Hydric soils are described in Chapter 2 as soils that formed under anaerobic conditions that develop while the soils are inundated or saturated near their surface. These soils can form under a variety of hydrologic regimes that include nearly continuous saturation (swamps, marshes), short-duration flooding (riparian systems), and periodic saturation by groundwater. The most significant effect of excess water is isolation of the soil from the atmosphere and the prevention of O

from entering the soil. The blockage of atmospheric O

induces biological and chemical processes that change the soil from an aerobic and oxidized state to an anaerobic and reduced state. This shift in the aeration status of the soil allows chemical reactions to occur that develop the common characteristics of hydric soils, such as the accumulation of organic carbon in A horizons, gray-colored subsoil horizons, and production of gases such as H

S and CH

. In addition, the creation of anaerobic conditions requires adaptations in plants if they are to survive in the anaerobic hydric soils.