The virtue of soil is one of only a few axioms of American culture. Respect for soil, which can be instilled at an early age without rousing political or religious controversy, is the mark of agrarian origins. Nevertheless, or perhaps for this very reason, the average American knows more about asteroids than about soil. Even scientists, who are responsible for knowing pretty much everything, seem generally less aware of soil than of water, air, or even rock. Thus, this chapter requires a primer on the nature of soil. The USDA, which is the center of gravity for soil science in the United States, has defined soil as “earthy materials . . . on the earth’s surface . . . capable of supporting plants out-of-doors.” To the thoughtful novice, this may seem a woefully unsatisfactory definition. In fact, one might feel downright indignant upon consulting a dictionary only to find that “earthy” means “having properties of soil.” Thus, the USDA seems to be saying that soil is something that is like soil. This could be a simple case of cheating or perhaps a devilishly clever means of suppressing debate about the definition of soil. Further reflection, however, suggests that the reference to plants is more significant than it first appears to be. Plants do not grow on bare rock, shifting sand, or the bottoms of oceans, all of which are non-soil substrates. Thus, the USDA definition may say it all, but is masterfully understated in doing so. Soil science textbooks are also a source of definitions, some of which are gratifyingly descriptive. Birkeland’s (1984) textbook, which gives a geological perspective on soil, specifies that soil is a natural entity; is composed of mineral or organic constituents, or both; differs from the material from which it was originally derived; and reflects the operation of pedogenic processes. Pedogenic processes, in turn, include the addition of organic matter from plants and other substances from the atmosphere; removal of substances by water through erosion or leaching; and a variety of internal transformations and transfers involving organic matter and minerals.
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