SOIL

earthen material that covers land surfaces and is formed by the action of natural physical, chemical, and biotic forces on the unconsolidated residue (regolith) of rocks and minerals on the Earth's surface. Among its many important functions, soil serves as a substratum of plant and, thus, further up the food chain, of animal life. The processes of soil formation create a soil profile that is composed of one to four biologically and chemically distinct master horizons (horizontal layers), conventionally designated from the surface down as A, B, and C and the unconsolidated rock R, of variable depths. A horizons are the most weathered and leached and generally contain the most organic matter and plant nutrients; B horizons are moderately weathered and accumulate the products removed from A horizons; C horizons contain parent material from which the above horizons are formed; and R layers, not part of the soil proper, exhibit properties much different from the overlying C horizons. The most fundamental process of soil formation and differentiation is weathering, which is the physical disintegration and chemical decomposition of rocks, minerals, and immature soils. Rates of weathering vary, depending on the material. Volcanic ash, for example, can support vegetation after only a few decades of weathering. The most important constituent of soil, crystalline clay, is formed by weathering of feldspars and other silicates and leaching (draining away by percolation of water through the soil) of bases and some silica. The two basic kinds of clays formed are known as 1:1 clays (composed of a tetrahedral and an octahedral layer), which result from rapid leaching, and 2:1 clays (composed of one octahedral layer between two tetrahedral layers), which are formed by gradual leaching. The transformation of organic matter, the second most important soil constituent, into humus (chemically stable, finely divided organic matter), called humification, is a rapid process: organic matter can be doubled or reduced by half in 5 to 50 years. Leaching has important effects on the soil profile. When there is no drainage it produces a soil profile stratified according to solubility of the dissolved substances, and when there is drainage it removes some substances from the soil profile. Dispersion mechanisms that affect soil formation include eluviation (transport of material out of a portion of the soil profile) and illuviation (transport of material into a portion of the soil profile), flooding, and podsolization (transport of alumina and iron with or without organic matter, resulting in the concentration of silica in the layer eluviated). Environmental factors influence soil formation in a variety of ways. Climatic factors such as leaching rainfall (rainfall minus potential evaporation), which determines leaching intensity, and temperature, which influences the solubility of some substances, affect the kind of clay that is formed. Leaching is impossible without drainage, and topography influences drainage, erosion, and flooding. Vegetational characteristics largely determine the distribution and kind of humus formed, soil organisms contribute to the decay of organic matter, and parent material determines the ultimate nature of soil. Time is also an important factor; as rocks age, their profiles become more differentiated and their soil properties change. These factors are interdependent, and very different soils may be formed from the same parent material. The question of soil classification is controversial since pedologists, engineers, agriculturalists, geologists, and others all have named and classified different kinds of soil, each group according to its own special knowledge and interests. Classification systems also must be manifold, recognizing that a soil may belong to more than one identifiable group. The 7th American Approximation System, introduced in 1960, is based on measurable soil properties (texture, chemical characteristics, colour, etc.) rather than on theories of soil formation. Soil groups are named by the aggregation of formative elements that denote special characteristics (horizons, source of parent material [river, lake, etc.], climatic regime). The system therefore provides precise definition of soil properties of groups and employs manifold classification. Because environmental factors affect soil formation, major soil regions in which the distribution of soils follow a definite pattern can be identified. Principal regions include: polar; podsolic, distinguished by evergreen forest; brunsolic, characterized by deciduous forest and including the regions of western Europe, the eastern United States, Washington state, and the adjacent Canadian coast; chernozemic (black earth), characterized by grassland vegetation and including the regions of the Danube Basin and parts of Ukraine and southern Russia, the Great Plains of the United States and the prairies of Canada, and the Pampas region of Argentina; cinnamonic, often found in areas having Mediterranean climates; desert; kaolinitic, marked by tropical climates; and montane. Transition from one region to another is gradual, and intermediate regions also can be identified. the biologically active, porous medium that has developed in the uppermost layer of the Earth's crust. Soil is one of the principal substrata of life on Earth, serving as a reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements through the global ecosystem. It has evolved through weathering processes driven by biological, climatic, geologic, and topographic influences. Since the rise of agriculture and forestry in the 8th millennium BC, there has also arisen by necessity a practical awareness of soils and their management. In the 18th and 19th centuries the Industrial Revolution brought increasing pressure on soil to produce raw materials demanded by commerce, while the development of quantitative science offered new opportunities for improved soil management. The study of soil as a separate scientific discipline began about the same time with systematic investigations of substances that enhance plant growth. This initial inquiry has expanded to an understanding of soils as complex, dynamic, biogeochemical systems that are vital to the life cycles of terrestrial vegetation and soil-inhabiting organismsand by extension to the human race as well. This article covers the structure, composition, and classification of soils and how these factors affect soil's role in the global ecosystem. In addition, the two most important phenomena that degrade soils, erosion and pollution, are discussed. Garrison Sposito Additional reading Perhaps the finest book on soils for the layperson is Milo I. Harpstead, Thomas J. Sauer, and William F. Bennett, Soil Science Simplified, 3rd ed. (1997). An in-depth introductory treatment is given by Frederick R. Troeh and Louis M. Thompson, Soils and Soil Fertility, 5th ed. (1993). A holistic account of soils in their ecological setting is presented in Hans Jenny, The Soil Resource (1980, reprinted with corrections, 1983). Technical terms are defined in Glossary of Soil Science Terms (1997), published by the Soil Science Society of America.An excellent technical discussion of pedology in an ecological context is Stanley W. Buol et al., Soil Genesis and Classification, 4th ed. (1997). Guides to soil classification are Keys to Soil Taxonomy, 8th ed. (1998), published by the U.S. Department of Agriculture; and Soil Map of the World: Revised Legend (1990, reprinted with corrections, 1994), published by the Food and Agriculture Organization (FAO) and UNESCO.The modern classic on soil formation is Hans Jenny, Factors of Soil Formation (1941, reissued 1994). See also the commentary volume, Factors of Soil Formation: A Fiftieth Anniversary Retrospective (1994), published by the Soil Science Society of America. The geomorphic processes related to soil formation and erosion are discussed by Cliff Ollier, Weathering (1984). Erosion is considered in more technical detail in Frederick R. Troeh, James Arthur Hobbs, and Roy L. Donahue, Soil and Water Conservation, 3rd ed. (1999). The special problems of soil loss in the humid tropics are brought out in R. Lal and Pedro A. Snchez (eds.), Myths and Science of Soils of the Tropics (1992); and, on a practical level, in Ted C. Sheng, Soil Conservation for Small Farmers in the Humid Tropics (1989).Soil pollution issues are given modern treatment in Garrison Sposito, The Chemistry of Soils (1989); Murray B. McBride, Environmental Chemistry of Soils (1994); Eldor A. Paul and Francis E. Clark (eds.) Soil Microbiology and Biochemistry, 2nd ed. (1996); and F.J. Stevenson and M.A. Cole, Cycles of Soil, 2nd ed. (1999). The last four texts also discuss biogeochemical cycling in soils at a technical level. Garrison Sposito