Wetlands soils are formed by complex biological-geological and chemical processes resulting from low-oxygen conditions created by being frequently saturated with water during the growing season over many decades. The depth and composition (pH, salinity nutrients, minerals) of wetland soils, in turn, helps determine what plants and subsurface organisms a wetland supports. Wetland soils may consist mostly of minerals in sands, silts, or clays, as in a riverine environment such as the Bronx River. More typically, wetland soils are characterized by the build up of inches of organic material through decades or centuries of plant material deposition and slow decomposition, as the peat soil of a salt marsh.
Soils formed under conditions of frequent water saturation during the growing season are called hydric soils. These soils show characteristics of anaerobic conditions (lacking oxygen) near the soil surface and the accumulation of dark organic matter, up to . A typical feature of hydric soils is a grey or dark color highlighted by splotches of brown or yellow where roots and oxygen have penetrated the soil and have oxidized minerals there. These soil color characteristic found within one and one-half feet of the soil surface are a strong indication of wetland conditions. Well-drained course mineral wetland soils, like sands, are not likely to have these mottles, even where frequent flooding may support wetland vegetation.
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