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Hardpan
:''For the geography term, a dry terminus of an internally drained basin in a dry climate (i.e. salt flat/playa), see Dry lake.''
In soil science, agriculture and gardening, hardpan or ouklip is a general term for a dense layer of soil, usually found below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water. Some hardpans are formed by deposits in the soil that fuse and bind the soil particles. These deposits can range from dissolved silica to matrices formed from iron oxides and calcium carbonate. Others are man-made, such as hardpan formed by compaction from repeated plowing, particularly with moldboard plows, or by heavy traffic or pollution.
==Formation==
Soil structure strongly affects its tendency to form a hard pan. One such common soil condition related to hardpan is soil pH. Acid soils are most often affected due to the propensity of certain mineral salts, most notably iron and calcium, to form hard complexes with soil particles under acid conditions.
Another major determinant is the soil particle size. Clay particles are some of the smallest particles commonly found in soils. Due to their structure the spaces between individual clay particles is small and already restricts the passage of water, reducing infiltration〔C.Michael Hogan. 2010. (''Abiotic factor''. Encyclopedia of Earth. eds Emily Monosson and C. Cleveland. National Council for Science and the Environment ). Washington DC〕 and hence drainage. Soils with a high clay content are also easily compacted and affected by man-made discharges. Clay particles have a strong negative electrostatic charge and will readily bond to positively charged ions dissolved in the soil-water matrix. Common salts such as sodium ions contained in wastewater can fulfil this role and lead to a localized hardpan in some soil types. This is a common cause of septic system failure due to the prevention of proper drainage in field.
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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