Soil chemistry

Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. Back in the early 1850's a consulting chemist to the Royal Agricultural Society in England, named J. Thomas Way, performed many experiments on how soils exchange ions. As a result of his diligent and strenuous work, he is considered the father of soil chemistry. But after him, many other big-name scientists also contributed to this branch of ecology including Edmund Ruffin, Linus Pauling, and many others.

History

Until the late 1960s, soil chemistry focused primarily on chemical reactions in the soil that contribute to pedogenesis or that affect plant growth. Since then, concerns have grown about environmental pollution, organic and inorganic soil contamination and potential ecological health and environmental health risks. Consequently, the emphasis in soil chemistry has shifted from pedology and agricultural soil science to an emphasis on environmental soil science.

Environmental soil chemistry

A knowledge of environmental soil chemistry is paramount to predicting the fate of contaminants, as well as the processes by which they are initially released into the soil. Once a chemical is exposed to the soil environment myriad chemical reactions can occur that may increase or decrease contaminant toxicity. These reactions include adsorption/desorption, precipitation, polymerization, dissolution, complexation and oxidation/reduction. These reactions are often disregarded by scientists and engineers involved with environmental remediation. Understanding these processes enable us to better predict the fate and toxicity of contaminants and provide the knowledge to develop scientifically correct, and cost-effective remediation strategies.

Concepts

• SOIL COMPONENTS: The Soil is made up of five components 1) Mineral Components 2) Organic matter or humus 3) Soil atmosphere 4) Soil water 5) Biological system or Soil microorganisms.
• Mineral Components: The mineral components of the soil are derived from the parental rocks or regolith. The minerals present about 90 % of the total weight of the soil. Some important elements, which are found in compound state, are O, Fe, Si, Al, N, P, K, Ca, Mg, C, H, etc.
• Organic matter or humus: Organic residues are obtained either from dead remains of plants and animals or through metabolic activities of living organisms present in the soil. When plants and animals die, their dead remain are acted upon by a number of microorganisms and are finally degraded or decomposed into simple organic compounds. A product of this microbial decomposition is called humus. Humus is fairly stable and amorphous, brown to black material, which is formed as a result of decomposition of plants and animal residues with no trace of the structure of the material from which it is derived. The chief elements found in humus are carbon, hydrogen, oxygen, Sulphur and nitrogen. The important compound found in humus are carbohydrates, phosphoric acid, some organic acids, resins, urea etc. Humus is a dynamic product and is constantly changing because of its oxidation, reduction and hydrolysis. Hence it has got much carbon content and less nitrogen. Humus is not soluble in water. It is present in the soil in the form of organic colloids. The amounts of humus in different soils vary greatly. Humus percentage in the soil is affected by climatic and biological factors.
• Soil Atmosphere: Gases found in soil profile are said to form the soil atmosphere. The soil atmosphere contains three main gases, namely oxygen, carbon dioxide and nitrogen. In the atmosphere, oxygen is 20 %, Nitrogen is 79 % and CO₂ is 0.15 to 0.65 % by volume. In the cultivated land, percentage of CO₂, is much higher than that of atmospheric CO₂, but oxygen content in such soils poorer than the percentage of oxygen in atmosphere air. The amount of CO₂ increases with the increase in the depth of soil due to decomposition of accumulated organic matter and abundance of plant roots. Presence of oxygen in the soil is important in the since that it helps in the process of breaking down of insoluble rocky mass into soluble minerals and organic humification. The accumulation of soluble nutrients in the soil makes it more productive. If the soil is deficient in oxygen, the rates of microbial activities are slowed down and may be eliminated. Under such conditions, several undesirable processes, such as evolution of nitrogen and methane, accumulation of sulphides, ferrous, manganese ions and organic inhibitors and so many other processes may come into play which may be injurious to the plants. The important factors, which bring about changes in the soil atmosphere are temperature, atmospheric pressure, wind and rainfall.
• Soil water: Soil water plays very important role in plant growth. Plants absorb a small quantity of rainwater and dew directly from their surfaces but most of the water absorbed by them comes from soil.
• Biological System: Organisms present in the soil are called soil organisms. Soil organisms are stable, mobile but some are held in the colloidal films of the soil particles. Protozoa, mites and insects are examples of moving organisms. Earthworm by their burrowing habit make the soil loose and fertile.
• SOIL TEXTURE: Soil texture is defined as the proportionate quantity of sand, silt and clay in the mineral fraction of a soil. Mineral fraction of soil consists of particles of various sizes. According to their size, soil particles are grouped into the three types i.e. sand, silt, clays. These soil separates differ not only in their sizes but also in their bearing on some of the important factors affecting plant growth such as soil aeration, work ability, movement and availability of water and nutrients.
• Sand: Sand particles consist of small pieces of primary un-weathered rock fragments. They are heaviest and coarsest of the mineral particles. They do not held water well, due to large pore spaces and do not stick together. When coated with clay, these sand particles take very active part in chemical reactions. Sand increases the size of pore spaces bet. The soil particles and thus, facilitate any nutrient for crop growth as they do not store nutrients. Silt:  Silt particles are bigger than clay particles but smaller than sand particles. They hold plant nutrients better than sand but not as well as clay. Silt particles allow water and air to pass readily, yet retain moisture for crop growth. Silty soil contains sufficient quantities of nutrients both organic and inorganic. Therefore, they are very fertile. Clay: Clay particles are smallest of all particles found in soil. They are the most active part of the soil and hold plant nutrients well. Clay soils have fine pores, poor drainage and aeration and thus they have highest water holding capacity. The clay acts as a storehouse for water and nutrients. Texture of soil for a given horizon is almost a permanent character because it remains unchanged over a long period of time.
• SOIL STRUCTURE: Soil structure should not be confused with soil texture. Texture denotes the size of individual soil particles; structure refers to the manner in which these individual soil particles; are grouped together to form clusters of particles called aggregates. Natural aggregates results in what are called peds, whereas artificial aggregates are called clods. Clods are formed due to disturbance of the field by ploughing or digging. Types of Soil structure: The classification of soil structural forms is based largely on shape. There are four types of soil structures.

  

  Spheroidal shape of soil particles

  i) Spheroidal: These are like a sphere or rounded. All the axes are approximately of the same dimensions, with curved and irregular faces. This group is further divided into crumb and granular structure. a) Crumb structure- The aggregates are small. They are weakly held together and are porous like crumbs of bread. b) Granular structure- This is similar to crumb structure except that the aggregates are harder, less porous and individual soil particles are more strongly held together than in the crumb structure. These are found commonly in cultivated fields.

  

  Plate like shape of soil particles

  ii) Platy structure: Platy structure, in which particles are arranged along a plant that is usually horizontal. If the units are thick they are called platy, while the units are thin, they are called laminar. Platy structures are usually found in the surface and sometimes in the lower sub-soils. iii) Block like structure: Particles are arranged around a central point are enclosed by surfaces that may be either flat or somewhat rounded. It can be divided into two subclasses. (a) Angular blocky (b) Sub-angular blocky.

  

  Block like structure of soil particles

  Angular blocky aggregates have angular corners. While sub-angular blocky are similar to angular blocky but the corners are rounded. These types are generally found in subsoil.

  

  Prism like structure of soil particles

  iv) Prism like structure: These aggregates are longer than they are wide, the vertical axis being greater than horizontal axis. They are commonly found in subsoil horizon of arid and semi-arid region soils. They may be further divided into two sub-groups.

• Anion and cation exchange capacity
• Soil pH
• Mineral formation and transformation processes and pedogenesis
• Clay mineralogy
• Sorption and precipitation reactions in soil
• Chemistry of problem soils

Methods of investigation

New knowledge about the chemistry of soils often comes from studies in the laboratory in which soil samples taken from undisturbed soil horizons in the field are used in experiments that include replicated treatments and controls. In many cases, the soil samples are air dried at ambient temperatures (e.g., 25^oC) and sieved to a 2 mm size prior to storage for further study. Such drying and sieving soil samples markedly disrupts soil structure, microbial population diversity, and chemical properties related to pH, oxidation-reduction status, manganese oxidation state, and dissolved organic matter; among other properties. Renewed interest in recent decades has led many soil chemists to maintain soil samples in a field-moist condition and stored at 4^oC under aerobic conditions before and during investigations.

Two approaches are frequently used in laboratory investigations in soil chemistry. The first is known as batch equilibration. The chemist adds a given volume of water or salt solution of known concentration of dissolved ions to a mass of soil (e.g., 25 mL of solution to 5 g of soil in a centrifuge tube or flask. The soil slurry then is shaken or swirled for a given amount of time (e.g., 15 minutes to many hours) to establish a steady state or equilibrium condition prior to filtering or centrifuging at high speed to separate sand grains, silt particles, and clay colloids from the equilibrated solution. The filtrate or centrifugate then is analyzed using one of several methods, including ion specific electrodes, atomic absorption spectrophotometry, inductively coupled plasma spectrometry, ion chromatography, and colorimetric methods. In each case, the analysis quantifies the concentration or activity of an ion or molecule in the solution phase, and by multiplying the measured concentration or activity (e.g., in mg ion/mL) by the solution-to-soil ratio (mL of extraction solution/g soil), the chemist obtains the result in mg ion/g soil. This result based on the mass of soil allows comparisons between different soils and treatments. A related approach uses a known volume to solution to leach (infiltrate) the extracting solution through a quantity of soil in small columns at a controlled rate to simulate how rain, snow meltwater, and irrigation water pass through soils in the field. The filtrate then is analyzed using the same methods as used in batch equilibrations.

Another approach to quantifying soil processes and phenomena uses in situ methods that do not disrupt the soil as occurs when the soil is shaken or leached with an extracting soil solution. These methods usually use surface spectroscopic techniques, such as Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), Mossbauer spectroscopy, and X-ray spectroscopy. These approaches aim to obtain information on the chemical nature of the mineralogy and chemistry of particle and colloid surfaces, and how ions and molecules are associated with such surfaces by adsorption, complexation, and precipitation.

These laboratory experiments and analyses have an advantage over field studies in that chemical mechanisms on how ions and molecules react in soils can be inferred from the data. One can draw conclusions or frame new hypotheses on similar reactions in different soils with diverse textures, organic matter contents, types of clay minerals and oxides, pH, and drainage condition. Laboratory studies have the disadvantage that they lose some of the realism and heterogeneity of undisturbed soil in the field, while gaining control and the power of extrapolation to unstudied soil. Mechanistic laboratory studies combined with more realistic, less controlled, observational field studies often yield accurate approximations of the behavior and chemistry of the soils that may be spatially heterogeneous and temporally variable. Another challenge faced by soil chemists is how microbial populations and enzyme activity in field soils may be changed when the soil is disturbed, both in the field and laboratory, particularly when soils samples are dried prior to laboratory studies and analysis.

References

• Sonon, L. S., M. A. Chappell and V.P. Evangelou (2000) The History of Soil Chemistry. Url accessed on 2006-04-11.

External links

• Media related to Soil chemistry at Wikimedia Commons