The Composition of Soils

Soils are composed of mineral matter, air, water, organic matter, and organisms. There are two general types of soils, mineral soils and organic soils. Mineral soils form from decomposed rocks or sediment derived from rocks. Organic soils form from the accumulation of plant material, usually in water-saturated, anaerobic conditions that retard decomposition. Mineral matter is described as texture and comprises half the volume of mineral soils. The other half of the soil volume is composed of voids or holes. These voids fill with water as the soil soaks up rain or flood waters, then are displaced with air as the water drains away, evaporates, or is absorbed by roots. Organic matter is generally very low in aridic soils, around 1% or less. Soil is a biologically active matrix. It is a home for plant roots, seeds, animals, bacteria, fungi, algae, and viruses. Because of its biological activity soil supplies plants needed nutrients, purifies water passing through (including wastewater effluent), and is an important carbon-sink that affects the concentration of atmospheric carbon dioxide.

Soils are complex and variable. Their properties and characteristics form a continuum with gradual to abrupt transitions. Repetitive patterns from continental to microscopic scales occur from the complex interactions of soil forming processes.

References

Brady, N.C. (1974). The Nature and Properties of Soils. New York: MacMillan. 639 pp.

Buol, S.W., F.D. Hole, and R.J. McCracken (1973). Soil Genesis and Classification. Ames, Iowa: The Iowa State University Press. 360 pp.

Soil Survey Staff (1975). Soil Taxonomy. USDA Agriculture Handbook No. 436. Washington, D.C.: U.S. Government Printing Office. 754 pp.

Soil Survey Staff (1993). Soil Survey Manual. USDA Handbook No. 18. Washington, D.C.: U.S. Government Printing Office. 437 pp.

Spangler, M.G. and R.L. Handy (1982). Soil Engineering. New York: Harper & Row. 819 pp.

URL:http://cals.arizona.edu/OALS/soils/composition.html
Last revised: Last revised: 24 August 2001
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