.016 |bibcode=2007Caten..69...65P |url=http://www.cern.ac.cn/ftp/0301%20Measurement%20of%20urbanization%20process%20and%20the%20paddy%20soil%20loss%20in%20Yixing%20city,%20China%20between%201949%20and%202000).pdf |access-date=15 August 2021}}</ref>
====Australia====
{{Main|Australian Soil Classification}}


Soil resources are critical to the environment, as well as to food and fibre production, producing 98.8% of food consumed by humans.<ref>{{cite journal |last1=Kopittke |first1=Peter M. |last2=Menzies |first2=Neal W. |last3=Wang |first3=Peng |last4=McKenna |first4=Brigid A. |last5=Lombi |first5=Enzo |year=2019 |title=Soil and the intensification of agriculture for global food security |journal=[[Environment International]] |volume=132 |pages=105078 |doi=10.1016/j.envint.2019.105078 |pmid=31400601 |issn=0160-4120 |doi-access=free|bibcode=2019EnInt.13205078K }}</ref> Soil provides minerals and water to plants according to several processes involved in plant nutrition. Soil absorbs rainwater and releases it later, thus preventing floods and drought, flood regulation being one of the major ecosystem services provided by soil.<ref>{{cite journal |last1=Stürck |first1=Julia |last2=Poortinga |first2=Ate |last3=Verburg |first3=Peter H. |year=2014 |title=Mapping ecosystem services: the supply and demand of flood regulation services in Europe |journal=Ecological Indicators |volume=38 |pages=198–211 |url=http://docs.gip-ecofor.org/public/Sturck_et_al_2014.pdf |doi=10.1016/j.ecolind.2013.11.010 |bibcode=2014EcInd..38..198S |access-date=15 August 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814121818/http://docs.gip-ecofor.org/public/Sturck_et_al_2014.pdf |url-status=dead }}</ref> Soil cleans water as it percolates through it.<ref>{{cite journal |last1=Van Cuyk |first1=Sheila |last2=Siegrist |first2=Robert |last3=Logan |first3=Andrew |last4=Masson |first4=Sarah |last5=Fischer |first5=Elizabeth |last6=Figueroa |first6=Linda |year=2001 |title=Hydraulic and purification behaviors and their interactions during wastewater treatment in soil infiltration systems |journal=[[Water Research]] |volume=35 |issue=4 |pages=953–964 |url=https://www.academia.edu/17525373 |doi=10.1016/S0043-1354(00)00349-3 |pmid=11235891 |bibcode=2001WatRe..35..953V |access-date=15 August 2021}}</ref> Soil is the habitat for many organisms: the major part of known and unknown [[biodiversity]] is in the soil, in the form of earthworms, [[woodlice]], [[millipede]]s, [[centipede]]s, [[snail]]s, [[slug]]s, [[mite]]s, [[springtail]]s, [[Enchytraeidae|enchytraeids]], [[nematode]]s, [[protist]]s), bacteria, [[archaea]], fungi and [[algae]]; and most organisms living above ground have part of them ([[plants]]) or spend part of their [[Biological life cycle|life cycle]] ([[insects]]) below-ground.<ref>{{cite book |title=European atlas of soil biodiversity |last1=Jeffery |first1=Simon |last2=Gardi |first2=Ciro |last3=Arwyn |first3=Jones |publisher=Publications Office of the European Union |location=Luxembourg, Luxembourg |year=2010 |isbn=978-92-79-15806-3 |doi=10.2788/94222 |url=https://op.europa.eu/en/publication-detail/-/publication/7161b2a1-f862-4c90-9100-557a62ecb908 |access-date=15 August 2021}}</ref> Above-ground and below-ground biodiversities are tightly interconnected,<ref name="Ponge2003" /><ref name="De Deyn2005">{{cite journal |last1=De Deyn |first1=Gerlinde B. |last2=Van der Putten |first2=Wim H. |year=2005 |title=Linking aboveground and belowground diversity |journal=[[Trends in Ecology and Evolution]] |volume=20 |issue=11 |pages=625–633 |url=https://www.researchgate.net/publication/7080980 |doi=10.1016/j.tree.2005.08.009 |pmid=16701446 |access-date=15 August 2021}}</ref> making [[soil protection]] of paramount importance for any [[Environmental restoration|restoration]] or [[Nature conservation|conservation]] plan.
There are fourteen soil orders at the top level of the Australian Soil Classification. They are: Anthroposols, Organosols, Podosols, Vertosols, Hydrosols, Kurosols, Sodosols, Chromosols, Calcarosols, Ferrosols, Dermosols, Kandosols, Rudosols and Tenosols.


The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even in deserts, cyanobacteria, [[lichen]]s and [[moss]]es form [[biological soil crust]]s which capture and sequester a significant amount of carbon by [[photosynthesis]]. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset the effect of increases in [[greenhouse gas emissions]] and slow global warming, while improving crop yields and reducing water needs.<ref>{{cite journal |last1= Hansen |first1=James |last2=Sato |first2=Makiko |last3=Kharecha |first3=Pushker |last4=Beerling |first4=David |last5=Berner |first5=Robert |last6=Masson-Delmotte |first6=Valerie |last7=Pagani |first7=Mark |last8=Raymo |first8=Maureen |last9=Royer |first9=Dana L. |last10=Zachos |first10=James C. |journal=[[Open Atmospheric Science Journal]] |year=2008 |volume=2 |pages=217–231 |title=Target atmospheric CO<sub>2</sub>: where should humanity aim? |issue=1 |arxiv=0804.1126 |bibcode=2008OASJ....2..217H |doi= 10.2174/1874282300802010217 |doi-access=free|s2cid=14890013 |url=https://benthamopen.com/contents/pdf/TOASCJ/TOASCJ-2-217.pdf |access-date=22 August 2021}}</ref><ref>{{cite journal |last=Lal |first=Rattan |date=11 June 2004 |title=Soil carbon sequestration impacts on global climate change and food security |journal=[[Science (journal)|Science]] |volume=304 |issue=5677 |pages=1623–1627 |doi=10.1126/science.1097396 |pmid=15192216 |bibcode=2004Sci...304.1623L |s2cid=8574723 |url=http://www.tinread.usarb.md:8888/jspui/bitstream/123456789/1067/1/soil_carbon.pdf |access-date=22 August 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814145106/http://www.tinread.usarb.md:8888/jspui/bitstream/123456789/1067/1/soil_carbon.pdf |url-status=dead }}</ref><ref>{{cite web |last=Blakeslee |first=Thomas |title=Greening deserts for carbon credits |date=24 February 2010 |access-date=22 August 2021 |publisher=[[Renewable Energy World]] |location=Orlando, Florida, USA |url=https://www.renewableenergyworld.com/om/greening-deserts-for-carbon-credits/#gref |url-status=live |archive-url=https://web.archive.org/web/20121101011735/http://www.renewableenergyworld.com/rea/news/article/2010/02/greening-deserts-for-carbon-credits |archive-date=1 November 2012}}</ref>
====European Union====
The EU's soil taxonomy is based on a new standard soil classification in the World Reference Base for Soil Resources produced by the [[United Nations|UN]]'s [[Food and Agriculture Organization]].<ref name=SEU>{{cite web |url=http://eusoils.jrc.ec.europa.eu/esdb_archive/eusoils_docs/other/EUR23439.pdf |title=Archived copy |accessdate=2013-10-08 |deadurl=yes |archiveurl=https://web.archive.org/web/20140630010420/http://eusoils.jrc.ec.europa.eu/esdb_archive/eusoils_docs/other/EUR23439.pdf |archivedate=30 June 2014 |df=dmy-all }} ''Soils of the European Union'' by the EU Institute for Environment and Sustainability. Accessed on 8 October 2013</ref>


[[Waste management]] often has a soil component. [[Septic drain field]]s treat [[septic tank]] effluent using [[Aerobic organism|aerobic]] soil processes. Land application of [[waste water]] relies on [[soil biology]] to aerobically treat [[Biochemical oxygen demand|BOD]]. Alternatively, [[landfill]]s use soil for [[daily cover]], isolating waste deposits from the atmosphere and preventing unpleasant smells. [[Composting]] is now widely used to treat aerobically solid domestic waste and dried effluents of [[settling basin]]s. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter.<ref>{{cite journal |last1=Mondini |first1=Claudio |last2=Contin |first2=Marco |last3=Leita |first3=Liviana |last4=De Nobili |first4=Maria |year=2002 |title=Response of microbial biomass to air-drying and rewetting in soils and compost |journal=Geoderma |volume=105 |issue=1–2 |pages=111–124 |url=https://www.academia.edu/5321925 |doi=10.1016/S0016-7061(01)00095-7 |bibcode=2002Geode.105..111M |access-date=22 August 2021}}</ref>
====United States====
{{Main|USDA soil taxonomy#Orders}}


Organic soils, especially peat, serve as a significant fuel and [[horticulture|horticultural]] resource. Peat soils are also commonly used for the sake of agriculture in Nordic countries, because peatland sites, when drained, provide fertile soils for food production.<ref>{{cite web |title=Peatlands and farming |date=6 July 2020 |access-date=22 August 2021 |publisher=[[National Farmers' Union of England and Wales]] |location=Stoneleigh, United Kingdom |url=https://www.countrysideonline.co.uk/food-and-farming/protecting-the-environment/peatlands-and-farming }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> However, wide areas of peat production, such as rain-fed [[sphagnum]] [[bog]]s, also called [[blanket bog]]s or [[raised bog]]s, are now protected because of their patrimonial interest. As an example, [[Flow Country]], covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now candidate for being included in the [[World Heritage List]]. Under present-day global warming peat soils are thought to be involved in a self-reinforcing (positive feedback) process of increased emission of greenhouse gases (methane and carbon dioxide) and increased temperature,<ref>{{cite journal |last1=van Winden |first1=Julia F. |last2=Reichart |first2=Gert-Jan |last3=McNamara |first3=Niall P. |last4=Benthien |first4=Albert |last5=Sinninghe Damste |first5=Jaap S. |journal=[[PLoS ONE]] |year=2012 |volume=7 |issue=6 |pages=e39614 |title=Temperature-induced increase in methane release from peat bogs: a mesocosm experiment |doi=10.1371/journal.pone.0039614 |pmid=22768100 |pmc=3387254 |bibcode=2012PLoSO...739614V |doi-access=free}}</ref> a contention which is still under debate when replaced at field scale and including stimulated plant growth.<ref>{{cite journal |last1=Davidson |first1=Eric A. |last2=Janssens |first2=Ivan A. |year=2006 |title=Temperature sensitivity of soil carbon decomposition and feedbacks to climate change |journal=[[Nature (journal)|Nature]] |volume=440 |issue=7081 |pages=165–173 |doi=10.1038/nature04514 |pmid=16525463 |bibcode=2006Natur.440..165D |s2cid=4404915 |doi-access=free }}</ref>
A taxonomy is an arrangement in a systematic manner; the [[USDA soil taxonomy]] has six levels of classification. They are, from most general to specific: order, suborder, great group, subgroup, family and series. Soil properties that can be measured quantitatively are used in this classification system – they include: depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content. There are 12 soil orders (the top hierarchical level) in soil taxonomy.<ref>[http://www.evsc.virginia.edu/~alm7d/soils/soilordr.html The Soil Orders] {{webarchive|url=https://web.archive.org/web/20100112143347/http://www.evsc.virginia.edu/~alm7d/soils/soilordr.html |date=12 January 2010 }}, Department of Environmental Sciences, University of Virginia, retrieved 23 October 2012.</ref>{{sfn|Donahue|Miller|Shickluna|1977|pp=411–32}}


[[Geophagy]] is the practice of eating soil-like substances. Both animals and humans occasionally consume soil for medicinal, recreational, or religious purposes.<ref>{{cite journal |last=Abrahams |first=Pter W. |year=1997 |title=Geophagy (soil consumption) and iron supplementation in Uganda |journal=[[Tropical Medicine and International Health]] |volume=2 |issue=7 |pages=617–623 |doi=10.1046/j.1365-3156.1997.d01-348.x |pmid=9270729 |s2cid=19647911 |doi-access=free}}</ref> It has been shown that some [[monkeys]] consume soil, together with their preferred food (tree [[foliage]] and [[fruits]]), in order to alleviate tannin toxicity.<ref name="Setz1999">{{cite journal|last1=Setz |first1=Eleonore Zulnara Freire |last2=Enzweiler |first2=Jacinta |last3=Solferini |first3=Vera Nisaka |last4=Amêndola |first4=Monica Pimenta |last5=Berton |first5=Ronaldo Severiano |year=1999 |title=Geophagy in the golden-faced saki monkey (Pithecia pithecia chrysocephala) in the Central Amazon |journal=[[Journal of Zoology]] |volume=247 |issue=1 |pages=91–103 |doi=10.1111/j.1469-7998.1999.tb00196.x |url=https://www.academia.edu/26464333 |access-date=22 August 2021}}</ref>
==Uses==
Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants; however, as demonstrated by [[hydroponics]], it is not essential to plant growth if the soil-contained nutrients can be dissolved in a solution. The types of soil and available moisture determine the species of plants that can be cultivated.


Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper [[Stratum|rock strata]], thus becoming [[groundwater]]. [[Pest (organism)|Pests]] ([[virus]]es) and [[pollutant]]s, such as [[persistent organic pollutant]]s ([[chlorinated]] [[pesticide]]s, [[polychlorinated biphenyl]]s), oils ([[hydrocarbon]]s), heavy metals ([[lead]], zinc, [[cadmium]]), and excess nutrients (nitrates, [[sulfate]]s, phosphates) are filtered out by the soil.<ref name="Kohne2009">{{cite journal |last1=Kohne |first1=John Maximilian |last2=Koehne |first2=Sigrid |last3=Simunek |first3=Jirka |date=2009 |title=A review of model applications for structured soils: a) Water flow and tracer transport |journal=Journal of Contaminant Hydrology |volume=104 |pages=4–35 |doi=10.1016/j.jconhyd.2008.10.002 |pmid=19012994 |issue=1–4 |bibcode=2009JCHyd.104....4K |url=http://www.pc-progress.com/documents/jirka/ko-ko_sim_2008_jcontamhydrol.pdf |url-status=live |archive-url=https://web.archive.org/web/20171107005433/http://www.soil.tu-bs.de/lehre/Master.Monitoring/2009/Daten/5_Literatur/A%20review%20of-Koehne-2009.pdf |archive-date=7 November 2017 |citeseerx=10.1.1.468.9149 |access-date=22 August 2021}}</ref> Soil organisms [[metabolise]] them or immobilise them in their biomass and necromass,<ref name="Diplock2009">{{Cite journal|last1=Diplock |first1=Elizabeth E. |last2=Mardlin |first2=Dave P. |last3=Killham |first3=Kenneth S. |last4=Paton |first4=Graeme Iain |year=2009 |title=Predicting bioremediation of hydrocarbons: laboratory to field scale |journal=[[Environmental Pollution (journal)|Environmental Pollution]] |volume=157 |pages=1831–1840 |doi=10.1016/j.envpol.2009.01.022 |pmid=19232804 |issue=6 |bibcode=2009EPoll.157.1831D |url=https://coek.info/pdf-predicting-bioremediation-of-hydrocarbons-laboratory-to-field-scale-.html |access-date=22 August 2021}}</ref> thereby incorporating them into stable humus.<ref name="Moeckel2008">{{cite journal |last1=Moeckel |first1=Claudia |last2=Nizzetto |first2=Luca |last3=Di Guardo |first3=Antonio |last4=Steinnes |first4=Eiliv |last5=Freppaz |first5=Michele |last6=Filippa |first6=Gianluca |last7=Camporini |first7=Paolo |last8=Benner |first8=Jessica |last9=Jones |first9=Kevin C. |date=2008 |title=Persistent organic pollutants in boreal and montane soil profiles: distribution, evidence of processes and implications for global cycling |journal=[[Environmental Science and Technology]] |volume=42 |pages=8374–8380 |doi=10.1021/es801703k |pmid=19068820 |issue=22 |bibcode=2008EnST...42.8374M |hdl=11383/8693 |url=https://www.academia.edu/15598352 |access-date=22 August 2021|hdl-access=free }}</ref> The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.<ref name="Rezaei2009">{{cite journal |last1=Rezaei |first1=Khalil |last2=Guest |first2=Bernard |last3=Friedrich |first3=Anke |last4=Fayazi |first4=Farajollah |last5=Nakhaei |first5=Mohamad |last6=Aghda |first6=Seyed Mahmoud Fatemi |last7=Beitollahi |first7=Ali |date=2009 |title=Soil and sediment quality and composition as factors in the distribution of damage at the December 26, 2003, Bam area earthquake in SE Iran (M (s)=6.6) |journal=Journal of Soils and Sediments |volume=9 |issue=1 |pages=23–32 |doi=10.1007/s11368-008-0046-9 |bibcode=2009JSoSe...9...23R |s2cid=129416733 |url=https://www.researchgate.net/publication/225752596 |access-date=22 August 2021}}</ref>
Soil material is also a critical component in the mining, construction and landscape development industries.<ref>{{cite book|title= Soils for Landscape Development |author1=Leake, Simon|author2=Haege, Elke|publisher=CSIRO Publishing|year=2014|isbn=978-0-643-10964-3}}</ref> Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in [[surface mining]], road building and dam construction. [[Earth sheltering]] is the architectural practice of using soil for external [[thermal mass]] against building walls. Many [[building material]]s are soil based.


==Degradation==
Soil resources are critical to the environment, as well as to food and fibre production. Soil provides minerals and water to plants. Soil absorbs rainwater and releases it later, thus preventing floods and drought. Soil cleans water as it percolates through it. Soil is the habitat for many organisms: the major part of known and unknown [[biodiversity]] is in the soil, in the form of [[invertebrates]] ([[earthworm]]s, [[woodlice]], [[millipede]]s, [[centipede]]s, [[snail]]s, [[slug]]s, [[mite]]s, [[springtail]]s, [[Enchytraeidae|enchytraeids]], [[nematode]]s, [[protist]]s), [[bacteria]], [[archaea]], fungi and [[algae]]; and most organisms living above ground have part of them ([[plants]]) or spend part of their [[Biological life cycle|life cycle]] ([[insects]]) below-ground. Above-ground and below-ground biodiversities are tightly interconnected,<ref name="Ponge2003">{{Cite journal| last = Ponge| first = Jean-François| year = 2003| title = Humus forms in terrestrial ecosystems: a framework to biodiversity| journal = Soil Biology and Biochemistry| volume = 35| issue = 7| pages = 935–45| doi = 10.1016/S0038-0717(03)00149-4| url = https://www.researchgate.net/publication/222567430| format = PDF| deadurl = no| archiveurl = https://web.archive.org/web/20160129153903/https://www.researchgate.net/publication/222567430| archivedate = 29 January 2016| df = dmy-all| citeseerx = 10.1.1.467.4937}}</ref><ref name="De Deyn2005">{{cite journal |last=De Deyn |first=Gerlinde B. |last2=Van der Putten |first2=Wim H. |date=2005 |title=Linking aboveground and belowground diversity |journal=Trends in Ecology & Evolution |volume=20 |pages=625–33 |doi=10.1016/j.tree.2005.08.009 |pmid=16701446 |issue=11}}</ref> making soil protection of paramount importance for any restoration or conservation plan.
{{Main|Soil retrogression and degradation|Soil conservation}}
[[Land degradation]] is a human-induced or natural process which impairs the capacity of [[land (economics)|land]] to function.<ref>{{cite journal |last1=Johnson |first1=Dan L. |last2=Ambrose |first2=Stanley H. |last3=Bassett |first3=Thomas J. |last4=Bowen |first4=Merle L. |last5=Crummey |first5=Donald E. |last6=Isaacson |first6=John S. |last7=Johnson |first7=David N. |last8=Lamb |first8=Peter |last9=Saul |first9=Mahir |last10=Winter-Nelson |first10=Alex E. |year=1997 |title=Meanings of environmental terms |url=https://www.researchgate.net/publication/240784159 |journal=[[Journal of Environmental Quality]] |volume=26 |issue=3 |pages=581–589 |doi=10.2134/jeq1997.00472425002600030002x |bibcode=1997JEnvQ..26..581J |access-date=29 August 2021}}</ref> [[Soil degradation]] involves [[Soil acidification|acidification]], [[soil contamination|contamination]], [[desertification]], [[erosion]] or [[Soil salinity|salination]].<ref>{{cite book |last=Oldeman |first=L. Roel |year=1993 |chapter=Global extent of soil degradation |title=ISRIC Bi-Annual Report 1991–1992 |pages=19–36 |chapter-url=https://library.wur.nl/WebQuery/wurpubs/fulltext/299739 |publisher=[[International Soil Reference and Information Centre]](ISRIC) |location=Wageningen, The Netherlands |access-date=29 August 2021}}</ref>


=== Acidification ===
The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even on desert crusts, cyanobacteria, lichens and mosses capture and sequester a significant amount of carbon by photosynthesis. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset the effect of increases in [[greenhouse gas]] emissions and slow global warming, while improving crop yields and reducing water needs.<ref>{{Cite journal|author = Hansen, J. |display-authors = etal |journal=Open Atmospheric Science Journal|year=2008|volume= 2|issue = 1 |pages= 217–31 | title=Target atmospheric CO2: Where should humanity aim?|arxiv = 0804.1126 |bibcode = 2008OASJ....2..217H |doi = 10.2174/1874282300802010217|postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}</ref><ref>{{cite journal |last=Lal |first=R. |date=11 June 2004 |title=Soil Carbon Sequestration Impacts on Global Climate Change and Food Security |journal=Science |volume=304 |issue=5677 |pages=1623–27 |doi=10.1126/science.1097396 |pmid=15192216|bibcode = 2004Sci...304.1623L }}</ref><ref>{{cite web|author=Blakeslee, Thomas R.|title=Greening Deserts for Carbon Credits|date=24 February 2010|accessdate=23 October 2012|publisher=Renewable Energy World.com|url=http://www.renewableenergyworld.com/rea/news/article/2010/02/greening-deserts-for-carbon-credits|deadurl=no|archiveurl=https://web.archive.org/web/20121101011735/http://www.renewableenergyworld.com/rea/news/article/2010/02/greening-deserts-for-carbon-credits|archivedate=1 November 2012|df=dmy-all}}</ref>
Soil acidification is beneficial in the case of [[alkaline soil]]s, but it degrades land when it lowers [[crop productivity]], soil biological activity and increases soil vulnerability to [[contamination]] and erosion. Soils are initially acid and remain such when their parent materials are low in basic [[cation]]s (calcium, magnesium, potassium and [[sodium]]). On parent materials richer in [[mineral weathering|weatherable minerals]] acidification occurs when basic cations are [[Leaching (pedology)|leached]] from the soil profile by rainfall or exported by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming [[nitrogenous fertilizer]]s and by the effects of [[acid precipitation]]. [[Deforestation]] is another cause of soil acidification, mediated by increased leaching of soil nutrients in the absence of [[tree canopies]].<ref>{{cite book |last1=Sumner |first1=Malcolm E. |last2=Noble |first2=Andrew D. |year=2003 |chapter=Soil acidification: the world story |title=Handbook of soil acidity |pages=1–28 |editor-last=Rengel |editor-first=Zdenko |chapter-url=https://pdf-drive.com/pdf/Zdenko20Rengel20-20Handbook20of20Soil20Acidity2028Books20in20Soils2C20Plants2C20and20the20Environment292028200329.pdf#page=16 |publisher=[[Marcel Dekker]] |location=New York, NY, USA |access-date=29 August 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814115102/https://pdf-drive.com/pdf/Zdenko20Rengel20-20Handbook20of20Soil20Acidity2028Books20in20Soils2C20Plants2C20and20the20Environment292028200329.pdf#page=16 |url-status=dead }}</ref>


===Contamination===
[[Waste management]] often has a soil component. [[Septic drain field]]s treat [[septic tank]] effluent using aerobic soil processes. [[Landfill]]s use soil for [[daily cover]]. Land application of waste water relies on soil biology to aerobically treat [[Biochemical oxygen demand|BOD]].
Soil [[contamination]] at low levels is often within a soil's capacity to treat and assimilate [[waste]] material. [[Soil biota]] can treat waste by transforming it, mainly through microbial [[Enzyme|enzymatic]] activity.<ref>{{cite journal |last1=Karam |first1=Jean |last2=Nicell |first2=James A. |year=1997 |title=Potential applications of enzymes in waste treatment |url=https://www.researchgate.net/publication/30002097 |journal=[[Journal of Chemical Technology & Biotechnology]] |volume=69 |issue=2 |pages=141–153 |doi=10.1002/(SICI)1097-4660(199706)69:2<141::AID-JCTB694>3.0.CO;2-U |bibcode=1997JCTB...69..141K |access-date=5 September 2021}}</ref> Soil organic matter and soil minerals can adsorb the waste material and decrease its [[toxicity]],<ref>{{cite journal |last1=Sheng |first1=Guangyao |last2=Johnston |first2=Cliff T. |last3=Teppen |first3=Brian J. |last4=Boyd |first4=Stephen A. |year=2001 |title=Potential contributions of smectite clays and organic matter to pesticide retention in soils |url=https://www.academia.edu/4875079 |journal=[[Journal of Agricultural and Food Chemistry]] |volume=49 |issue=6 |pages=2899–2907 |doi=10.1021/jf001485d |pmid=11409985 |access-date=5 September 2021}}</ref> although when in colloidal form they may transport the adsorbed contaminants to subsurface environments.<ref>{{cite journal |last1=Sprague |first1=Lori A. |last2=Herman |first2=Janet S. |last3=Hornberger |first3=George M. |last4=Mills |first4=Aaron L. |year=2000 |title=Atrazine adsorption and colloid-facilitated transport through the unsaturated zone |url=https://lmecol.evsc.virginia.edu/pubs/73-Sprague_JEQ2000.pdf |journal=[[Journal of Environmental Quality]] |volume=29 |issue=5 |pages=1632–1641 |doi=10.2134/jeq2000.00472425002900050034x |bibcode=2000JEnvQ..29.1632S |access-date=5 September 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814121821/https://lmecol.evsc.virginia.edu/pubs/73-Sprague_JEQ2000.pdf |url-status=dead }}</ref> Many waste treatment processes rely on this natural [[bioremediation]] capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. [[Environmental remediation|Remediation]] of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore [[soil functions]] and values. Techniques include [[Leaching (chemistry)|leaching]], [[air sparging]], [[soil conditioner]]s, [[phytoremediation]], bioremediation and [[In situ bioremediation|Monitored Natural Attenuation]]. An example of diffuse pollution with contaminants is copper accumulation in [[vineyard]]s and [[orchard]]s to which fungicides are repeatedly applied, even in [[organic farming]].<ref>{{Cite journal |last1=Ballabio |first1=Cristiano |last2=Panagos |first2=Panos |last3=Lugato |first3=Emanuele |last4=Huang |first4=Jen-How |last5=Orgiazzi |first5=Alberto |last6=Jones |first6=Arwyn |last7=Fernández-Ugalde |first7=Oihane |last8=Borrelli |first8=Pasquale |last9=Montanarella |first9=Luca |date=15 September 2018 |title=Copper distribution in European topsoils: an assessment based on LUCAS soil survey |journal=[[Science of the Total Environment]] |volume=636 |pages=282–298 |doi=10.1016/j.scitotenv.2018.04.268 |pmid=29709848 |issn=0048-9697 |bibcode=2018ScTEn.636..282B |doi-access=free}}</ref>


[[Microfiber|Microfibres]] from synthetic textiles are another type of plastic soil contamination, 100% of agricultural soil samples from southwestern China contained plastic particles, 92% of which were microfibres. Sources of microfibres likely included string or twine, as well as irrigation water in which clothes had been washed.<ref name="auto">{{Cite web |last=Environment |first=U. N. |date=2021-10-21 |title=Drowning in Plastics – Marine Litter and Plastic Waste Vital Graphics |url=http://www.unep.org/resources/report/drowning-plastics-marine-litter-and-plastic-waste-vital-graphics |access-date=2022-03-23 |website=UNEP - UN Environment Programme |language=en}}</ref>
Organic soils, especially [[peat]], serve as a significant fuel resource; but wide areas of peat production, such as [[sphagnum]] [[bog]]s, are now protected because of patrimonial interest.


The application of biosolids from sewage sludge and compost can introduce [[microplastics]] to soils. This adds to the burden of microplastics from other sources (e.g. the atmosphere). Approximately half the sewage sludge in Europe and North America is applied to agricultural land. In Europe it has been estimated that for every million inhabitants 113 to 770 tonnes of microplastics are added to agricultural soils each year.<ref name="auto"/>
[[Geophagy]] is the practice of eating soil-like substances. Both animals and human cultures occasionally consume soil for medicinal, recreational, or religious purposes. It has been shown that some [[monkeys]] consume soil, together with their preferred food (tree [[foliage]] and [[fruits]]), in order to alleviate [[tannin]] toxicity.<ref name="Setz1999">{{Cite journal| last = Setz| first = EZF| authorlink = |author2=Enzweiler J |author3=Solferini VN |author4=Amendola MP |author5=Berton RS | year = 1999| title = Geophagy in the golden-faced saki monkey (Pithecia pithecia chrysocephala) in the Central Amazon| journal = Journal of Zoology| volume = 247| issue=1|pages = 91–103| doi = 10.1111/j.1469-7998.1999.tb00196.x | url = https://repository.si.edu/bitstream/handle/10088/1059/Setz-et-al-1999.pdf| format = Submitted manuscript}}</ref>


===Desertification===
Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper [[Stratum|rock strata]], thus becoming groundwater. [[Pest (organism)|Pests]] ([[virus]]es) and [[pollutant]]s, such as persistent organic pollutants ([[chlorinated]] [[pesticide]]s, [[polychlorinated biphenyls]]), oils ([[hydrocarbon]]s), heavy metals ([[lead]], [[zinc]], [[cadmium]]), and excess nutrients ([[nitrate]]s, [[sulfate]]s, [[phosphate]]s) are filtered out by the soil.<ref name="Kohne2009">{{cite journal |last=Kohne |first=John Maximilian |last2=Koehne |first2=Sigrid |last3=Simunek |first3=Jirka |date=2009 |title=A review of model applications for structured soils: a) Water flow and tracer transport |journal=Journal of Contaminant Hydrology |volume=104 |pages=4–35 |doi=10.1016/j.jconhyd.2008.10.002 |pmid=19012994 |issue=1–4 |bibcode=2009JCHyd.104....4K |url=http://www.soil.tu-bs.de/lehre/Master.Monitoring/2009/Daten/5_Literatur/A%20review%20of-Koehne-2009.pdf |deadurl=yes |archiveurl=https://web.archive.org/web/20171107005433/http://www.soil.tu-bs.de/lehre/Master.Monitoring/2009/Daten/5_Literatur/A%20review%20of-Koehne-2009.pdf |archivedate=7 November 2017 |df=dmy-all |citeseerx=10.1.1.468.9149 |access-date=1 November 2017 }}</ref> Soil organisms [[metabolise]] them or immobilise them in their [[biomass]] and necromass,<ref name="Diplock2009">{{Cite journal| last = Diplock| first = EE| authorlink = |author2=Mardlin DP |author3=Killham KS |author4=Paton GI | year = 2009| title = Predicting bioremediation of hydrocarbons: laboratory to field scale| journal = Environmental Pollution| volume = 157| pages = 1831–40| doi = 10.1016/j.envpol.2009.01.022| pmid = 19232804| issue = 6}}</ref> thereby incorporating them into stable humus.<ref name="Moeckel2008">{{cite journal |last=Moeckel |first=Claudia |last2=Nizzetto |first2=Luca |last3=Di Guardo |first3=Antonio |last4=Steinnes |first4=Eiliv |last5=Freppaz |first5=Michele |last6=Filippa |first6=Gianluca |last7=Camporini |first7=Paolo |last8=Benner |first8=Jessica |last9=Jones |first9=Kevin C. |date=2008 |title=Persistent organic pollutants in boreal and montane soil profiles: distribution, evidence of processes and implications for global cycling |journal=Environmental Science and Technology |volume=42 |pages=8374–80 |doi=10.1021/es801703k |pmid=19068820 |issue=22 |bibcode=2008EnST...42.8374M}}</ref> The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.<ref name="Rezaei2009">{{cite journal |last=Rezaei |first=Khalil |last2=Guest |first2=Bernard |last3=Friedrich |first3=Anke |last4=Fayazi |first4=Farajollah |last5=Nakhaei |first5=Mohamad |last6=Aghda |first6=Seyed Mahmoud Fatemi |last7=Beitollahi |first7=Ali |date=2009 |title=Soil and sediment quality and composition as factors in the distribution of damage at the December 26, 2003, Bam area earthquake in SE Iran (M (s)=6.6) |journal=Journal of Soils and Sediments |volume=9 |pages=23–32 |doi=10.1007/s11368-008-0046-9}}</ref>
[[File:Soil erosion, Southfield - geograph.org.uk - 367917.jpg|thumb|Desertification]]


[[Desertification]], an environmental process of ecosystem degradation in arid and semi-arid regions, is often caused by badly adapted human activities such as [[overgrazing]] or excess harvesting of [[firewood]]. It is a common misconception that [[drought]] causes desertification.<ref>{{Cite journal |last=Le Houérou |first=Henry N. |year=1996 |title=Climate change, drought and desertification |journal=[[Journal of Arid Environments]] |volume=34 |issue=2 |pages=133–185 |doi=10.1006/jare.1996.0099 |bibcode=1996JArEn..34..133L |url=http://www7.nau.edu/mpcer/direnet/publications/publications_l/files/LeHouerou_1996.pdf |access-date=5 September 2021}}</ref> Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. [[Soil management]] tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover.<ref>{{Cite journal |last1=Lyu |first1=Yanli |last2=Shi |first2=Peijun |last3=Han |first3=Guoyi |last4=Liu |first4=Lianyou |last5=Guo |first5=Lanlan |last6=Hu |first6=Xia |last7=Zhang |first7=Guoming |year=2020 |title=Desertification control practices in China |journal=[[Sustainability (journal)|Sustainability]] |volume=12 |issue=8 |pages=3258 |doi=10.3390/su12083258 |issn=2071-1050 |doi-access=free}}</ref> These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases [[land degradation]]. Increased population and livestock pressure on marginal lands accelerates desertification.<ref>{{Cite journal |last1=Kéfi |first1=Sonia |last2=Rietkerk |first2=Max |last3=Alados |first3=Concepción L. |last4=Pueyo |first4=Yolanda |last5=Papanastasis |first5=Vasilios P. |last6=El Aich |first6=Ahmed |last7=de Ruiter |first7=Peter C. |year=2007 |title=Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems |journal=[[Nature (journal)|Nature]] |volume=449 |issue=7159 |pages=213–217 |doi=10.1038/nature06111 |pmid=17851524 |bibcode=2007Natur.449..213K |hdl=1874/25682 |s2cid=4411922 |url=https://www.researchgate.net/publication/232801317 |access-date=5 September 2021}}</ref> It is now questioned whether present-day climate warming will favour or disfavour desertification, with contradictory reports about predicted rainfall trends associated with increased temperature, and strong discrepancies among regions, even in the same country.<ref>{{Cite journal |last1=Wang |first1=Xunming |last2=Yang |first2=Yi |last3=Dong |first3=Zhibao |last4=Zhang |first4=Caixia |year=2009 |title=Responses of dune activity and desertification in China to global warming in the twenty-first century |journal=[[Global and Planetary Change]] |volume=67 |issue=3–4 |pages=167–185 |doi=10.1016/j.gloplacha.2009.02.004 |bibcode=2009GPC....67..167W |url=https://www.researchgate.net/publication/229103975 |access-date=5 September 2021}}</ref>
==Degradation==
{{Main|Soil retrogression and degradation|Soil conservation}}
[[Land degradation]]<ref>{{cite journal | last1 = Johnson | first1 = D.L. | last2 = Ambrose | first2 = S.H. | last3 = Bassett | first3 = T.J. | last4 = Bowen | first4 = M.L. | last5 = Crummey | first5 = D.E. | last6 = Isaacson | first6 = J.S. | last7 = Johnson | first7 = D.N. | last8 = Lamb | first8 = P. | last9 = Saul | first9 = M. | year = 1997 | title = Meanings of environmental terms | url = | journal = Journal of Environmental Quality | volume = 26 | issue = 3| pages = 581–89 | doi=10.2134/jeq1997.00472425002600030002x | last10 = Winter-Nelson | first10 = A. E.}}</ref> refers to a human-induced or natural process which impairs the capacity of [[land (economics)|land]] to function. Soils degradation involves the [[Soil acidification|acidification]], [[soil contamination|contamination]], [[desertification]], [[erosion]] or [[Soil salinity|salination]].


===Erosion===
[[Soil acidification]] is beneficial in the case of alkaline soils, but it degrades land when it lowers crop productivity and increases soil vulnerability to contamination and erosion. Soils are often initially acid because their [[parent material]]s were acid and initially low in the [[Base (chemistry)|basic]] [[cation]]s ([[calcium]], [[magnesium]], [[potassium]] and [[sodium]]). Acidification occurs when these elements are leached from the soil profile by rainfall or by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming [[nitrogenous fertilizer]]s and by the effects of [[acid precipitation]].
[[File:Riparian buffer on Bear Creek in Story County, Iowa.JPG|thumb|upright|Erosion control]]


[[Erosion]] of soil is caused by [[Water erosion#Rainfall|water]], [[Water erosion#Wind erosion|wind]], [[Water erosion#Glaciers|ice]], and [[Water erosion#Mass movement|movement in response to gravity]]. More than one kind of erosion can occur simultaneously. Erosion is distinguished from [[weathering]], since erosion also transports eroded soil away from its place of origin (soil in transit may be described as [[sediment]]). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially unsuitable land use practices.<ref>{{Cite journal |last1=Yang |first1=Dawen |last2=Kanae |first2=Shinjiro |last3=Oki |first3=Taikan |last4=Koike |first4=Toshio |last5=Musiake |first5=Katumi |year=2003 |title=Global potential soil erosion with reference to land use and climate changes |journal=Hydrological Processes |volume=17 |issue=14 |pages=2913–28 |doi=10.1002/hyp.1441 |bibcode=2003HyPr...17.2913Y |s2cid=129355387 |url=https://www.oieau.org/eaudoc/system/files/documents/38/191115/191115_doc.pdf |access-date=5 September 2021 |archive-date=18 August 2021 |archive-url=https://web.archive.org/web/20210818043117/https://www.oieau.org/eaudoc/system/files/documents/38/191115/191115_doc.pdf |url-status=dead }}</ref> These include [[agriculture|agricultural]] activities which leave the soil bare during times of heavy rain or strong winds, [[overgrazing]], [[deforestation]], and improper [[construction]] activity. Improved management can limit erosion. [[Soil conservation#Erosion prevention|Soil conservation techniques]] which are employed include changes of land use (such as replacing erosion-prone [[crop]]s with [[grass]] or other soil-binding plants), changes to the timing or type of agricultural operations, [[Terrace (agriculture)|terrace]] building, use of erosion-suppressing cover materials (including [[Cover crop#Water management|cover crops]] and other plants), limiting disturbance during construction, and avoiding construction during erosion-prone periods and in erosion-prone places such as steep slopes.<ref>{{Cite journal |last1=Sheng |first1=Jian-an |last2=Liao |first2=An-zhong |year=1997 |title=Erosion control in South China |journal=Catena |issn=0341-8162 |volume=29 |issue=2 |pages=211–221 |doi=10.1016/S0341-8162(96)00057-4 |bibcode=1997Caten..29..211S |url=https://coek.info/pdf-erosion-control-in-south-china-.html |access-date=5 September 2021}}</ref> Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion (the so-called [[Dust Bowl|dust bowl]]) which ruined American and Canadian prairies during the 1930s, when immigrant farmers, encouraged by the federal government of both countries, settled and converted the original [[shortgrass prairie]] to [[agricultural crops]] and [[cattle ranching]].
[[Soil contamination]] at low levels is often within a soil's capacity to treat and assimilate waste material. Soil biota can treat waste by transforming it; soil colloids can adsorb the waste material. Many waste treatment processes rely on this treatment capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. [[Environmental remediation|Remediation]] of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore [[soil functions]] and values. Techniques include leaching, air sparging, chemical amendments, [[phytoremediation]], [[bioremediation]] and natural degradation. An example of diffuse pollution with contaminants is the copper distribution in agricultural soils mainly due to fungicide applications in vineyards and other permanent crops.<ref>{{Cite journal|last=Ballabio|first=Cristiano|last2=Panagos|first2=Panos|last3=Lugato|first3=Emanuele|last4=Huang|first4=Jen-How|last5=Orgiazzi|first5=Alberto|last6=Jones|first6=Arwyn|last7=Fernández-Ugalde|first7=Oihane|last8=Borrelli|first8=Pasquale|last9=Montanarella|first9=Luca|date=2018-09-15|title=Copper distribution in European topsoils: An assessment based on LUCAS soil survey|url=http://www.sciencedirect.com/science/article/pii/S0048969718314451|journal=Science of the Total Environment|volume=636|pages=282–298|doi=10.1016/j.scitotenv.2018.04.268|issn=0048-9697}}</ref>
[[File:Soil erosion, Southfield - geograph.org.uk - 367917.jpg|thumb|Desertification]]


A serious and long-running water erosion problem occurs in [[China]], on the middle reaches of the [[Yellow River]] and the upper reaches of the [[Yangtze River]]. From the Yellow River, over 1.6&nbsp;billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion (gully erosion) in the [[Loess Plateau]] region of northwest China.<ref>{{Cite journal |last1=Ran |first1=Lishan |last2=Lu |first2=Xi Xi |last3=Xin |first3=Zhongbao |year=2014 |title=Erosion-induced massive organic carbon burial and carbon emission in the Yellow River basin, China |journal=[[Biogeosciences]] |volume=11 |issue=4 |pages=945–959 |doi=10.5194/bg-11-945-2014 |bibcode=2014BGeo...11..945R |url=https://bg.copernicus.org/articles/11/945/2014/bg-11-945-2014.pdf |access-date=5 September 2021 |hdl=10722/228184 |hdl-access=free |doi-access=free }}</ref>
[[Desertification]] is an environmental process of ecosystem degradation in arid and semi-arid regions, often caused by human activity. It is a common misconception that [[drought]]s cause desertification. Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Soil management tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover. These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands accelerates desertification.

[[File:Riparian buffer on Bear Creek in Story County, Iowa.JPG|thumb|upright|Erosion control]]
Soil piping is a particular form of soil erosion that occurs below the soil surface.<ref>{{Cite journal |last1=Verachtert |first1=Els |last2=Van den Eeckhaut |first2=Miet |last3=Poesen |first3=Jean |last4=Deckers |first4=Jozef |year=2010 |title=Factors controlling the spatial distribution of soil piping erosion on loess-derived soils: a case study from central Belgium |journal=[[Geomorphology (journal)|Geomorphology]] |volume=118 |issue=3 |pages=339–348 |doi=10.1016/j.geomorph.2010.02.001 |bibcode=2010Geomo.118..339V |url=https://lirias.kuleuven.be/retrieve/109942 |access-date=5 September 2021}}</ref> It causes [[levee]] and dam failure, as well as [[Sinkhole|sink hole]] formation. Turbulent flow removes soil starting at the mouth of the [[Seep (hydrology)|seep]] flow and the [[subsoil]] erosion advances up-gradient.<ref>{{Cite journal |last=Jones |first=Anthony |title=Soil piping and stream channel initiation |journal=[[Water Resources Research]] |volume=7 |issue=3 |pages=602–610 |year=1976 |doi=10.1029/WR007i003p00602 |bibcode=1971WRR.....7..602J |url=https://booksc.eu/book/20668631/3ac27a |access-date=5 September 2021 |archive-date=5 September 2021 |archive-url=https://web.archive.org/web/20210905084319/https://booksc.eu/book/20668631/3ac27a |url-status=dead }}</ref> The term [[sand boil]] is used to describe the appearance of the discharging end of an active soil pipe.<ref>{{cite web|last=Dooley |first=Alan |title=Sandboils 101: Corps has experience dealing with common flood danger |website=Engineer Update |publisher=[[United States Army Corps of Engineers|US Army Corps of Engineers]] |date=June 2006 |url=http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm |archive-url=https://web.archive.org/web/20080418185527/http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm |archive-date=18 April 2008 |url-status=dead}}</ref>

===Salination===
[[Soil salination]] is the accumulation of free [[salt]]s to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include [[corrosion]] damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and [[water quality]] problems due to [[sedimentation]]. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. [[Surface irrigation|Irrigation]] of arid lands is especially problematic.<ref>{{cite web |last=Oosterbaan |first=Roland J. |title=Effectiveness and social/environmental impacts of irrigation projects: a critical review |series=Annual Reports of the International Institute for Land Reclamation and Improvement (ILRI) |year=1988 |pages=18–34 |location=Wageningen, The Netherlands |url=http://www.waterlog.info/pdf/irreff.pdf |url-status=live |archive-url=https://web.archive.org/web/20090219070320/http://waterlog.info/pdf/irreff.pdf |archive-date=19 February 2009 |df=dmy-all |access-date=5 September 2021}}</ref> All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying [[water table]]. Rapid salination occurs when the land surface is within the [[capillary fringe]] of saline groundwater. [[Soil salinity control]] involves [[watertable control]] and [[leaching model|flushing]] with higher levels of applied water in combination with [[tile drainage]] or another form of [[Drainage system (agriculture)|subsurface drainage]].<ref>{{Cite book |title=Drainage manual: a guide to integrating plant, soil, and water relationships for drainage of irrigated lands |year=1993 |publisher=[[United States Department of the Interior]], [[United States Bureau of Reclamation|Bureau of Reclamation]] |location=Washington, D.C. |url=https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/DrainMan.pdf |isbn=978-0-16-061623-5 |access-date=5 September 2021}}</ref><ref name="Waterlog">{{cite web |last=Oosterbaan |first=Roland J. |url=http://www.waterlog.info |title=Waterlogging, soil salinity, field irrigation, plant growth, subsurface drainage, groundwater modelling, surface runoff, land reclamation, and other crop production and water management aspects |access-date=5 September 2021 |url-status=live |archive-url=https://web.archive.org/web/20100816225219/http://www.waterlog.info/ |archive-date=16 August 2010}}</ref>

== Reclamation ==
{{Main|Soil regeneration}}
Soils which contain high levels of particular clays with high swelling properties, such as [[smectite]]s, are often very fertile. For example, the smectite-rich [[Paddy field|paddy]] soils of Thailand's [[Central Thailand|Central Plains]] are among the most productive in the world. However, the overuse of mineral nitrogen [[fertilizer]]s and pesticides in [[Irrigation|irrigated]] intensive [[Rice production in Thailand|rice production]] has endangered these soils, forcing farmers to implement [[integrated farming|integrated practices]] based on Cost Reduction Operating Principles.<ref>{{cite journal |last1=Stuart |first1=Alexander M. |last2=Pame |first2=Anny Ruth P. |last3=Vithoonjit |first3=Duangporn |last4=Viriyangkura |first4=Ladda |last5=Pithuncharurnlap |first5=Julmanee |last6=Meesang |first6=Nisa |last7=Suksiri |first7=Prarthana |last8=Singleton |first8=Grant R. |last9=Lampayan |first9=Rubenito M. |year=2018 |title=The application of best management practices increases the profitability and sustainability of rice farming in the central plains of Thailand |url=https://www.researchgate.net/publication/314091782 |journal=Field Crops Research |volume=220 |pages=78–87 |doi=10.1016/j.fcr.2017.02.005 |bibcode=2018FCrRe.220...78S |access-date=12 September 2021}}</ref>

Many farmers in tropical areas, however, struggle to retain organic matter and clay in the soils they work. In recent years, for example, productivity has declined and soil erosion has increased in the low-clay soils of northern Thailand, following the abandonment of [[shifting cultivation]] for a more permanent land use.<ref>{{cite journal |last1=Turkelboom |first1=Francis |last2=Poesen |first2=Jean |last3=Ohler |first3=Ilse |last4=Van Keer |first4=Koen |last5=Ongprasert |first5=Somchai |last6=Vlassak |first6=Karel |year=1997 |title=Assessment of tillage erosion rates on steep slopes in northern Thailand |url=https://www.academia.edu/17993140 |journal=Catena |volume=29 |issue=1 |pages=29–44 |doi=10.1016/S0341-8162(96)00063-X |bibcode=1997Caten..29...29T |access-date=12 September 2021}}</ref> Farmers initially responded by adding organic matter and clay from [[Mound-building termites|termite mound]] material, but this was [[Sustainability|unsustainable]] in the long-term because of rarefaction of termite mounds. Scientists experimented with adding [[bentonite]], one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the [[International Water Management Institute]] (IWMI) in cooperation with [[Khon Kaen University]] and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of {{convert|200|kg/rai|kg/hectare lb/acre|lk=in}} of bentonite resulted in an average yield increase of 73%.<ref>{{cite journal |last1=Saleth |first1=Rathinasamy Maria |last2=Inocencio |first2=Arlene |last3=Noble |first3=Andrew |last4=Ruaysoongnern |first4=Sawaeng |year=2009 |title=Economic gains of improving soil fertility and water holding capacity with clay application: the impact of soil remediation research in Northeast Thailand |url=https://ageconsearch.umn.edu/record/53064/files/RR130.pdf |journal=Journal of Development Effectiveness |volume=1 |issue=3 |pages=336–352 |doi=10.1080/19439340903105022 |s2cid=18049595 |access-date=12 September 2021}}</ref> Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.<ref>{{cite journal |last1=Semalulu |first1=Onesmus |last2=Magunda |first2=Matthias |last3=Mubiru |first3=Drake N. |year=2015 |title=Amelioration of sandy soils in drought stricken areas through use of Ca-bentonite |url=https://www.ajol.info/index.php/ujas/article/download/141752/131487 |journal=Uganda Journal of Agricultural Sciences |volume=16 |issue=2 |pages=195–205 |doi=10.4314/ujas.v16i2.5 |access-date=12 September 2021 |doi-access=free}}</ref>

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.<ref name="Water Management Institute2010">{{cite journal |year=2010 |url=http://www.iwmi.cgiar.org/Publications/Success_Stories/PDF/2010/Issue%202%20-%20Improving_soils_and_boosting_yields_in_Thailand.pdf |title=Improving soils and boosting yields in Thailand |doi=10.5337/2011.0031 |journal=Success Stories |issue=2 |url-status=live |archive-url=https://web.archive.org/web/20120607030912/http://www.iwmi.cgiar.org/Publications/Success_Stories/PDF/2010/Issue%202%20-%20Improving_soils_and_boosting_yields_in_Thailand.pdf |archive-date=7 June 2012 |author=[[International Water Management Institute]] |doi-access=free |access-date=12 September 2021}}</ref>

If the soil is too high in clay or salts (e.g. [[saline sodic soil]]), adding gypsum, washed river sand and organic matter (e.g.[[municipal solid waste]]) will balance the composition.<ref>{{cite journal |last1=Prapagar |first1=Komathy |last2=Indraratne |first2=Srimathie P. |last3=Premanandharajah |first3=Punitha |year=2012 |title=Effect of soil amendments on reclamation of saline-sodic soil |url=https://www.researchgate.net/publication/267202667 |journal=Tropical Agricultural Research |volume=23 |issue=2 |pages=168–176 |doi=10.4038/tar.v23i2.4648 |access-date=12 September 2021 |doi-access=free}}</ref>

Adding organic matter, like [[ramial chipped wood]] or [[compost]], to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.<ref>{{cite web |last1=Lemieux |first1=Gilles |last2=Germain |first2=Diane |title=Ramial chipped wood: the clue to a sustainable fertile soil |publisher=[[Université Laval]], Département des Sciences du Bois et de la Forêt, Québec, Canada |date=December 2000 |url=https://www.healthy-vegetable-gardening.com/support-files/rcw-the-clue-to-a-sustainable-fertile-soil.pdf |access-date=12 September 2021 |archive-date=28 September 2021 |archive-url=https://web.archive.org/web/20210928080056/https://www.healthy-vegetable-gardening.com/support-files/rcw-the-clue-to-a-sustainable-fertile-soil.pdf |url-status=dead }}</ref><ref>{{cite journal |last1=Arthur |first1=Emmanuel |last2=Cornelis |first2=Wim |last3=Razzaghi |first3=Fatemeh |year=2012 |title=Compost amendment of sandy soil affects soil properties and greenhouse tomato productivity |url=https://www.academia.edu/31660161 |journal=Compost Science and Utilization |volume=20 |issue=4 |pages=215–221 |doi=10.1080/1065657X.2012.10737051 |bibcode=2012CScUt..20..215A |s2cid=96896374 |access-date=12 September 2021}}</ref>


Special mention must be made of the use of [[charcoal]], and more generally [[biochar]] to improve nutrient-poor tropical soils, a process based on the higher fertility of anthropogenic [[Pre-Columbian era|pre-Columbian]] Amazonian [[Dark earth|Dark Earths]], also called [[Terra Preta]] de Índio, due to interesting physical and chemical properties of soil black carbon as a source of stable humus.<ref>{{cite journal |last1=Glaser |first1=Bruno |last2=Haumaier |first2=Ludwig |last3=Guggenberger |first3=Georg |last4=Zech |first4=Wolfgang |year=2001 |title=The 'Terra Preta' phenomenon: a model for sustainable agriculture in the humid tropics |url=https://www.researchgate.net/publication/12032464 |journal=[[The Science of Nature|Naturwissenschaften]] |volume=88 |issue=1 |pages=37–41 |doi=10.1007/s001140000193 |pmid=11302125 |bibcode=2001NW.....88...37G |s2cid=26608101 |access-date=12 September 2021}}</ref> However, the uncontrolled application of [[Charring|charred]] waste products of all kinds may endanger soil life and human health.<ref>{{cite journal |last1=Kavitha |first1=Beluri |last2=Pullagurala Venkata Laxma |first2=Reddy |last3=Kim |first3=Bojeong |last4=Lee |first4=Sang Soo |last5=Pandey |first5=Sudhir Kumar |last6=Kim |first6=Ki-Hyun |year=2018 |title=Benefits and limitations of biochar amendment in agricultural soils: a review |url=https://booksc.eu/book/72239607/440436 |journal=[[Journal of Environmental Management]] |volume=227 |pages=146–154 |doi=10.1016/j.jenvman.2018.08.082 |pmid=30176434 |bibcode=2018JEnvM.227..146K |s2cid=52168678 |access-date=12 September 2021 |archive-date=12 September 2021 |archive-url=https://web.archive.org/web/20210912081933/https://booksc.eu/book/72239607/440436 |url-status=dead }}</ref>
[[Erosion]] of soil is caused by [[Water erosion#Rainfall|water]], [[Water erosion#Wind erosion|wind]], [[Water erosion#Glaciers|ice]], and [[Water erosion#Mass movement|movement in response to gravity]]. More than one kind of erosion can occur simultaneously. Erosion is distinguished from [[weathering]], since erosion also transports eroded soil away from its place of origin (soil in transit may be described as [[sediment]]). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially poor [[land use]] practices. These include [[agriculture|agricultural]] activities which leave the soil bare during times of heavy rain or strong winds, [[overgrazing]], [[deforestation]], and improper [[construction]] activity. Improved management can limit erosion. [[Soil conservation#Erosion prevention|Soil conservation techniques]] which are employed include changes of land use (such as replacing erosion-prone [[crop]]s with [[grass]] or other soil-binding plants), changes to the timing or type of agricultural operations, [[Terrace (agriculture)|terrace]] building, use of erosion-suppressing cover materials (including [[Cover crop#Water management|cover crops]] and [[Soil bioengineering#Technical functions|other plants]]), limiting disturbance during construction, and avoiding construction during erosion-prone periods.


== History of studies and research ==
A serious and long-running water erosion problem occurs in [[China]], on the middle reaches of the [[Yellow River]] and the upper reaches of the [[Yangtze River]]. From the Yellow River, over 1.6&nbsp;billion tons of sediment flow each year into the ocean. The [[sediment]] originates primarily from water erosion (gully erosion) in the [[Loess Plateau]] region of northwest China.
The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.<ref>{{cite book |last=Hillel |first=Daniel |year=1992 |title=Out of the Earth: civilization and the life of the soil |publisher=[[University of California Press]] |location=Berkeley, California |isbn=978-0-520-08080-5}}</ref>


===Studies of soil fertility===
Soil piping is a particular form of soil erosion that occurs below the soil surface. It causes levee and dam failure, as well as [[Sinkhole|sink hole]] formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient.<ref>{{Cite journal | last = Jones | first = j. a. a. | title = Soil piping and stream channel initiation | journal = Water Resources Research | volume = 7 | issue = 3 | pages = 602–10 | year = 1976 | doi = 10.1029/WR007i003p00602 | postscript = . | bibcode=1971WRR.....7..602J}}</ref> The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.<ref>{{cite web
{{Main|Soil fertility}}
|last=Dooley
The Greek historian [[Xenophon]] (450–355&nbsp;[[Before the Common Era|BCE]]) was the first to expound upon the merits of green-manuring crops: 'But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much as dung.'{{sfn|Donahue|Miller|Shickluna|1977|p=4}}
|first=Alan
|title=Sandboils 101: Corps has experience dealing with common flood danger | website = Engineer Update
|publisher=US Army Corps of Engineers
|date=June 2006
|url=http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm
|accessdate = 14 May 2008
|archiveurl = https://web.archive.org/web/20080418185527/http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm <!-- Bot retrieved archive --> |archivedate = 18 April 2008}}</ref>


[[Columella]]'s ''Of husbandry'', {{circa|60&nbsp;[[Common Era|CE]]}}, advocated the use of lime and that [[clover]] and [[alfalfa]] ([[green manure]]) should be turned under,<ref>{{cite book |last=Columella |first=Lucius Junius Moderatus |year=1745 |title=Of husbandry, in twelve books, and his book concerning trees, with several illustrations from Pliny, Cato, Varro, Palladius, and other antient and modern authors, translated into English |publisher=[[Andrew Millar]] |location=London, United Kingdom |url=https://catalog.hathitrust.org/Record/005783003 |access-date=19 September 2021}}</ref> and was used by 15 generations (450&nbsp;years) under the [[Roman Empire]] until its collapse.{{sfn|Donahue|Miller|Shickluna|1977|p=4}}{{sfn|Kellogg|1957|p=1}} From the [[fall of Rome]] to the [[French Revolution]], knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European [[Middle Ages]], [[Ibn al-'Awwam|Yahya Ibn al-'Awwam]]'s handbook,<ref>{{cite book |language=fr |last=[[Ibn al-'Awwam]] |year=1864 |title=Le livre de l'agriculture, traduit de l'arabe par Jean Jacques Clément-Mullet |series=Filāḥah.French. |publisher=Librairie A. Franck |location=Paris, France |url=https://catalog.hathitrust.org/Record/009953450 |access-date=19 September 2021}}</ref> with its emphasis on irrigation, guided the people of North Africa, Spain and the [[Middle East]]; a translation of this work was finally carried to the southwest of the United States when under Spanish influence.<ref>{{cite book |last=Jelinek |first=Lawrence J. |year=1982 |title=Harvest empire: a history of California agriculture |publisher=Boyd and Fraser |location=San Francisco, California |isbn=978-0-87835-131-2}}</ref> [[Olivier de Serres]], considered the father of French [[agronomy]], was the first to suggest the abandonment of [[fallowing]] and its replacement by hay [[meadows]] within [[crop rotation]]s. He also highlighted the importance of soil (the French [[terroir]]) in the management of vineyards. His famous book {{Lang|fr|Le Théâtre d'Agriculture et mesnage des champs}}<ref>{{cite book |language=fr |last=de Serres |first=Olivier |year=1600 |title=Le Théâtre d'Agriculture et mesnage des champs |publisher=Jamet Métayer |location=Paris, France |url=https://gallica.bnf.fr/ark:/12148/bpt6k738381/f1.image |access-date=19 September 2021}}</ref> contributed to the rise of modern, [[sustainable agriculture]] and to the collapse of old [[agricultural practices]] such as [[soil amendment]] for crops by the lifting of [[forest litter]] and [[assarting]], which ruined the soils of western Europe during the Middle Ages and even later on according to regions.<ref>{{cite journal |last1=Virto |first1=Iñigo |last2=Imaz |first2=María José |last3=Fernández-Ugalde |first3=Oihane |last4=Gartzia-Bengoetxea |first4=Nahia |last5=Enrique |first5=Alberto |last6=Bescansa |first6=Paloma |journal=[[Sustainability (journal)|Sustainability]] |volume=7 |issue=1 |title=Soil degradation and soil quality in western Europe: current situation and future perspectives |year=2015 |pages=313–365 |doi=10.3390/su7010313 |doi-access=free}}</ref>
[[Soil salination]] is the accumulation of free [[salt]]s to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. [[Surface irrigation|Irrigation]] of arid lands is especially problematic.<ref>{{cite web | author = ILRI | title = Effectiveness and Social/Environmental Impacts of Irrigation Projects: a Review | series = In: Annual Report 1988 of the International Institute for Land Reclamation and Improvement (ILRI) | year = 1989 | pages = 18–34 | location = Wageningen, The Netherlands | url = http://www.waterlog.info/pdf/irreff.pdf | format = pdf | deadurl = no | archiveurl = https://web.archive.org/web/20090219070320/http://waterlog.info/pdf/irreff.pdf | archivedate = 19 February 2009 | df = dmy-all }}</ref> All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying [[water table]]. Rapid salination occurs when the land surface is within the [[capillary fringe]] of saline groundwater. [[Soil salinity control]] involves [[watertable control]] and [[leaching model|flushing]] with higher levels of applied water in combination with [[tile drainage]] or another form of [[Drainage system (agriculture)|subsurface drainage]].<ref>{{Cite book | title = Drainage Manual: A Guide to Integrating Plant, Soil, and Water Relationships for Drainage of Irrigated Lands | year = 1993 | publisher = Interior Dept., Bureau of Reclamation | isbn = 978-0-16-061623-5}}</ref><ref name="Waterlog">{{cite web |url=http://www.waterlog.info |title=Free articles and software on drainage of waterlogged land and soil salinity control |accessdate=28 July 2010 |deadurl=no |archiveurl=https://web.archive.org/web/20100816225219/http://www.waterlog.info/ |archivedate=16 August 2010 |df=dmy-all }}</ref>


Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.<ref>{{cite journal |last1=Van der Ploeg |first1=Rienk R. |last2=Schweigert |first2=Peter |last3=Bachmann |first3=Joerg |journal=[[Scientific World Journal]] |volume=1 |issue=S2 |title=Use and misuse of nitrogen in agriculture: the German story |year=2001 |pages=737–744 |doi=10.1100/tsw.2001.263 |pmid=12805882 |pmc=6084271 |doi-access=free}}</ref> In about 1635, the Flemish chemist [[Jan Baptist van Helmont]] thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight.<ref>{{cite web |url=https://www.bbc.co.uk/bitesize/clips/zpgb4wx |title=Van Helmont's experiments on plant growth |website=[[BBC World Service]] |access-date=19 September 2021}}</ref><ref name="Brady"/>{{sfn|Kellogg|1957|p=3}} [[John Woodward (naturalist)|John Woodward]] ({{abbr|d.|died}}&nbsp;1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, [[Jethro Tull (agriculturist)|Jethro Tull]] demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.<ref name="Brady">{{cite book |last=Brady |first=Nyle C. |title=The nature and properties of soils |edition=9th |year=1984 |publisher=[[Collier Macmillan]] |location=New York, New York |isbn=978-0-02-313340-4 |url=https://archive.org/details/natureproperties00brad_0 |access-date=19 September 2021}}</ref>{{sfn|Kellogg|1957|p=2}}
==Reclamation==
Soils which contain high levels of particular clays, such as [[smectite]]s, are often very fertile. For example, the smectite-rich clays of Thailand's [[Central Thailand|Central Plains]] are among the most productive in the world.


As chemistry developed, it was applied to the investigation of soil fertility. The French chemist [[Antoine Lavoisier]] showed in about 1778 that plants and animals must [[Combustion|combust]] oxygen internally to live. He was able to deduce that most of the {{convert|165|lb|adj=on}} weight of van Helmont's willow tree derived from air.<ref>{{cite journal |language=fr |last=de Lavoisier |first=Antoine-Laurent |journal=Mémoires de l'Académie Royale des Sciences |title=Mémoire sur la combustion en général |year=1777 |url=http://www.academie-sciences.fr/pdf/dossiers/Franklin/Franklin_pdf/Mem1777_p592.pdf |access-date=19 September 2021}}</ref> It was the French agriculturalist [[Jean-Baptiste Boussingault]] who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil.<ref>{{cite book |language=fr |last=Boussingault |first=Jean-Baptiste |title=Agronomie, chimie agricole et physiologie, volumes 1–5 |year=1860–1874 |publisher=Mallet-Bachelier |location=Paris, France |url=https://archive.org/details/8TSUP364_1 |access-date=19 September 2021}}</ref> [[Justus von Liebig]] in his book ''Organic chemistry in its applications to agriculture and physiology'' (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced.<ref>{{cite book |last=von Liebig |first=Justus |title=Organic chemistry in its applications to agriculture and physiology |year=1840 |publisher=Taylor and Walton |location=London |url=https://archive.org/details/organicchemistry00liebrich |access-date=19 September 2021}}</ref> Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by [[Alexander von Humboldt]]. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.<ref>{{cite journal |last=Way |first=J. Thomas |journal=Journal of the Royal Agricultural Society of England |title=On the composition and money value of the different varieties of guano |year=1849 |volume=10 |pages=196–230 |url=https://www.biodiversitylibrary.org/item/37078#page/220/mode/1up |access-date=19 September 2021}}</ref>
Many farmers in tropical areas, however, struggle to retain organic matter in the soils they work. In recent years, for example, productivity has declined in the low-clay soils of northern Thailand. Farmers initially responded by adding organic matter from termite mounds, but this was unsustainable in the long-term. Scientists experimented with adding [[bentonite]], one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the [[International Water Management Institute]] in cooperation with [[Khon Kaen University]] and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of 200&nbsp;kg bentonite per rai (6.26 rai = 1 hectare) resulted in an average yield increase of 73%. More work showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.


The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England [[John Bennet Lawes]] and [[Joseph Henry Gilbert]] worked in the [[Rothamsted Research|Rothamsted Experimental Station]], founded by the former, and {{Not a typo|(re)discovered}} that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the [[superphosphate]], consisting in the acid treatment of phosphate rock.{{sfn|Kellogg|1957|p=4}} This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of [[coke (fuel)|coke]] was recovered and used as fertiliser.<ref>{{cite web |last=Tandon |first=Hari L.S. |url=http://www.tandontech.net/fertilisers.html |title=A short history of fertilisers |website=Fertiliser Development and Consultation Organisation |access-date=17 December 2017 |archive-url=https://web.archive.org/web/20170123214241/http://www.tandontech.net/fertilisers.html |archive-date=23 January 2017 |url-status=dead}}</ref> Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms was still not understood.
In 2008, three years after the initial trials, [[International Water Management Institute|IWMI]] scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.<ref name="Water Management Institute2010">{{cite journal |year=2010 |url=http://www.iwmi.cgiar.org/Publications/Success_Stories/PDF/2010/Issue%202%20-%20Improving_soils_and_boosting_yields_in_Thailand.pdf |title=Improving soils and boosting yields in Thailand |doi=10.5337/2011.0031 |journal=Success Stories |issue=2 |deadurl=no |archiveurl=https://web.archive.org/web/20120607030912/http://www.iwmi.cgiar.org/Publications/Success_Stories/PDF/2010/Issue%202%20-%20Improving_soils_and_boosting_yields_in_Thailand.pdf |archivedate=7 June 2012 |df=dmy-all |author=International Water Management Institute}}</ref>


In 1856, J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates,<ref>{{cite journal |last=Way |first=J. Thomas |journal=Journal of the Royal Agricultural Society of England |title=On the power of soils to absorb manure |year=1852 |volume=13 |pages=123–143 |url=https://biodiversitylibrary.org/page/45583402 |access-date=19 September 2021}}</ref> and twenty years later [[Robert Warington]] proved that this transformation was done by living organisms.<ref>{{cite book |last=Warington |first=Robert |title=Note on the appearance of nitrous acid during the evaporation of water: a report of experiments made in the Rothamsted laboratory |url=https://books.google.com/books?id=NlISAQAAMAAJ |year=1878 |publisher=[[Harrison and Sons]] |location=London, United Kingdom |access-date=19 September 2021}}</ref> In 1890 [[Sergei Winogradsky]] announced he had found the bacteria responsible for this transformation.<ref>{{cite journal |last=Winogradsky |first=Sergei |journal=[[Comptes Rendus de l'Académie des Sciences|Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences]] |title=Sur les organismes de la nitrification |language=fr |trans-title=On the organisms of nitrification |year=1890 |volume=110 |issue=1 |pages=1013–1016 |url=https://gallica.bnf.fr/ark:/12148/bpt6k30663/f1087?lang=EN |access-date=19 September 2021}}</ref>
If the soil is too high in clay, adding gypsum, washed river sand and organic matter will balance the composition. Adding organic matter (like [[ramial chipped wood]] for instance) to soil which is depleted in nutrients and too high in sand will boost its quality.<ref>{{Cite journal|title=Provide for your garden's basic needs ... and the plants will take it from there|date=10 March 2011|journal=USA Weekend|url=http://www.usaweekend.com/article/20110311/HOME04/103130305|archive-url=https://archive.today/20130209012951/http://www.usaweekend.com/article/20110311/HOME04/103130305|dead-url=yes|archive-date=9 February 2013}}</ref>


It was known that certain [[legume]]s could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist [[Hermann Hellriegel]] and the Dutch microbiologist [[Martinus Beijerinck]].{{sfn|Kellogg|1957|p=4}}
==See also==


Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.{{sfn|Kellogg|1957|pp=1–4}}

===Studies of soil formation===
{{See also|Soil formation}}
Scientists who studied soil in connection with agricultural practices considered it mainly a static substrate. However, the soil is the result of evolution from more ancient geological materials under the action of biotic and abiotic processes. After studies of soil improvement commenced, other researchers began to study soil genesis and, as a result, soil types and classifications.

In 1860, while in Mississippi, [[Eugene W. Hilgard]] (1833–1916) studied the relationship between rock material, climate, vegetation, and the type of soils that were developed. He realised that the soils were dynamic and considered the classification of soil types.<ref>{{cite book |last=Hilgard |first=Eugene W. |title=Soils: their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions |year=1907 |publisher=[[The Macmillan Company]] |location=London, United Kingdom |url=https://www.biodiversitylibrary.org/bibliography/24461 |access-date=19 September 2021}}</ref> (See also at [https://www.gutenberg.org/ebooks/73975 Project Gutenberg]). His work was not continued. At about the same time, [[Friedrich Albert Fallou]] described soil profiles and related soil characteristics to their formation as part of his professional work evaluating forest and farmland for the principality of [[Saxony]]. His 1857 book, {{Lang|de|Anfangsgründe der Bodenkunde}} (First Principles of soil science), established modern soil science.<ref>{{cite book |language=de |last=Fallou |first=Friedrich Albert |title=Anfangsgründe der Bodenkunde |year= 1857 |publisher=G. Schönfeld's Buchhandlung |location= Dresden, Germany |url=http://digital.slub-dresden.de/fileadmin/data/321768043/321768043_tif/jpegs/321768043.pdf |access-date=15 December 2018 |archive-url=https://web.archive.org/web/20181215223343/http://digital.slub-dresden.de/fileadmin/data/321768043/321768043_tif/jpegs/321768043.pdf |archive-date=15 December 2018 |url-status=dead}}</ref> Contemporary with Fallou's work, and driven by the same need to accurately assess land for equitable taxation, Vasily Dokuchaev led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to Western Europe until 1914 through a publication in German by [[Konstantin Glinka]], a member of the Russian team.<ref>{{cite book |language=de |last=Glinka |first=Konstantin Dmitrievich |title=Die Typen der Bodenbildung: ihre Klassifikation und geographische Verbreitung |year=1914 |publisher=[[Borntraeger]] |location=Berlin, Germany}}</ref>

[[Curtis F. Marbut]], influenced by the work of the Russian team, translated Glinka's publication into English,<ref>{{cite book |last=Glinka |first=Konstantin Dmitrievich |title=The great soil groups of the world and their development |url=http://reader.library.cornell.edu/docviewer/digital?id=chla3055800#mode/1up |year=1927 |publisher=Edwards Brothers |location=Ann Arbor, Michigan |access-date=19 September 2021}}</ref> and, as he was placed in charge of the U.S. [[National Cooperative Soil Survey]], applied it to a national soil classification system.<ref name="Brady"/>

==See also==
{{portal|Environment|Geology}}
{{div col|content=
{{div col|content=
* [[Acid sulfate soil]]
*[[Acid sulfate soil]]
*[[Agricultural science]]
* [[Agrophysics]]
* [[Alkaline soil]]
*[[Agrophysics]]
*[[Crust (geology)|Crust]]
* [[Biochar]]
*[[Factors affecting permeability of soils]]
* [[Crust (geology)|Crust]]
*[[Index of soil-related articles]]
* [[Geoponic]]
*[[Lunar soil]] and [[martian soil]]
* [[Factors affecting permeability of soils]]
* [[Index of soil-related articles]]
*[[Mycorrhizal fungi and soil carbon storage]]
*[[Red soil]]
* [[Mineral matter in plants]]
*[[Shrink–swell capacity]]
* [[Mycorrhizal fungi and soil carbon storage]]
* [[Nitrogen cycle]]
*[[Soil biodiversity]]
*[[Soil liquefaction]]
* [[Red Mediterranean soil]]
*[[Soil moisture velocity equation]]
* [[Saline soil]]
* [[Shrink-swell capacity]]
*[[Soil zoology]]
* [[Soil management]]
*[[Tillage erosion]]
* [[Soil zoology]]
*[[World Soil Museum]]
* [[World Soil Museum]]
}}
}}
{{wikiquote}}
{{Commons category|Soils}}


==References==
== References ==
{{clear}}
;Citations
{{reflist}}
{{reflist}}


;Sources
==Sources==
{{Free-content attribution
| title = Drowning in Plastics – Marine Litter and Plastic Waste Vital Graphics
| publisher = United Nations Environment Programme
| documentURL = https://www.unep.org/resources/report/drowning-plastics-marine-litter-and-plastic-waste-vital-graphics
| license statement URL = https://commons.wikimedia.org/wiki/File:United_Nations_Environment_Programme_Drowning_in_Plastics_%E2%80%93_Marine_Litter_and_Plastic_Waste_Vital_Graphics.pdf
| license = Cc BY-SA 3.0 IGO
}}

==Bibliography==
{{refbegin}}
{{refbegin}}
* {{cite book |title=Soils: An Introduction to Soils and Plant Growth |last1=Donahue |first1=Roy Luther |last2=Miller |first2=Raymond W. |last3=Shickluna |first3=John C. |year=1977 |publisher=Prentice-Hall |isbn=978-0-13-821918-5 |ref=harv}}
*{{cite book |last1=Donahue |first1=Roy Luther |title=Soils: An Introduction to Soils and Plant Growth |last2=Miller |first2=Raymond W. |last3=Shickluna |first3=John C. |year=1977 |publisher=[[Prentice-Hall]] |isbn=978-0-13-821918-5 |url=https://archive.org/details/soilsintroductio00dona}}
* {{cite web|title=Arizona Master Gardener|url=http://ag.arizona.edu/pubs/garden/mg/soils/soils.html|publisher=Cooperative Extension, College of Agriculture, University of Arizona|accessdate=27 May 2013}}
*{{cite web |title=Arizona Master Gardener |url=http://ag.arizona.edu/pubs/garden/mg/soils/soils.html|publisher=Cooperative Extension, College of Agriculture, [[University of Arizona]] |access-date=27 May 2013}}
* {{cite book |title=Soil: The Yearbook of Agriculture 1957 |editor-last=Stefferud |editor-first=Alfred |year=1957 |publisher=United States Department of Agriculture |url=//archive.org/stream/yoa1957#page/n18/mode/1up |oclc=704186906 |ref=harv}}
*{{cite book |editor-last=Stefferud |editor-first=Alfred |title=Soil: The Yearbook of Agriculture 1957 |year=1957 |publisher=United States Department of Agriculture |url=https://archive.org/stream/yoa1957#page/n18/mode/1up |oclc=704186906}}
** {{harvc |name-list-format=harv |last=Kellogg |first=Charles E. |chapter=We Seek; We Learn |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n17/mode/1up }}
**{{harvc |name-list-style=harv |last=Kellogg |first=Charles E. |chapter=We Seek; We Learn |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n17/mode/1up}}
** {{harvc |name-list-format=harv |last=Simonson |first=Roy W. |chapter=What Soils Are |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n34/mode/1up }}
**{{harvc |name-list-style=harv |last=Simonson |first=Roy W. |chapter=What Soils Are |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n34/mode/1up}}
** {{harvc |name-list-format=harv |last=Russell |first=M.B. |chapter=Physical Properties |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n49/mode/1up }}
**{{harvc |name-list-style=harv |last=Russell |first=M.B. |chapter=Physical Properties |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n49/mode/1up}}
** {{harvc |name-list-format=harv |last=Richards |first=L.A. |last2=Richards |chapter=Soil Moisture |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n68/mode/1up }}
**{{harvc |name-list-style=harv |last=Dean |first=L.A. |chapter=Plant Nutrition and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n100/mode/1up}}
** {{harvc |name-list-format=harv |last=Wadleigh |first=C.H. |chapter=Growth of Plants |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n57/mode/1up }}
**{{harvc |name-list-style=harv |last=Russel |first=Darrell A. |chapter=Boron and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n145/mode/1up |oclc=704186906}}
** {{harvc |name-list-format=harv |last=Allaway |first=W.H. |chapter=pH, Soil Acidity, and Plant Growth |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n87/mode/1up }}
** {{harvc |name-list-format=harv |last=Coleman |first=N.T. |last2=Mehlich |chapter=The Chemistry of Soil pH |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n92/mode/1up }}
** {{harvc |name-list-format=harv |last=Dean |first=L.A. |chapter=Plant Nutrition and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n100/mode/1up }}
** {{harvc |name-list-format=harv |last=Allison |first=Franklin E. |chapter=Nitrogen and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n105/mode/1up |oclc=704186906|ref=harv}}
** {{harvc |name-list-format=harv |last=Olsen |first=Sterling R. |last2=Fried |chapter=Soil Phosphorus and Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n115/mode/1up }}
** {{harvc |name-list-format=harv |last=Reitemeier |first=R.F. |chapter=Soil Potassium and Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n123/mode/1up }}
** {{harvc |name-list-format=harv |last=Jordan |first=Howard V. |last2=Reisenauer |chapter=Sulfur and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n129/mode/1up }}
** {{harvc |name-list-format=harv |last=Holmes |first=R.S. |last2=Brown |chapter=Iron and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n133/mode/1up }}
** {{harvc |name-list-format=harv |last=Seatz |first=Lloyd F. |last2=Jurinak |chapter=Zinc and Soil Fertility|in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n138/mode/1up }}
** {{harvc |name-list-format=harv |last=Russel |first=Darrell A. |chapter=Boron and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n145/mode/1up |oclc=704186906|ref=harv}}
** {{harvc |name-list-format=harv |last=Reuther |first=Walter |chapter=Copper and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n154/mode/1up |oclc=704186906|ref=harv}}
** {{harvc |name-list-format=harv |last=Sherman |first=G. Donald |chapter=Manganese and Soil Fertility|in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n162/mode/1up }}
** {{harvc |name-list-format=harv |last=Stout |first=P.R. |last2=Johnson |chapter=Trace Elements |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n167/mode/1up }}
** {{harvc |name-list-format=harv |last=Broadbent |first=F.E. |chapter=Organic Matter |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n179/mode/1up }}
** {{harvc |name-list-format=harv |last=Clark |first=Francis E. |chapter=Living Organisms in the Soil |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n185/mode/1up }}
** {{harvc |name-list-format=harv |last=Flemming |first=Walter E. |chapter=Soil Management and Insect Control |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n367/mode/1up }}


{{refend}}
{{refend}}
Line 1,046: Line 463:
==Further reading==
==Further reading==
{{refbegin|colwidth=33em}}
{{refbegin|colwidth=33em}}
* [http://www.soil-net.com/ Soil-Net.com] A free schools-age educational site teaching about soil and its importance.
*[http://www.soil-net.com/ Soil-Net.com] {{Webarchive|url=https://web.archive.org/web/20080710061716/http://www.soil-net.com/ |date=10 July 2008 }} A free schools-age educational site teaching about soil and its importance.
* Adams, J.A. 1986. ''Dirt''. College Station, Texas: Texas A&M University Press {{ISBN|0-89096-301-0}}
*Adams, J.A. 1986. ''Dirt''. College Station, Texas: Texas A&M University Press {{ISBN|0-89096-301-0}}
* Certini, G., Scalenghe, R. 2006. Soils: Basic concepts and future challenges. Cambridge Univ Press, Cambridge.
*Certini, G., Scalenghe, R. 2006. Soils: Basic concepts and future challenges. Cambridge Univ Press, Cambridge.
* [[David R. Montgomery]], ''Dirt: The Erosion of Civilizations'', {{ISBN|978-0-520-25806-8}}
*[[David R. Montgomery|Montgomery, David R.]], ''Dirt: The Erosion of Civilizations'' (U of California Press, 2007), {{ISBN|978-0-520-25806-8}}
* Faulkner, Edward H. 1943. Plowman's Folly. New York, Grosset & Dunlap. {{ISBN|0-933280-51-3}}
*Faulkner, Edward H. ''Plowman's Folly'' (New York, Grosset & Dunlap, 1943). {{ISBN|0-933280-51-3}}
* [http://www.landis.org.uk/soilscapes/ LandIS Free Soilscapes Viewer] Free interactive viewer for the Soils of England and Wales
*[https://web.archive.org/web/20080705133103/http://www.landis.org.uk/soilscapes LandIS Free Soilscapes Viewer] Free interactive viewer for the Soils of England and Wales
* Jenny, Hans. 1941. [http://www.soilandhealth.org/01aglibrary/010159.Jenny.pdf Factors of Soil Formation: A System of Quantitative Pedology]
*Jenny, Hans. 1941. [https://web.archive.org/web/20130225050838/http://soilandhealth.org/01aglibrary/010159.Jenny.pdf Factors of Soil Formation: A System of Quantitative Pedology]
* Logan, W.B. 1995. Dirt: The ecstatic skin of the earth. {{ISBN|1-57322-004-3}}
*Logan, W.B. ''Dirt: The ecstatic skin of the earth'' (1995). {{ISBN|1-57322-004-3}}
* Mann, Charles C. September 2008. " Our good earth" ''National Geographic Magazine''
*Mann, Charles C. September 2008. " Our good earth" ''National Geographic Magazine''
* {{cite web|url=http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm |title=97 Flood |publisher=USGS |accessdate=8 July 2008 |deadurl=yes |archiveurl=https://web.archive.org/web/20080624040143/http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm |archivedate=24 June 2008 }} Photographs of sand boils.
* Soil Survey Division Staff. 1999. ''Soil survey manual''. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
* Soil Survey Staff. 1975. ''Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys.'' USDA-SCS Agric. Handb. 436. United States Government Printing Office, Washington, DC.
* [https://web.archive.org/web/20060828063956/http://forages.oregonstate.edu/is/ssis/main.cfm?PageID=3 Soils (Matching suitable forage species to soil type)], Oregon State University
* {{cite web|url= http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html|title= Lecture 1 Chapter 1 Why Study Soils?|access-date= 2019-01-07|last= Gardiner|first= Duane T|work= ENV320: Soil Science Lecture Notes|publisher= Texas A&M University-Kingsville|archive-url= https://web.archive.org/web/20180209052922/http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html|archive-date= 9 February 2018|dead-url= yes|df= dmy-all}}
* Janick, Jules. 2002. [https://web.archive.org/web/20050317030248/http://www.hort.purdue.edu/newcrop/tropical/lecture_06/chapter_12l_R.html Soil notes], Purdue University
* [http://www.landis.org.uk/ LandIS Soils Data for England and Wales] a pay source for GIS data on the soils of England and Wales and soils data source; they charge a handling fee to researchers.
{{refend}}


==External links==
==External links==
*{{cite web|url=http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm |title=97 Flood |publisher=USGS |access-date=8 July 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080624040143/http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm |archive-date=24 June 2008}} Photographs of sand boils.
*Soil Survey Division Staff. 1999. ''Soil survey manual''. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
*Soil Survey Staff. 1975. ''Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys.'' USDA-SCS Agric. Handb. 436. United States Government Printing Office, Washington, DC.
*[https://web.archive.org/web/20060828063956/http://forages.oregonstate.edu/is/ssis/main.cfm?PageID=3 Soils (Matching suitable forage species to soil type)], Oregon State University
*{{cite web|url= http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html|title= Lecture 1 Chapter 1 Why Study Soils?|access-date= 7 January 2019|last= Gardiner|first= Duane T|website= ENV320: Soil Science Lecture Notes|publisher= Texas A&M University-Kingsville|archive-url= https://web.archive.org/web/20180209052922/http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html|archive-date= 9 February 2018|url-status=dead|df= dmy-all}}
*Janick, Jules. 2002. [https://web.archive.org/web/20050317030248/http://www.hort.purdue.edu/newcrop/tropical/lecture_06/chapter_12l_R.html Soil notes], Purdue University
*[http://www.landis.org.uk/ LandIS Soils Data for England and Wales] {{Webarchive|url=https://web.archive.org/web/20070716033248/http://www.landis.org.uk/ |date=16 July 2007 }} a pay source for GIS data on the soils of England and Wales and soils data source; they charge a handling fee to researchers.
{{refend}}

{{wiktionary|soil}}
{{wikiversity|Soil Formation}}
{{wikiversity|Soil Formation}}
{{Wikibooks |Historical Geology|Soils and paleosols}}
{{Wikibooks |Historical Geology|Soils and paleosols}}
{{Commons category|Soils}}
{{Wikiquote}}
{{div col|content=
{{div col|content=
*[https://www.theguardian.com/environment/video/2019/jul/11/its-time-we-stopped-treating-soil-like-dirt-video Short video explaining soil basics]
* [http://www.edaphic.com.au/soil-water-compendium/ The Soil Water Compendium (soil water content sensors explained)]
*[http://www.edaphic.com.au/soil-water-compendium/ The Soil Water Compendium (soil water content sensors explained)]
* [http://www.fao.org/globalsoilpartnership/en/ Global Soil Partnership]
* [http://www.fao.org/soils-portal/en/ FAO Soils Portal]
*[http://www.fao.org/globalsoilpartnership/en/ Global Soil Partnership]
* [http://www.fao.org/ag/agl/agll/wrb/ World Reference Base for Soil Resources]
*[http://www.fao.org/soils-portal/en/ FAO Soils Portal]
* [http://www.isric.org/ ISRIC – World Soil Information (ICSU World Data Centre for Soils)]
*[https://wrb.isric.org/ World Reference Base for Soil Resources]
* [http://www.isric.org/explore/library World Soil Library and Maps]
*[https://www.isric.org/ ISRIC – World Soil Information (ISC World Data Centre for Soils)]
* [http://www.wossac.com/ Wossac the world soil survey archive and catalogue]
*[https://www.isric.org/explore/library ISRIC -World Soil Library and Maps]
* [http://csss.ca/ Canadian Society of Soil Science]
*[https://wsm.isric.org/ ISRIC - World Soil Museum (WSM virtual)]
* [https://www.soils.org/ Soil Science Society of America]
*[https://data.isric.org/ ISRIC - Soil data hub]
*[http://www.wossac.com/ Wossac the world soil survey archive and catalogue]
* [http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm USDA-NRCS Web Soil Survey]
* [http://eusoils.jrc.ec.europa.eu/ European Soil Portal] (wiki)
*[http://csss.ca/ Canadian Society of Soil Science]
*[https://www.soils.org/ Soil Science Society of America]
* [http://www.cranfield.ac.uk/sas/nsri National Soil Resources Institute UK]
* [http://passel.unl.edu/ Plant and Soil Sciences eLibrary]
*[http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm USDA-NRCS Web Soil Survey]
*[http://eusoils.jrc.ec.europa.eu/ European Soil Portal] (wiki)
* [https://archive.org/details/yoa1957 Copies of the reference 'Soil: The Yearbook of Agriculture 1957' in multiple formats]
*[http://www.cranfield.ac.uk/sas/nsri National Soil Resources Institute UK]
*[http://passel.unl.edu/ Plant and Soil Sciences eLibrary]
*[https://archive.org/details/yoa1957 Copies of the reference 'Soil: The Yearbook of Agriculture 1957' in multiple formats]
}}
}}


{{Use dmy dates|date=November 2014}}
{{Use dmy dates|date=June 2019}}

{{Soil science topics}}
{{Soil science topics}}
{{Geotechnical engineering}}
{{Geotechnical engineering}}
{{Natural resources}}
{{Natural resources}}

{{Authority control}}
{{Authority control}}


[[Category:Soil| ]]
[[Category:Soil| ]]
[[Category:Land management]]
[[Category:Land management]]
[[Category:Horticulture and gardening]]
[[Category:Horticulture]]
[[Category:Granularity of materials]]
[[Category:Granularity of materials]]
[[Category:Natural materials]]
[[Category:Natural materials]]

Latest revision as of 16:26, 22 December 2024

Surface-water-gley developed in glacial till in Northern Ireland

Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.

Soil measuring and surveying device

Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a porous phase that holds gases (the soil atmosphere) and water (the soil solution).[1][2] Accordingly, soil is a three-state system of solids, liquids, and gases.[3] Soil is a product of several factors: the influence of climate, relief (elevation, orientation, and slope of terrain), organisms, and the soil's parent materials (original minerals) interacting over time.[4] It continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion.[5] Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem.[6]

Most soils have a dry bulk density (density of soil taking into account voids when dry) between 1.1 and 1.6 g/cm3, though the soil particle density is much higher, in the range of 2.6 to 2.7 g/cm3.[7] Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic,[8] although fossilized soils are preserved from as far back as the Archean.[9]

Collectively the Earth's body of soil is called the pedosphere. The pedosphere interfaces with the lithosphere, the hydrosphere, the atmosphere, and the biosphere.[10] Soil has four important functions:

All of these functions, in their turn, modify the soil and its properties.

Soil science has two basic branches of study: edaphology and pedology. Edaphology studies the influence of soils on living things.[11] Pedology focuses on the formation, description (morphology), and classification of soils in their natural environment.[12] In engineering terms, soil is included in the broader concept of regolith, which also includes other loose material that lies above the bedrock, as can be found on the Moon and other celestial objects.[13]

Processes

[edit]

Soil is a major component of the Earth's ecosystem. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, with effects ranging from ozone depletion and global warming to rainforest destruction and water pollution. With respect to Earth's carbon cycle, soil acts as an important carbon reservoir,[14] and it is potentially one of the most reactive to human disturbance[15] and climate change.[16] As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback (amplification).[17] This prediction has, however, been questioned on consideration of more recent knowledge on soil carbon turnover.[18]

Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, and a medium for plant growth, making it a critically important provider of ecosystem services.[19] Since soil has a tremendous range of available niches and habitats, it contains a prominent part of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored.[20][21] Soil has a mean prokaryotic density of roughly 108 organisms per gram,[22] whereas the ocean has no more than 107 prokaryotic organisms per milliliter (gram) of seawater.[23] Organic carbon held in soil is eventually returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter; tillage usually increases the rate of soil respiration, leading to the depletion of soil organic matter.[24] Since plant roots need oxygen, aeration is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected soil pores, which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival.[25]

Soils can effectively remove impurities,[26] kill disease agents,[27] and degrade contaminants, this latter property being called natural attenuation.[28] Typically, soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide.[29] Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins.[30] Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.[31]

Composition

[edit]
This is a diagram and related photograph of soil layers from bedrock to soil.
A, B, and C represent the soil profile, a notation firstly coined by Vasily Dokuchaev (1846–1903), the father of pedology. Here, A is the topsoil; B is a regolith; C is a saprolite (a less-weathered regolith); the bottom-most layer represents the bedrock.

Components of a silt loam soil by percent volume

  Water (25%)
  Gases (25%)
  Sand (18%)
  Silt (18%)
  Clay (9%)
  Organic matter (5%)

A typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas.[32] The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other.[33] The pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil.[34] Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.[35]

Given sufficient time, an undifferentiated soil will evolve a soil profile that consists of two or more layers, referred to as soil horizons. These differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity.[8] The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type of parent material, the processes that modify those parent materials, and the soil-forming factors that influence those processes. The biological influences on soil properties are strongest near the surface, though the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The solum normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon.[36] It has been suggested that the pedon, a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the humipedon (the living part, where most soil organisms are dwelling, corresponding to the humus form), the copedon (in intermediary position, where most weathering of minerals takes place) and the lithopedon (in contact with the subsoil).[37]

The soil texture is determined by the relative proportions of the individual particles of sand, silt, and clay that make up the soil.

A soil texture triangle plot is a visual representation of the proportions of sand, silt, and clay in a soil sample.

The interaction of the individual mineral particles with organic matter, water, gases via biotic and abiotic processes causes those particles to flocculate (stick together) to form aggregates or peds.[38] Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction (acidity), etc.

Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed.[39] The mixture of water and dissolved or suspended materials that occupy the soil pore space is called the soil solution. Since soil water is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called the soil solution. Water is central to the dissolution, precipitation and leaching of minerals from the soil profile. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.[40]

Soils supply plants with nutrients, most of which are held in place by particles of clay and organic matter (colloids)[41] The nutrients may be adsorbed on clay mineral surfaces, bound within clay minerals (absorbed), or bound within organic compounds as part of the living organisms or dead soil organic matter. These bound nutrients interact with soil water to buffer the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.[42]

Plant nutrient availability is affected by soil pH, which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acidic) where weathering is more advanced.[43]

Most plant nutrients, with the exception of nitrogen, originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute nitric acid and ammonia,[44] but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and the soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby to soil formation and soil fertility.[45] Microbial soil enzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil by volatilisation (loss to the atmosphere as gases) or leaching.[46]

Formation

[edit]

Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum, producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time.[47] These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons. However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars[48] and analogous conditions in planet Earth deserts.[49]

An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage nitrogen-fixing lichens and cyanobacteria then epilithic higher plants) become established very quickly on basaltic lava, even though there is very little organic material.[50] Basaltic minerals commonly weather relatively quickly, according to the Goldich dissolution series.[51] The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering mycorrhizal fungi[52] that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes,[53] inselbergs,[54] and glacial moraines.[55]

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil: parent material, climate, topography (relief), organisms, and time.[56] When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.[57]

Physical properties

[edit]

The physical properties of soils, in order of decreasing importance for ecosystem services such as crop production, are texture, structure, bulk density, porosity, consistency, temperature, colour and resistivity.[58] Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: sand, silt, and clay. At the next larger scale, soil structures called peds or more commonly soil aggregates are created from the soil separates when iron oxides, carbonates, clay, silica and humus, coat particles and cause them to adhere into larger, relatively stable secondary structures.[59] Soil bulk density, when determined at standardized moisture conditions, is an estimate of soil compaction.[60] Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil.[61] These properties vary through the depth of a soil profile, i.e. through soil horizons. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.[62]

Soil moisture

[edit]

Soil water content can be measured as volume or weight. Soil moisture levels, in order of decreasing water content, are saturation, field capacity, wilting point, air dry, and oven dry. Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity. Irrigating soil above field capacity risks percolation losses. Wilting point describes the dry limit for growing plants. During growing season, soil moisture is unaffected by functional groups or specie richness.[63]

Available water capacity is the amount of water held in a soil profile available to plants. As water content drops, plants have to work against increasing forces of adhesion and sorptivity to withdraw water. Irrigation scheduling avoids moisture stress by replenishing depleted water before stress is induced.[64][65]

Capillary action is responsible for moving groundwater from wet regions of the soil to dry areas. Subirrigation designs (e.g., wicking beds, sub-irrigated planters) rely on capillarity to supply water to plant roots. Capillary action can result in an evaporative concentration of salts, causing land degradation through salination.

Soil moisture measurement—measuring the water content of the soil, as can be expressed in terms of volume or weight—can be based on in situ probes (e.g., capacitance probes, neutron probes), or remote sensing methods. Soil moisture measurement is an important factor in determining changes in soil activity.[63]

Soil gas

[edit]

The atmosphere of soil, or soil gas, is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO2 concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.[66] Calcareous soils regulate CO2 concentration by carbonate buffering, contrary to acid soils in which all CO2 respired accumulates in the soil pore system.[67] At extreme levels, CO2 is toxic.[68] This suggests a possible negative feedback control of soil CO2 concentration through its inhibitory effects on root and microbial respiration (also called soil respiration).[69] In addition, the soil voids are saturated with water vapour, at least until the point of maximal hygroscopicity, beyond which a vapour-pressure deficit occurs in the soil pore space.[34] Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction.[70] Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including greenhouse gases) as well as water.[71] Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total pore space (porosity) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil.[72][71] Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO3 to the gases N2, N2O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called denitrification.[73] Aerated soil is also a net sink of methane (CH4)[74] but a net producer of methane (a strong heat-absorbing greenhouse gas) when soils are depleted of oxygen and subject to elevated temperatures.[75]

Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots,[76] bacteria,[77] fungi,[78] animals.[79] These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks[80][81] playing a decisive role in the stability, dynamics and evolution of soil ecosystems.[82] Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.[83]

Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,[84] a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin.

Solid phase (soil matrix)

[edit]

Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential,[85] but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.[86]

Soil biodiversity

[edit]

Large numbers of microbes, animals, plants and fungi are living in soil. However, biodiversity in soil is much harder to study as most of this life is invisible, hence estimates about soil biodiversity have been unsatisfactory. A recent study suggested that soil is likely home to 59 ± 15% of the species on Earth. Enchytraeidae (worms) have the greatest percentage of species in soil (98.6%), followed by fungi (90%), plants (85.5%), and termites (Isoptera) (84.2%). Many other groups of animals have substantial fractions of species living in soil, e.g. about 30% of insects, and close to 50% of arachnids.[87] While most vertebrates live above ground (ignoring aquatic species), many species are fossorial, that is, they live in soil, such as most blind snakes.

Chemistry

[edit]

The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic colloids that determines soil's chemical properties.[88] A colloid is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer, thus small enough to remain suspended by Brownian motion in a fluid medium without settling.[89] Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays. The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions. Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. Cation-exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil (or centimoles of positive charge per kilogram of soil; cmolc/kg). Similarly, positively charged sites on colloids can attract and release anions in the soil, giving the soil anion exchange capacity.

Cation and anion exchange

[edit]

The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.

The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.[90]

  1. Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.[91] Substitutions in the outermost layers are more effective than for the innermost layers, as the electric charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
  2. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.[92]
  3. Hydroxyls may substitute for oxygens of the silica layers, a process called hydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).[93]
  4. Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.[94]

Cations held to the negatively charged colloids resist being washed downward by water and are out of reach of plant roots, thereby preserving the soil fertility in areas of moderate rainfall and low temperatures.[95][96]

There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another (ion exchange). If present in equal amounts in the soil water solution:

Al3+ replaces H+ replaces Ca2+ replaces Mg2+ replaces K+ same as NH+
4
replaces Na+[97]

If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action. This is largely what occurs with the addition of cationic fertilisers (potash, lime).[98]

As the soil solution becomes more acidic (low pH, meaning an abundance of H+), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites (protonation). A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxy groups on the surface of soil colloids creates what is described as pH-dependent surface charges.[99] Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.[100] Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.[101] Plants are able to excrete H+ into the soil through the synthesis of organic acids and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.[102]

Cation exchange capacity (CEC)

[edit]

Cation exchange capacity is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution.[103] CEC is the amount of exchangeable hydrogen cation (H+) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40 ÷ 2) × 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g.[104] The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.

Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (such as tropical rainforests), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.[105] Live plant roots also have some CEC, linked to their specific surface area.[106]

Cation exchange capacity for soils; soil textures; soil colloids[107]
Soil State CEC meq/100 g
Charlotte fine sand Florida 1.0
Ruston fine sandy loam Texas 1.9
Glouchester loam New Jersey 11.9
Grundy silt loam Illinois 26.3
Gleason clay loam California 31.6
Susquehanna clay loam Alabama 34.3
Davie mucky fine sand Florida 100.8
Sands 1–5
Fine sandy loams 5–10
Loams and silt loams 5–15
Clay loams 15–30
Clays over 30
Sesquioxides 0–3
Kaolinite 3–15
Illite 25–40
Montmorillonite 60–100
Vermiculite (similar to illite) 80–150
Humus 100–300

Anion exchange capacity (AEC)

[edit]

Anion exchange capacity is the soil's ability to remove anions (such as nitrate, phosphate) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution.[108] Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC,[109] followed by the iron oxides.[110] Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils.[111] Phosphates tend to be held at anion exchange sites.[112]

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH) for other anions.[108] The order reflecting the strength of anion adhesion is as follows:

H
2
PO
4
replaces SO2−
4
replaces NO
3
replaces Cl

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil.[107] As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).[113]

Reactivity (pH)

[edit]

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydronium concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.[114]

At 25 °C an aqueous solution that has a pH of 3.5 has 10−3.5 moles H3O+ (hydronium ions) per litre of solution (and also 10−10.5 moles per litre OH). A pH of 7, defined as neutral, has 10−7 moles of hydronium ions per litre of solution and also 10−7 moles of OH per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10−9.5 moles hydronium ions per litre of solution (and also 10−2.5 moles per litre OH). A pH of 3.5 has one million times more hydronium ions per litre than a solution with pH of 9.5 (9.5 − 3.5 = 6 or 106) and is more acidic.[115]

The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese.[116] As a result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH,[117] although most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5.[118] Given that at low pH toxic metals (e.g. cadmium, zinc, lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both made more available to organisms,[119][120] it has been suggested that plants, animals and microbes commonly living in acid soils are pre-adapted to every kind of pollution, whether of natural or human origin.[121]

In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual rain acidity against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in tropical rainforests.[122] Once the colloids are saturated with H3O+, the addition of any more hydronium ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity.[123] In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil.[124] In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH 10.[125] Beyond a pH of 9, plant growth is reduced.[126] High pH results in low micro-nutrient mobility, but water-soluble chelates of those nutrients can correct the deficit.[127] Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.[128][129]

Base saturation percentage

[edit]

There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called base saturation. If a soil has a CEC of 20 meq and 5 meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids (20 − 5 = 15 meq) are assumed occupied by base-forming cations, so that the base saturation is 15 ÷ 20 × 100% = 75% (the compliment 25% is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH).[130] It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity).[131] The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.[132]

Buffering

[edit]

The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, though soils high in colloids (whether mineral or organic) have high buffering capacity.[133] Buffering occurs by cation exchange and neutralisation. However, colloids are not the only regulators of soil pH. The role of carbonates should be underlined, too.[134] More generally, according to pH levels, several buffer systems take precedence over each other, from calcium carbonate buffer range to iron buffer range.[135]

The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydronium ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.

The addition of a small amount of lime, Ca(OH)2, will displace hydronium ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO2 and water, with little permanent change in soil pH.

The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.[136]

Redox

[edit]

Soil chemical reactions involve some combination of proton and electron transfer. Oxidation occurs if there is a loss of electrons in the transfer process while reduction occurs if there is a gain of electrons. Reduction potential is measured in volts or millivolts. Soil microbial communities develop along electron transport chains, forming electrically conductive biofilms, and developing networks of bacterial nanowires.

Redox factors in soil development, where formation of redoximorphic color features provides critical information for soil interpretation. Understanding the redox gradient is important to managing carbon sequestration, bioremediation, wetland delineation, and soil-based microbial fuel cells.

Nutrients

[edit]
Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake[137]
Element Symbol Ion or molecule
Carbon C CO2 (mostly through leaves)
Hydrogen H H+, H2O (water)
Oxygen O O2−, OH, CO2−
3
, SO2−
4
, CO2
Phosphorus P H
2
PO
4
, HPO2−
4
(phosphates)
Potassium K K+
Nitrogen N NH+
4
, NO
3
(ammonium, nitrate)
Sulfur S SO2−
4
Calcium Ca Ca2+
Iron Fe Fe2+, Fe3+ (ferrous, ferric)
Magnesium Mg Mg2+
Boron B H3BO3, H
2
BO
3
, B(OH)
4
Manganese Mn Mn2+
Copper Cu Cu2+
Zinc Zn Zn2+
Molybdenum Mo MoO2−
4
(molybdate)
Chlorine Cl Cl (chloride)

Seventeen elements or nutrients are essential for plant growth and reproduction. They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl).[138][139][140] Nutrients required for plants to complete their life cycle are considered essential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation,[140] the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant.[141] A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.[142]

Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an ionic form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.[143]

The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.[144]

Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals, most of the soil cation exchange capacity arising from charged carboxylic groups on organic matter.[145] However, despite the great capacity of humus to retain water once water-soaked, its high hydrophobicity decreases its wettability once dry.[146] All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.[147][144]

Soil organic matter

[edit]

The organic material in soil is made up of organic compounds and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 lb of earthworms, 2400 lb of fungi, 1500 lb of bacteria, 133 lb of protozoa and 890 lb of arthropods and algae.[148]

A few percent of the soil organic matter, with small residence time, consists of the microbial biomass and metabolites of bacteria, molds, and actinomycetes that work to break down the dead organic matter.[149][150] Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. However, in the same time soil microbes contribute to carbon sequestration in the topsoil through the formation of stable humus.[151] In the aim to sequester more carbon in the soil for alleviating the greenhouse effect it would be more efficient in the long-term to stimulate humification than to decrease litter decomposition.[152]

The main part of soil organic matter is a complex assemblage of small organic molecules, collectively called humus or humic substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete.[153] Other studies showed that the classical notion of molecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct from polysaccharides, lignins and proteins.[154]

Most living things in soils, including plants, animals, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition, the rate of which is dependent on the temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by protozoa, which in turn are fed upon by nematodes, annelids and arthropods, themselves able to consume and transform raw or humified organic matter. This has been called the soil food web, through which all organic matter is processed as in a digestive system.[155] Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat (histosols), are infertile.[156] In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.

In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker A horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor (O horizon) and thin A horizon.[157]

Humus

[edit]

Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to soil health and plant growth.[158] Humus also feeds arthropods, termites and earthworms which further improve the soil.[159] The end product, humus, is suspended in colloidal form in the soil solution and forms a weak acid that can attack silicate minerals by chelating their iron and aluminum atoms.[160] Humus has a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.[161]

Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus.[162] As the residues break down, only molecules made of aliphatic and aromatic hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus.[154] Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure.[161] Although the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.[163]

Lignin is resistant to breakdown and accumulates within the soil. It also reacts with proteins,[164] which further increases its resistance to decomposition, including enzymatic decomposition by microbes.[165] Fats and waxes from plant matter have still more resistance to decomposition and persist in soils for thousand years, hence their use as tracers of past vegetation in buried soil layers.[166] Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay.[167] Proteins normally decompose readily, to the exception of scleroproteins, but when bound to clay particles they become more resistant to decomposition.[168] As for other proteins clay particles absorb the enzymes exuded by microbes, decreasing enzyme activity while protecting extracellular enzymes from degradation.[169] The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years.[170] A study showed increased soil fertility following the addition of mature compost to a clay soil.[171] High soil tannin content can cause nitrogen to be sequestered as resistant tannin-protein complexes.[172][173]

Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present.[157] Soils with humus can vary in nitrogen content but typically have 3 to 6 percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting soil fertility.[156] Humus also absorbs water, and expands and shrinks between dry and wet states to a higher extent than clay, increasing soil porosity.[174] Humus is less stable than the soil's mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminishes without the addition of new organic matter. However, humus in its most stable forms may persist over centuries if not millennia.[175] Charcoal is a source of highly stable humus, called black carbon,[176] which had been used traditionally to improve the fertility of nutrient-poor tropical soils. This very ancient practice, as ascertained in the genesis of Amazonian dark earths, has been renewed and became popular under the name of biochar. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.[177]

Climatological influence

[edit]

The production, accumulation and degradation of organic matter are greatly dependent on climate. For example, when a thawing event occurs, the flux of soil gases with atmospheric gases is significantly influenced.[178] Temperature, soil moisture and topography are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where decomposer activity is impeded by low temperature[179] or excess moisture which results in anaerobic conditions.[180] Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients. Forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter by the living plant and microbial biomass to maintain their productivity, a process which is disturbed by human activities.[181] Excessive slope, in particular in the presence of cultivation for the sake of agriculture, may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.[182]

Plant residue

[edit]

Typical types and percentages of plant residue components

  Cellulose (45%)
  Lignin (20%)
  Hemicellulose (18%)
  Protein (8%)
  Sugars and starches (5%)
  Fats and waxes (2%)

Cellulose and hemicellulose undergo fast decomposition by fungi and bacteria, with a half-life of 12–18 days in a temperate climate.[183] Brown rot fungi can decompose the cellulose and hemicellulose, leaving the lignin and phenolic compounds behind. Starch, which is an energy storage system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists of polymers composed of 500 to 600 units with a highly branched, amorphous structure, linked to cellulose, hemicellulose and pectin in plant cell walls. Lignin undergoes very slow decomposition, mainly by white rot fungi and actinomycetes; its half-life under temperate conditions is about six months.[183]

Horizons

[edit]

A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a soil horizon. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions.[184] No soil profile has all the major horizons. Some, called entisols, may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimed mining waste deposits,[185] moraines,[186] volcanic cones[187] sand dunes or alluvial terraces.[188] Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage.[189] The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died.[190] By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like in sediment layers. Sampling pollen, testate amoebae and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change, land use change) which occurred in the course of soil formation.[191] Soil horizons can be dated by several methods such as radiocarbon, using pieces of charcoal provided they are of enough size to escape pedoturbation by earthworm activity and other mechanical disturbances.[192] Fossil soil horizons from paleosols can be found within sedimentary rock sequences, allowing the study of past environments.[193]

The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils.[194] The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts (leaf litter) or are directly produced belowground for subterranean plant organs (root litter), and then release dissolved organic matter.[195] The remaining surficial organic layer, called the O horizon, produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live.[196] After sufficient time, humus moves downward and is deposited in a distinctive organic-mineral surface layer called the A horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called pedoturbation. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil[197] and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.[198]

Classification

[edit]

One of the first soil classification systems was developed by Russian scientist Vasily Dokuchaev around 1880.[199] It was modified a number of times by American and European researchers and was developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on soil morphology instead of parental materials and soil-forming factors. Since then, it has undergone further modifications. The World Reference Base for Soil Resources[200] aims to establish an international reference base for soil classification.

Uses

[edit]

Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants. The types of soil and available moisture determine the species of plants that can be cultivated. Agricultural soil science was the primeval domain of soil knowledge, long time before the advent of pedology in the 19th century. However, as demonstrated by aeroponics, aquaponics and hydroponics, soil material is not an absolute essential for agriculture, and soilless cropping systems have been claimed as the future of agriculture for an endless growing mankind.[201]

Soil material is also a critical component in mining, construction and landscape development industries.[202] Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in surface mining, road building and dam construction. Earth sheltering is the architectural practice of using soil for external thermal mass against building walls. Many building materials are soil based. Loss of soil through urbanization is growing at a high rate in many areas and can be critical for the maintenance of subsistence agriculture.[203]

Soil resources are critical to the environment, as well as to food and fibre production, producing 98.8% of food consumed by humans.[204] Soil provides minerals and water to plants according to several processes involved in plant nutrition. Soil absorbs rainwater and releases it later, thus preventing floods and drought, flood regulation being one of the major ecosystem services provided by soil.[205] Soil cleans water as it percolates through it.[206] Soil is the habitat for many organisms: the major part of known and unknown biodiversity is in the soil, in the form of earthworms, woodlice, millipedes, centipedes, snails, slugs, mites, springtails, enchytraeids, nematodes, protists), bacteria, archaea, fungi and algae; and most organisms living above ground have part of them (plants) or spend part of their life cycle (insects) below-ground.[207] Above-ground and below-ground biodiversities are tightly interconnected,[157][208] making soil protection of paramount importance for any restoration or conservation plan.

The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even in deserts, cyanobacteria, lichens and mosses form biological soil crusts which capture and sequester a significant amount of carbon by photosynthesis. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset the effect of increases in greenhouse gas emissions and slow global warming, while improving crop yields and reducing water needs.[209][210][211]

Waste management often has a soil component. Septic drain fields treat septic tank effluent using aerobic soil processes. Land application of waste water relies on soil biology to aerobically treat BOD. Alternatively, landfills use soil for daily cover, isolating waste deposits from the atmosphere and preventing unpleasant smells. Composting is now widely used to treat aerobically solid domestic waste and dried effluents of settling basins. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter.[212]

Organic soils, especially peat, serve as a significant fuel and horticultural resource. Peat soils are also commonly used for the sake of agriculture in Nordic countries, because peatland sites, when drained, provide fertile soils for food production.[213] However, wide areas of peat production, such as rain-fed sphagnum bogs, also called blanket bogs or raised bogs, are now protected because of their patrimonial interest. As an example, Flow Country, covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now candidate for being included in the World Heritage List. Under present-day global warming peat soils are thought to be involved in a self-reinforcing (positive feedback) process of increased emission of greenhouse gases (methane and carbon dioxide) and increased temperature,[214] a contention which is still under debate when replaced at field scale and including stimulated plant growth.[215]

Geophagy is the practice of eating soil-like substances. Both animals and humans occasionally consume soil for medicinal, recreational, or religious purposes.[216] It has been shown that some monkeys consume soil, together with their preferred food (tree foliage and fruits), in order to alleviate tannin toxicity.[217]

Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, thus becoming groundwater. Pests (viruses) and pollutants, such as persistent organic pollutants (chlorinated pesticides, polychlorinated biphenyls), oils (hydrocarbons), heavy metals (lead, zinc, cadmium), and excess nutrients (nitrates, sulfates, phosphates) are filtered out by the soil.[218] Soil organisms metabolise them or immobilise them in their biomass and necromass,[219] thereby incorporating them into stable humus.[220] The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.[221]

Degradation

[edit]

Land degradation is a human-induced or natural process which impairs the capacity of land to function.[222] Soil degradation involves acidification, contamination, desertification, erosion or salination.[223]

Acidification

[edit]

Soil acidification is beneficial in the case of alkaline soils, but it degrades land when it lowers crop productivity, soil biological activity and increases soil vulnerability to contamination and erosion. Soils are initially acid and remain such when their parent materials are low in basic cations (calcium, magnesium, potassium and sodium). On parent materials richer in weatherable minerals acidification occurs when basic cations are leached from the soil profile by rainfall or exported by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming nitrogenous fertilizers and by the effects of acid precipitation. Deforestation is another cause of soil acidification, mediated by increased leaching of soil nutrients in the absence of tree canopies.[224]

Contamination

[edit]

Soil contamination at low levels is often within a soil's capacity to treat and assimilate waste material. Soil biota can treat waste by transforming it, mainly through microbial enzymatic activity.[225] Soil organic matter and soil minerals can adsorb the waste material and decrease its toxicity,[226] although when in colloidal form they may transport the adsorbed contaminants to subsurface environments.[227] Many waste treatment processes rely on this natural bioremediation capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. Remediation of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore soil functions and values. Techniques include leaching, air sparging, soil conditioners, phytoremediation, bioremediation and Monitored Natural Attenuation. An example of diffuse pollution with contaminants is copper accumulation in vineyards and orchards to which fungicides are repeatedly applied, even in organic farming.[228]

Microfibres from synthetic textiles are another type of plastic soil contamination, 100% of agricultural soil samples from southwestern China contained plastic particles, 92% of which were microfibres. Sources of microfibres likely included string or twine, as well as irrigation water in which clothes had been washed.[229]

The application of biosolids from sewage sludge and compost can introduce microplastics to soils. This adds to the burden of microplastics from other sources (e.g. the atmosphere). Approximately half the sewage sludge in Europe and North America is applied to agricultural land. In Europe it has been estimated that for every million inhabitants 113 to 770 tonnes of microplastics are added to agricultural soils each year.[229]

Desertification

[edit]
Desertification

Desertification, an environmental process of ecosystem degradation in arid and semi-arid regions, is often caused by badly adapted human activities such as overgrazing or excess harvesting of firewood. It is a common misconception that drought causes desertification.[230] Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Soil management tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover.[231] These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands accelerates desertification.[232] It is now questioned whether present-day climate warming will favour or disfavour desertification, with contradictory reports about predicted rainfall trends associated with increased temperature, and strong discrepancies among regions, even in the same country.[233]

Erosion

[edit]
Erosion control

Erosion of soil is caused by water, wind, ice, and movement in response to gravity. More than one kind of erosion can occur simultaneously. Erosion is distinguished from weathering, since erosion also transports eroded soil away from its place of origin (soil in transit may be described as sediment). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially unsuitable land use practices.[234] These include agricultural activities which leave the soil bare during times of heavy rain or strong winds, overgrazing, deforestation, and improper construction activity. Improved management can limit erosion. Soil conservation techniques which are employed include changes of land use (such as replacing erosion-prone crops with grass or other soil-binding plants), changes to the timing or type of agricultural operations, terrace building, use of erosion-suppressing cover materials (including cover crops and other plants), limiting disturbance during construction, and avoiding construction during erosion-prone periods and in erosion-prone places such as steep slopes.[235] Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion (the so-called dust bowl) which ruined American and Canadian prairies during the 1930s, when immigrant farmers, encouraged by the federal government of both countries, settled and converted the original shortgrass prairie to agricultural crops and cattle ranching.

A serious and long-running water erosion problem occurs in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion (gully erosion) in the Loess Plateau region of northwest China.[236]

Soil piping is a particular form of soil erosion that occurs below the soil surface.[237] It causes levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient.[238] The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.[239]

Salination

[edit]

Soil salination is the accumulation of free salts to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. Irrigation of arid lands is especially problematic.[240] All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying water table. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater. Soil salinity control involves watertable control and flushing with higher levels of applied water in combination with tile drainage or another form of subsurface drainage.[241][242]

Reclamation

[edit]

Soils which contain high levels of particular clays with high swelling properties, such as smectites, are often very fertile. For example, the smectite-rich paddy soils of Thailand's Central Plains are among the most productive in the world. However, the overuse of mineral nitrogen fertilizers and pesticides in irrigated intensive rice production has endangered these soils, forcing farmers to implement integrated practices based on Cost Reduction Operating Principles.[243]

Many farmers in tropical areas, however, struggle to retain organic matter and clay in the soils they work. In recent years, for example, productivity has declined and soil erosion has increased in the low-clay soils of northern Thailand, following the abandonment of shifting cultivation for a more permanent land use.[244] Farmers initially responded by adding organic matter and clay from termite mound material, but this was unsustainable in the long-term because of rarefaction of termite mounds. Scientists experimented with adding bentonite, one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the International Water Management Institute (IWMI) in cooperation with Khon Kaen University and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of 200 kilograms per rai (1,300 kg/ha; 1,100 lb/acre) of bentonite resulted in an average yield increase of 73%.[245] Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.[246]

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.[247]

If the soil is too high in clay or salts (e.g. saline sodic soil), adding gypsum, washed river sand and organic matter (e.g.municipal solid waste) will balance the composition.[248]

Adding organic matter, like ramial chipped wood or compost, to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.[249][250]

Special mention must be made of the use of charcoal, and more generally biochar to improve nutrient-poor tropical soils, a process based on the higher fertility of anthropogenic pre-Columbian Amazonian Dark Earths, also called Terra Preta de Índio, due to interesting physical and chemical properties of soil black carbon as a source of stable humus.[251] However, the uncontrolled application of charred waste products of all kinds may endanger soil life and human health.[252]

History of studies and research

[edit]

The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.[253]

Studies of soil fertility

[edit]

The Greek historian Xenophon (450–355 BCE) was the first to expound upon the merits of green-manuring crops: 'But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much as dung.'[254]

Columella's Of husbandry, c. 60 CE, advocated the use of lime and that clover and alfalfa (green manure) should be turned under,[255] and was used by 15 generations (450 years) under the Roman Empire until its collapse.[254][256] From the fall of Rome to the French Revolution, knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European Middle Ages, Yahya Ibn al-'Awwam's handbook,[257] with its emphasis on irrigation, guided the people of North Africa, Spain and the Middle East; a translation of this work was finally carried to the southwest of the United States when under Spanish influence.[258] Olivier de Serres, considered the father of French agronomy, was the first to suggest the abandonment of fallowing and its replacement by hay meadows within crop rotations. He also highlighted the importance of soil (the French terroir) in the management of vineyards. His famous book Le Théâtre d'Agriculture et mesnage des champs[259] contributed to the rise of modern, sustainable agriculture and to the collapse of old agricultural practices such as soil amendment for crops by the lifting of forest litter and assarting, which ruined the soils of western Europe during the Middle Ages and even later on according to regions.[260]

Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.[261] In about 1635, the Flemish chemist Jan Baptist van Helmont thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight.[262][263][264] John Woodward (d. 1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, Jethro Tull demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.[263][265]

As chemistry developed, it was applied to the investigation of soil fertility. The French chemist Antoine Lavoisier showed in about 1778 that plants and animals must combust oxygen internally to live. He was able to deduce that most of the 165-pound (75 kg) weight of van Helmont's willow tree derived from air.[266] It was the French agriculturalist Jean-Baptiste Boussingault who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil.[267] Justus von Liebig in his book Organic chemistry in its applications to agriculture and physiology (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced.[268] Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by Alexander von Humboldt. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.[269]

The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England John Bennet Lawes and Joseph Henry Gilbert worked in the Rothamsted Experimental Station, founded by the former, and (re)discovered that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the superphosphate, consisting in the acid treatment of phosphate rock.[270] This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of coke was recovered and used as fertiliser.[271] Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms was still not understood.

In 1856, J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates,[272] and twenty years later Robert Warington proved that this transformation was done by living organisms.[273] In 1890 Sergei Winogradsky announced he had found the bacteria responsible for this transformation.[274]

It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist Hermann Hellriegel and the Dutch microbiologist Martinus Beijerinck.[270]

Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.[275]

Studies of soil formation

[edit]

Scientists who studied soil in connection with agricultural practices considered it mainly a static substrate. However, the soil is the result of evolution from more ancient geological materials under the action of biotic and abiotic processes. After studies of soil improvement commenced, other researchers began to study soil genesis and, as a result, soil types and classifications.

In 1860, while in Mississippi, Eugene W. Hilgard (1833–1916) studied the relationship between rock material, climate, vegetation, and the type of soils that were developed. He realised that the soils were dynamic and considered the classification of soil types.[276] (See also at Project Gutenberg). His work was not continued. At about the same time, Friedrich Albert Fallou described soil profiles and related soil characteristics to their formation as part of his professional work evaluating forest and farmland for the principality of Saxony. His 1857 book, Anfangsgründe der Bodenkunde (First Principles of soil science), established modern soil science.[277] Contemporary with Fallou's work, and driven by the same need to accurately assess land for equitable taxation, Vasily Dokuchaev led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to Western Europe until 1914 through a publication in German by Konstantin Glinka, a member of the Russian team.[278]

Curtis F. Marbut, influenced by the work of the Russian team, translated Glinka's publication into English,[279] and, as he was placed in charge of the U.S. National Cooperative Soil Survey, applied it to a national soil classification system.[263]

See also

[edit]

References

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