Soil Classification and Processes

Soils are extremely important environmentally, for they provide the medium and the essential nutrients for plant growth and strongly influence the chemical composition of surface- and groundwaters. Soils are also called upon to remove or attenuate contaminants derived from the atmosphere or released from the surface or near-surface disposal of wastes. We will limit our discussion of soils to a few basic principles. For a more thorough treatment of soil geochemistry the reader is referred to books by Loughnan (1969), Greenland and Hayes (1978), Lindsay (1979), Rose et al. (1979), Bolt (1979), Drever (1985), and Sposito (1989). 

Soils are the products of chemical weathering of parent rocks and sediments and, usually to a les.ser extent, the physical breakdown of those materials. When the parent material for soil formation is underlying bedrock, the soil so formed in place is called a residual soil. Soils may also develop in materials that have been transported and deposited by wind (loess), streams (alluvium), ice (till), or by landslides (colluvium). 

As soil is formed from the weathering of underlying bedrock, the O, A, E, and B (together called the solum), and especially C horizons tend to thicken with time unless surface soil is eroded away, in which case O, A, E, and B horizons will migrate downward as underlying materials near the land surface. 

Low rainfall and high moisture losses due to evapotranspiration are typical featuresljf arid regions. Thus, following precipitation events and resultant infiltration of moisture, arid soils often experience net moisture movement upward toward the land surface driven by capillary forces. For such 

O A surface layer composed primarily of organic matter, black or dark brown in color. 

---