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A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · X · Y · Z   entire Web this site soil The uppermost surface layer of the Earth, in which plants grow and on which, directly or indirectly, almost all life on Earth depends. Soil consists, in the upper layers, of organic material mixed with inorganic matter (see humus) resultant from weathering. Soil depth, where soil exists, may reach to many meters. Between the soil and the bedrock is a layer called the subsoil. Mature soil may be described in terms of four soil horizons: A, the uppermost layer, containing organic matter, though most of the soluble chemicals have been leached (washed out); B, strongly leached and with little or no organic matter; C, the subsoil, a layer of weathered and shattered rock; and D, the bedrock. Three main types of soil are commonly distinguished: pedalfers, associated with temperate, humid climates, have a leached A-horizon but contain iron and aluminum salts with clay in the B-horizon; pedocals, associated with low-rainfall regions, contain soluble substances such as calcium carbonate (soluble in rainwater, which contains carbon dioxide) and other salts; laterites, tropical red or yellow soils, heavily leached and rich in iron and aluminum. Soils may also be classified in terms of texture (e.g., clay, silt, sand). Loams, with roughly equal proportions of sand, silt, and clay, together with humus, are among the richest agricultural soils. Related categories    • AGRICULTURE    • HORTICULTURE    • ECOLOGY AND THE ENVIRONMENT Also on this site: Encyclopedia of Science Encyclopedia of History BACK TO TOP