One of the last great frontiers in biological and ecological research is in the soil. Civilizations, so dependent on soils as a source of nutrients for food—consumed directly as vegetables, fruit, and grain, or animals that feed on plants—owe a considerable debt to soils.
Over the span of several millennia, there has been concern about the use and misuse of soils. There is ample evidence that numerous civilizations, from ancient Sumeria and Babylonia to those that use modern high-intensity, high-input agriculture, have suffered with problems of long-term sustainability (Whitney, 1925; Pesek, 1989).
Indeed, one prominent soil physicist has been moved to comment that "the plow has caused more destruction to civilizations than the sword" (Hillel, 1991). Perhaps the adage of "beating swords into plowshares" needs rethinking. As we will discover in the course of this book, it is truly a time to be working with nature and to cease treating soils as a "black box."
Soil is a unique entity. It has its origins in physical, chemical, and biological interactions between the parent materials and the atmosphere. The simplest definitions of soil follow the most common understanding, such as "the upper layer of earth which can be dug or plowed and in which plants grow." (Webster's New Universal Unabridged Dictionary, 1983). The soil scientist defines it as "a natural body, synthesized in profile form from a variable mixture of broken and weathered minerals and decaying organic matter, which covers the earth in a thin layer and which supplies, when containing the proper amounts of air and water, mechanical support and, in part, sustenance for plants" (Buckman and Brady, 1970). This definition recognizes that soil has vertical structure, is composed of a variety of materials, and has a biological nature as well; it is derived in part from decaying organic matter. Nevertheless, uncertainties emerge when more restrictive definitions are attempted. How deep is soil or when is nonsoil encountered? Working definitions of soil depth range from 1 meter to many meters, depending on the ecosystem and the nature of the investigations. Are barren, rocky areas excluded if they do not allow growth of higher plants? Lindeman (1942) considered the substratum of a lake as a benthic soil (see also Jenny, 1980). When do simple sediments become soil? When can they support plant growth? Only after biological, physical, and chemical interactions convert sediments into an organized profile?
Soils are composed of a variable combination of four key constituents: minerals, organic matter, water, and air. Of the Greco-Roman concepts of fundamental constituents—earth, air, fire, and water—three of the four are contained with the broad concept of soil. Indeed, if the energetic process of life ("the fire of life") (Kleiber, 1961) is included within the soil, then all four of the ancient "elements" are present therein.
Are living organisms part of soil? We would include the phrase "with its living organisms" in the general definition of soil. Thus, from our viewpoint soil is alive and is composed of living and nonliving components having many interactions.
It is as a part of that larger unit, the terrestrial ecosystem, that soil must be studied and conserved. The interdependence of terrestrial vegetation and animals, soils, atmosphere, and hydrosphere is complex, with many feedback mechanisms. When we view the soil system as an environment for organisms, we must remember that the biota have been involved in its creation, as well as adapting to life within it. The principles by which organisms in soils are distributed, interact, and carry on their lives are far from completely known, and the importance of the biota for soil processes is not often appreciated.
This book on soil ecology emphasizes the interdisciplinary nature of studies in ecology as well as soils. Considerable "niche overlap" (similarity in what they do, i.e., their "profession") (Elton, 1927) exists between the two disciplines of ecology and soil science. Ecology, which is heavily organism-oriented, is concerned with all forms of life in relation to their environment. Soil science, in contrast, contains several other aspects in addition to soil biology, such as soil genesis and classification, soil physics, and soil chemistry. A broader view of ecology asks: How do systems work? From that perspective, ecology and soil science share similar objectives.
The overlap between ecology and soil science is both extensive and interesting. Aspects of soil physics, chemistry, and mineralogy have a great impact on how many different kinds of organisms coexist in the opaque, complex, semiaquatic milieu that we call soil. We first describe what soils are and how they are formed, and then discuss some of the current research being done in soil ecology.
With a rising tide of interest worldwide in soils, and in belowground processes in general, numerous types of studies using tools in all ranges of the size and electromagnetic energy spectra, and encompassing from microsites to the biosphere, are now possible. Significant achievements during the past 5 to 10 years make a book of this sort both timely and useful. This book is intended primarily as a source of ideas and concepts and thus is intended as a supplemental reference for courses in ecology, soil science, and soil microbiology.
We hope that we will interest a new generation of ecologists and soil scientists in the world of soil ecology: the interface between biology, chemistry, and physics of soil systems.
Paul F. Hendrix Athens, Georgia, February 2004
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