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This rating indicates the percentage of map units that meets the criteria
for hydric soils. Map units are composed of one or more map unit components
or soil types, each of which is rated as hydric soil or not hydric. Map units
that are made up dominantly of hydric soils may have small areas of minor
nonhydric components in the higher positions on the landform, and map units
that are made up dominantly of nonhydric soils may have small areas of minor
hydric components in the lower positions on the landform. Each map unit is
rated based on its respective components and the percentage of each component
within the map unit.
The thematic map is color coded based on the composition of hydric components.
The five color classes are separated as 100 percent hydric components, 66 to
99 percent hydric components, 33 to 65 percent hydric components, 1 to 32 percent
hydric components, and less than one percent hydric components.
In Web Soil Survey, the Summary by Map Unit table that is displayed below the
map pane contains a column named 'Rating'.In this column the percentage of each
map unit that is classified as hydric is displayed.
Hydric soils are defined by the National Technical Committee for Hydric Soils (NTCHS)
as soils that formed under conditions of saturation, flooding, or ponding long enough
during the growing season to develop anaerobic conditions in the upper part (Federal
Register, 1994). Under natural conditions, these soils are either saturated or
inundated long enough during the growing season to support the growth and reproduction
of hydrophytic vegetation.
The NTCHS definition identifies general soil properties that are associated with
wetness. In order to determine whether a specific soil is a hydric soil or nonhydric
soil, however, more specific information, such as information about the depth and
duration of the water table, is needed. Thus, criteria that identify those estimated
soil properties unique to hydric soils have been established (Federal Register, 2002).
These criteria are used to identify map unit components that normally are associated
with wetlands. The criteria used are selected estimated soil properties that are
described in "Soil Taxonomy" (Soil Survey Staff, 1999) and "Keys to Soil Taxonomy"
(Soil Survey Staff, 2006) and in the "Soil Survey Manual" (Soil Survey Division Staff, 1993).
If soils are wet enough for a long enough period of time to be considered hydric,
they should exhibit certain properties that can be easily observed in the field.
These visible properties are indicators of hydric soils. The indicators used to make
onsite determinations of hydric soils are specified in "Field Indicators of Hydric
Soils in the United States" (Hurt and Vasilas, 2006).