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Soil in the United States
==Factors Contributing to Soil Diversity==
Soils are the product of climate, organisms and topography, acting on parent (geologic) material over time. Thus the great diversity of geologic materials, geomorphic processes, climatic conditions, biotic assemblages and land surface ages in the United States is responsible for the presence of an enormous variety of mineral and organic soils. (Most of the mineral soils contain significant quantities of organic matter, but not enough to qualify for classification as organic soils.) The inorganic particles of different mineral soils vary greatly in size distribution, often as a result of transport and deposition of the parent material from which the soil is formed. Examples include loess (wind-deposited silt), dune sands, alluvial (river-deposited) sands and silts, and glacial till (which may include substantial amounts of clay, silt, sand, gravel and larger particles). Compared with sands (0.05 to 2 mm in diameter), silts (0.002 to 0.05 mm in diameter) have a very much larger specific surface (i.e. particle surface area per unit mass). At the surface of a particle, weathering processes occur. If the particle contains potential plant nutrients in mineral form, such processes result in release of the nutrients in readily available, ionic form. Thus, high specific surface is a major reason why silty soils tend to be relatively fertile. Clay particles are finer than silt, being less than 0.002 mm in diameter. Water retention tends to be greater in the finer-textured soils. If a fine-textured soil is well aggregated (with aggregates consisting of numerous organic and inorganic particles bonded together), the large pores between aggregates will facilitate drainage and aeration. (In contrast, drainage and aeration can be poor in poorly aggregated fine-textured soils in which nearly all of the pore space consists of fine pores.) Drainage is usually good and trafficability is usually superior in the coarser-textured soils. While some of the clay in a soil may have been inherited in the parent material, older soils might contain a significant amount of clay formed by weathering processes during soil formation. Soils with a high concentration of clay and organic matter tend to have considerable net negative electrical charge, conferring ability to retain many plant nutrient cations (e.g. Ca2+, Mg2+, K+, NH4+), readily available to plants by ion exchange. Plant nutrients are also released from soil organic matter by decomposition, and organic matter is particularly significant as the major form in which soil nitrogen is stored. Organic matter contributes to aggregation and water-retention properties of soil. Soil chemical composition reflects not only the original geologic materials (e.g. limestone, granite, basalt), but also soil-forming processes since deposition. In much of the northern US, soil formation commenced either shortly after glacial retreat at the end of the last Ice Age, or even more recently. Elsewhere in the US, one may find some older land surfaces where soil formation has occurred over a much longer period, in addition to some young soils.〔Brady, N. C. and R. R. Weil. 1999. The Nature and Properties of Soils. 12th Ed. Prentice-Hall. 881 pp.〕〔Miller, R. W. and D. T. Gardiner. 2001. Soils in Our Environment. 9th Ed. Prentice-Hall. 642 pp.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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