Hydric soil issoil which is permanently or seasonally saturated by water, resulting inanaerobic conditions, as found inwetlands.
Mostsoils areaerobic. This is important becauseplant roots respire (that is, they consumeoxygen andcarbohydrates while releasingcarbon dioxide) and there must be sufficient air—especially oxygen—in the soil to support most forms ofsoil life. Air normally moves through interconnectedpores by forces such as changes inatmospheric pressure, the flushing action of rainwater, and by simplediffusion.
In addition to plantroots, most forms of soilmicroorganisms need oxygen to survive. This is true of the more well-knownsoil animals as well, such asants,earthworms andmoles. But soils can often become saturated with water due to rainfall and flooding.Gas diffusion in soil slows (some 10,000 times slower) when soil becomes saturated with water because there are no open passageways for air to travel. When oxygen levels become limited, intense competition arises between soil life forms for the remaining oxygen. When this anaerobic environment continues for long periods during the growing season, quite different biological and chemical reactions begin to dominate, compared with aerobic soils. In soils where saturation with water is prolonged and is repeated for many years, unique soil properties usually develop that can be recognized in the field. Soils with these unique properties are called hydric soils, and although they may occupy a relatively small portion of the landscape, they maintain importantsoil functions in the environment.[1]
The plants found in hydric soils often haveaerenchyma, internal spaces in stems and rhizomes, that allow atmospheric oxygen to be transported to the rooting zone.[2] Hence, manywetlands are dominated by plants with aerenchyma;[3] common examples include cattails, sedges and water-lilies.
A hydric soil is defined by federal law[4] to mean "soil that, in its undrained condition, is saturated, flooded, or ponded long enough during a growing season to develop an anaerobic condition that supports the growth and regeneration of hydrophytic vegetation". This term is part of the legal definition of awetland included in theUnited StatesFood Security Act of 1985 (P.L. 99-198). This definition is provided in the controlling regulations to the Wetland Conservation Provisions of the FSA of 1985(7 C.F.R 12) and is used by the U.S.D.A. Natural Resources Conservation Service in the administration of the Wetland Conservation Compliance provisions ("Swampbuster") contained in the FSA of 1985. In adopting this definition in 1985, Congress attempted to capture the duration of waterlogged condition of a hydric soil by adding that a hydric soil is waterlogged long enough to support not only the growth of plants adapted to life in anaerobic conditions but also the regeneration of such plants.
Another common definition of a hydric soils is provided by the National Technical Committee of Hydric Soils (NTCHS) as "a soil that formed under conditions of saturation, flooding, orponding long enough during the growing season to develop anaerobic conditions in the upper part."[5] The NTCHS hydric soil definition is used by the U.S. Army Corps of Engineers and the Environmental Protection Agency in their joint responsibilities in the administration of Section 404 of the Clean Water Act (1972).
