Soils, which are composed of minerals, organic material, water, and air, serve as the foundation for terrestrial ecosystems. The rates at which nitrogen, phosphorus, and other nutrients are released into the soil and transported to plants drive several major processes within an ecosystem, including respiration, nutrient recycling, and the production of plants, animals, and microbes.
Living entities in the soil, including microbes, microbe-feeding fauna, and vegetation, are influenced by soil processes, and these organisms in turn have an impact on the soil system. Maintaining a good soil cover in forests and croplands facilitates the retention of organic matter and the cycling of nutrients within the ecosystem. These activities occur through the process of decomposition, which drives complex food webs in the soil. Decomposition begins when dead organic litter, representing 80 to 90 percent of the new plant growth previously produced by an ecosystem, is returned to the soil. The litter, consisting of leaves, roots, and wood from trees, as well as organic residues from agricultural fields, is decomposed on or in the soil, and the nutrients contained within it are recycled for use by living plants.
The breakdown of organic litter begins with the chewing action of both large and small animals, which derive nutrients from the plant material, and continues as the organic compounds within these smaller pieces are broken down by the activities of bacteria, fungi, and other fauna. Throughout this
Soils provide a wide range and variety of microhabitats accommodating diverse flora and fauna. An enormous amount of surface area (hundreds of square meters per gram of soil) exists on the soil particles, which range in size, from small to large in diameter, as sands (0.05-2 micrometer), clays (0.1-2 micrometer), and silts (2-50 micrometer). Numerous microbes, protozoa, and nematodes live in water films on these particles, while the more-mobile fauna, from collembola and mites to such macrofauna as earthworms, millipedes, ants, termites, and moles, move through pores and create burrows in the soil. Because of their role in moving the soil around, macrofauna are often called ecological engineers.
Much of the early history of Georgia and other eastern states consists of the successive clearing and conversion of forests to croplands, followed by continual agricultural use of the land. Such activities led to the extensive erosion of upland areas in Georgia's Piedmont, with the top foot or more of soil disappearing over several decades. Many of the streams in the Piedmont and Coastal Plain were clear before Europeans settled there but were steadily eroded when settlers began cultivating crops in the late eighteenth and early nineteenth centuries. Many of the cropland soils farmed today, when compared with the soils as they existed before colonization, are depleted in organic matter.
A map of principal land-use types in Georgia, created at Iowa State University in Ames, shows agricultural land (including cropland and pastureland), as well as deciduous, coniferous, and mixed-species forests. Relatively high amounts of organic matter per unit area exist in the very wet soils of the Lower Coastal Plain and estuaries, and the largest amounts are found in the Okefenokee Swamp, on the Florida border. Low levels of organic matter predominate around urban areas, particularly in the Atlanta metropolitan area. Fortunately, because of several developments in Georgia, including the extensive conversion of cropland into forest, conservation tillage in croplands, and the Conservation Reserve Program, organic matter is now increasing in the soils of many areas of Georgia.
David C. Coleman, D. A. Crossley Jr., and Paul F. Hendrix, Fundamentals of Soil Ecology, 2d ed. (Boston, Mass.: Elsevier Academic Press, 2004).
E. T. Elliott and D. C. Coleman, "Let the Soil Work for Us," Ecological Bulletins 39 (1988): 23-32.
Stanley Wayne Trimble, Man-induced Soil Erosion on the Southern Piedmont, 1700-1970 (Ankeny, Iowa: Soil Conservation Society of America, 1974).
David C. Coleman, University of Georgia
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