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Soils can process and contain considerable amounts of water. They can take in water, and will keep doing so until they are full, or until the rate at which they can transmit water into and through the pores is exceeded. Some of this water will steadily drain through the soil (via gravity) and end up in the waterways and streams, but much of it will be retained, away from the influence of gravity, for use of plants and other organisms to contribute to land productivity and soil health. ==Soil water retention== Pores (the spaces that exist between soil particles) provide for the passage and/or retention of gasses and moisture within the soil profile. The soil's ability to retain water is strongly related to particle size; water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil, so clays generally retain more water.〔Leeper, G.W. & Uren, N.C., 1993. ''Soil Science: An Introduction'', 5th edn. Melbourne University Press, Melbourne.〕 Conversely, sands provide easier passage or transmission of water through the profile. Clay type, organic content, and soil structure also influence soil water retention.〔Charman, P.E.V. & Murphy, B.W., 1998. ''Soils: Their Properties and Management'', 5th edn. Oxford University Press, Melbourne.〕 The maximum amount of water that a given soil can retain is called field capacity, whereas a soil so dry that plants cannot liberate the remaining moisture from the soil particles is said to be at wilting point.〔 Available water is that which the plants can utilize from the soil within the range of field capacity and wilting point. Roughly speaking for agriculture (top layer soil), soil is %25water, %25 air, %45 mineral, %5 other; water varies widely from about %1 to %90 due to several retention and drainage properties of a given soil. The role of soil water retention is profound; its effects are far reaching and relationships are invariably complex. This section focuses on a few key roles and recognizes that it is beyond the scope of this discussion to encompass all roles that can be found in the literature. The process by which soil absorbs water and water goes down is called percolation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Soil water (retention)」の詳細全文を読む スポンサード リンク
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