6 - Chemistry of the pedosphere

Summary

The pedosphere consists of soil and constitutes the interface between the lithosphere, the hydrosphere, and the atmosphere. Instead of attempting a short definition of soil, we shall look at the main disciplines of soil science: edaphology and pedology, both of which were founded in the 19th century. Edaphology deals with the conditions necessary for the growth of plants (Liebig, around 1840), that is, their nutrition, in particular the types of chemical species they consume and produce. Pedology is the study of soil genesis, morphology, and classification (Dokuchaev, 1883).

The upper layer of the Earth's crust consists of fragments of rocks, mainly silicates, combined with water, air, and organic material. The largest pieces of rock, boulders and cobbles, have sizes ranging from meters to decimeters, whereas small fragments are coarse gravel in the centimeter range, followed in order of decreasing size by fine gravel, sand, silt, and clay (maximum size 4 μm), as shown later in Table 6.3. This heterogeneous, solid material extending from the outer surface, in contact with the atmosphere or ocean, downward to the solid bedrock, is called the regolith. In some cases the regolith results from weathering of bedrock at a given location, but more often it has been transported to a given site; in a few cases there is no regolith. Material at a given site may have been deposited by ice, water, or wind, individually or in combination. The regolith is often stratified; for example, Figure 6.1 shows layers formed by water and air in material deposited by glaciers.