How Soil is Defined

Soil is the surface of the earth that is not water, air, or rock. More precise definitions diverge in the direction of interest and need. Interest in soil is primarily for agricultural or engineering purposes and this bias is reflected in soil definitions, classification systems, and research.

An agricultural definition of soil is "a dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms" (Brady, 1974, p. 617). The USDA Soil Taxonomy definition is:

The collection of natural bodies on the earth's surface, in places modified or even made by man of earthly materials, containing living matter and supporting or capable of supporting plants out-of-doors. Its upper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice. Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through time, of climate, living organisms, parent materials, and relief. In the few places where it contains thin cemented horizons that are impermeable to roots, animals, or marks of other biologic activity. The lower limit of soil, therefore, is normally the lower limit of biologic activity, which generally coincides with the common rooting depth of native perennial plants (Soil Survey Staff, 1975, p. 1).

An engineering definition of soil is "all the fragmented mineral material at or near the surface of the earth, the moon, or other planetary body, plus the air, water, organic matter, and other substances which may be included therein" (Spangler and Handy, 1982, p. 67). This is the same definition for regolith.

Geological definitions of soil depend on the interest of the geologist. "Hard rock" geologists tend to view soil as regolith, employing the engineering definition. Geomorphologists, however, are interested in soil forming processes and adopt definitions and classification systems developed primarily for agriculturists.

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/defined.html
Last revised: Last revised: 24 August 2001
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