A hydric soil is a soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part” (59 Fed. Reg. 35680, 7/13/94)
The definition of anaerobic for hydric soils is the “virtual” lack of oxygen, as almost no soil will become completely devoid of oxygen. We use soil morphology, mostly based on the reduction of Iron, to identify hydric soils. Iron is reduced much “lower” on the redox scale than Oxygen. In the photo above, the Iron within the soil changes to a bright orange as it comes back into contact with Oxygen.
As soil becomes waterlogged, the microbes use up O2 as part of their biological process. This can be in a little as two days in a laboratory setting and less than one week in the field. Scientists consider the “upper part” as the major part of the rooting zone, or about 6″ in sandy soils and 12″ in loamy soils.
There are several indicators used to identify the hydric soil component of a wetland on-site. Iron oxidation is one of the most visible indicators of a hydric soil. Certain criteria are also used to generate lists off-site to determine if an area is likely or not to contain hydric soils. Combined, scientists use these indicators and specific criteria to determine if an area is hydric. This data is utilized for wetland delineation and regulation.
Want to learn more about hydric soils? Visit these sites:
Natural Resources Conservation Service, Wetland Science and Soils Division
Soil Science Society of America
Society of Wetland Scientists