Formation of soils

So far we have discussed the mechanisms and solid products of chemical weathering without precise consideration of the environment in which these reactions occur. While chemical attack of exposed bedrock surfaces can happen, most weathering reactions occur in (or under) soils. We have already noted that the oxidation of soil organic matter causes acidity of natural waters (Section 4.4.2), promoting chemical weathering. This acknowledges the important role of soils in environmental chemistry. So what exactly are soils? A glance at a dictionary suggests that soils constitute the upper layer of the Earth's continental crust in which plants grow, usually consisting of disintegrated rock with admixture of organic remains.

Soil formation is influenced by geological (G), environmental (E) and biological (B) factors such that the product, soil (S), is a function of all of these factors with respect to time (t), i.e.:

In fact a number of key factors can be identified in soil formation, including parent material (p), climate (cl), relief (r), vegetation (v) and the influence of organisms (o). Thus, equation 4.15 above can be more precisely written:

The key factors in equation 4.16 are summarized in Fig. 4.13 where the soil is depicted as a simple box. Various inputs and outputs govern the box's content while processes within the box generate the outputs. It is important to note that of all of these factors only time is an independent variable, the others being inextricably linked. For clarity, these factors are discussed individually below, although in nature they operate together.

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