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Life In The Soil

Soils teem with life. In fact, more creatures live below the surface of the earth than live above. Among these soil dwellers are bacteria, fungi, and algae, which feed on plant and animal remains breaking them down into humus, the organic component of soil, in the process. The numbers of these microscopic soil organisms is vast—a gram of soil, which would fit into a peanut shell, can contain from several hundred million to a few billion microorganisms. The importance of their actions to the health of the soil is equally large.

Bacteria are the most abundant life form in most soils and are responsible for the decay of the residue from crops. Certain bacteria convert ammonia in soils into nitrogen, a fundamental plant nutrient. Some algae perform the same function (assuring a nitrogen supply) in rice paddy soils. Algae are numerous on the surfaces of moist soil. Fungi teem in soils, and range from several celled fungi to the large wild mushrooms that grow on moist soil. Fungi are capable of decomposing a greater variety of organic compounds than bacteria.

Nematodes are also abundant in most soils, and these eel-shaped, colorless worms are slightly larger than bacteria, algae, and fungi. An acre of soil may hold as many as 1 million nematodes. Most nematodes feed on dead plants, but some are parasites, and eat the roots of crops such as citrus, cotton, alfalfa, and corn.

Ants abound in soils. They create mazes of tunnels and construct mounds, mixing soils and bringing up subsurface soils in the process. They also gather vegetation into their mounds, which, as a result, become rich in organic matter. By burrowing and recolonizing, ants can eventually rework and fertilize the soil covering an entire prairie.

Earthworms burrow through soils, mixing organics with minerals as they go, and aerating the soil. Some earthworms pull leaves from the forest floor into their burrows, called middens, enriching the soil. The burrowing of the 4,000 or so worms that can inhabit an acre of soil turns and aerates soil, bringing 7-18 tons of soil to the surface annually.

Larger animals inhabit soils, including moles, which tunnel just below the surface eating earthworms, grubs, and plant roots, loosening the soil and making it more porous. Mice also burrow, as do ground squirrels, marmots, and prairie dogs; all bringing tons of subsoil material to the surface. These animals all prefer dry areas, so the soils they unearth are often sandy and gravelly.



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Brady, Nyle C. The Nature and Properties of Soils. New York: Macmillan, 1989.

Foth, Henry D. Fundamentals of Soil Science. New York: John Wiley & Sons, 1990.

Hamblin, W.K., and Christiansen, E.H. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Harpstead, M.I., F.D. Hole, and W.F. Bennet. Soil Science Simplified. Ames, IA: Iowa State University Press, 1988.

Hillel, Daniel. Out of the Earth. New York: The Free Press, 1991.

Middleton, Gerard V., and Celestina V. Cotti Ferrero. Encyclopedia of Sediments & Sedimentary Rocks. Boston: Kluwer Academic Publishers, 2003.

Spearks, Donald L. Environmental Soil Chemistry. 2nd ed. New York: Academic Press, 2002.


Rhonda, R. J. "Spatial Heterogeneity of The Soil Moisture Content and Its Impact." Journal of Hydrometeorology 3, no. 5 (2002): 556-570.

Beth Hanson


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Alluvial soils

—Soils containing sand, silt, and clay, which are brought by flooding onto lands along rivers; these young soils are high in mineral content, and are the most productive soils for agriculture.


—The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.


—The portion of soil comprising the smallest soil particles, those with diameters less than 0.002 mm, which is composed mainly of hydrous aluminum silicates and other minerals.


—Layers of soil that have built up over time and lie parallel to the surface of the earth; these are composed of soils of varying thickness, color, and composition.


—The portion of the soil necessary to plants for growth, including nitrogen, potassium, and other minerals.

Organic matter

—The carbonaceous portion of the soil that derives from once living matter, including, for the most part, plants.

Parent material

—Loose mineral matter scattered over the Earth by wind, water, or glacial ice, or weathered in place from rocks.


—The movement of water down through soil layers, through which minerals and nutrients are moved through soil.


—Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.


—Soil particles derived mainly from sedimentary materials that range between 0.0002–0.05 mm in size.

Soil series

—Soils that share a defined set of characteristics and share the same name.


—The uppermost layer of soil, to a depth of approximately 7.1-7.9 in (18-20 cm), which is the primary feeding zone for agricultural plants.

Additional topics

Science EncyclopediaScience & Philosophy: Adam Smith Biography to Spectroscopic binarySoil - Soil Formation, Soil Profiles And Horizons, Aging Soils, Soil Categories, Soil Groups And Agriculture