Organic Components of Soil

It cannot be emphasized enough that soil is formed by the interaction of the mineral weathering product of rocks and the organic material 

Dominant land use Tonnes carbon added to soil ha year  

Source adapted from White, Principles and Practice of Soil Science, Blackwell Science, 1997. 

The pK of Equation (5.5) is in the range 3-5, and of Equation (5.6) in the range 7-8. Thus, carboxyl groups are important in acid soils, while phenolic groups become important above pH 7. 

Although there is considerable variation between soils, the ratio of the major elements, C N P S, in humic material is approximately 100 10 2 1, forming a major reservoir of N, P, and S that is made available to plants as soil organic matter, is broken down by soil micoorganisms. 

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Rates of hydrolysis may be influenced by the presence of dissolved organic carbon, or organic components of soil and sediment. The magnitude of the effect is determined by the structure of the compound and by the kinetics of its association with these components. For example, whereas the neutral hydrolysis of chlorpyrifos was unaffected by sorption to sediments, the rate of alkaline hydrolysis was considerably slower (Macalady and Wolf 1985) humic acid also reduced the rate of alkaline hydrolysis of 1-octyl 2,4-dichlo-rophenoxyacetate (Perdue and Wolfe 1982). Conversely, sediment sorption had no effect on the neutral hydrolysis of 4-chlorostilbene oxide, although the rate below pH 5 where acid hydrolysis dominates was reduced (Metwally and Wolfe 1990). 

Biological Mechanisms for Association with Organic Components of Soil and Sediment... 

The detailed chemical structures of the organic components of soil and sediments are largely unknown, and terms such as humic acid and fulvic acid are primarily descriptive rather than representing chemically defined entities. A brief summary is given of studies aimed at providing information on the structural components of humic and fulvic acids. Further studies using specific chemical reactions are given below. 

It is more strongly adsorbed by the organic components of soils than monuron or diuron, and is therefore of poor efficiency in soils with high humus content, particularly under dry weather conditions. Low doses are persistent in the soil for 3-6 months, high doses for as long as 2 years. It is a nontoxic substance. 

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