Hydric Soils of the United States
Kenai Hydric Soils and definitions
Draft Alaska Hydric Field Indicators
(The Army Corps of Engineers is in the process of regionalizing the 1987
Delineation Manual, when that is complete, these indicators will be incorporated
into the Alaska Manual)
The definition of 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. The concept of hydric
soils includes soils developed under sufficiently wet conditions to support
the growth and regeneration of hydrophytic vegetation. Soils that are sufficiently
wet because of artificial measures are included in the concept of hydric
soils. Also, soils in which the hydrology has been artificially modified
are hydric if the soil, in an unaltered state, was hydric. Some series,
designated as hydric, have phases that are not hydric depending on water
table, flooding, and ponding characteristics.
The list of hydric soils was created by computer using
criteria that were developed by the
Technical Committee for Hydric Soils. The criteria are selected soil
properties that are documented in Soil Taxonomy (Soil Survey Staff , 1999)
and were designed primarily to generate a list of hydric soils from
the national database of Map Unit Interpretation Records (MUIR). Criteria
1, 3, and 4 serve as both database criteria and as indicators for identification
of hydric soils. Criterion 2 serves only to retrieve soils from
In addition, the wording of criteria 1 & 2 were changed in
2000 to incorporate recent changes in Soil Taxonomy ( Soil Survey Staff,
IT IS IMPORTANT TO NOTE THAT THESE CHANGES DO NOT CAUSE ANY SOILS TO
BE ADDED OR DELETED FROM THE LIST.
Field Indicators are soil characteristics which are documented to be
strictly associated only with hydric soils. Field Indicators are
an efficient on-site means to confirm the presence of hydric soil.
The Field Indicators are designed to identify soils which meet the hydric
soil definition without further data collection. Some hydric soils
exist for which no Field Indicators have yet been recorded and documented,
and to identify these soils as hydric, evidence must be gathered to demonstrate
that the definition is met. Additional Field Indicators are being developed
Hydric Soil lists have a number of agricultural and nonagricultural
applications. These include assistance in land-use planning, conservation
planning, and assessment of potential wildlife habitat. A combination of
the hydric soil, hydrophytic vegetation, and hydrology criteria
defines wetlands as described in the National Food Security Act Manual
(Soil Conservation Service, 1994) and the Corps of Engineers Wetlands Delineation
Manual (Environmental Laboratory, 1987). Therefore an area that meets the
hydric soil criteria must also meet the hydrophytic vegetation and
wetland hydrology criteria in order for it to be classified as a jurisdictional
The general list of hydric soils is maintained in a computer
file and is updated periodically. The most current list of hydric soils
may be obtained directly from the USDA-NRCS Hydric Soils Homepage, electronically,
or as hardcopy, for the cost of printing from the Natural Resources Conservation
Service (NRCS) Project Manager, Statistical Laboratory, Iowa State University,
217 Snedecor Hall, Ames, IA 50011. State lists of hydric soils are
also available electronically from the USDA-NRCS Hydric Soils Homepage
or as hardcopy from the NRCS State Conservationist in each state. The NRCS
also maintains, for each conservation district in the United States, lists
of map units that contain, or may, in some delineations, contain
soils. These detailed lists are available by contacting the respective
NRCS State Conservationist and are recommended for preliminary use in making
wetland determinations. Field Indicators should be used for on-site determinations
of hydric soils.
DEFINITION OF HYDRIC SOIL
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.
The following criteria reflect those soils that are likely to
meet this definition.
CRITERIA FOR HYDRIC SOILS
All Histels except Folistels and Histosols except Folists, or
Soils in Aquic suborders, great groups, or subgroups, Albolls suborder,
Historthels great group, Histoturbels great group, Pachic subgroups,
or Cumulic subgroups that are:
a. Somewhat poorly drained with a water table* equal to
0.0 foot (ft) from the surface during the growing season, or
b. poorly drained or very poorly drained and have either:
(1) water table* equal to 0.0 ft during the growing
season if textures are coarse sand, sand, or fine sand in all layers
within 20 inches (in),
or for other soils
(2) water table* at less than or equal to 0.5 ft from the
surface during the growing season if permeability is equal to or
greater than 6.0 in/hour (h) in all layers within 20 in, or
(3) water table* at less than or equal to 1.0 ft from the
surface during the growing season if permeability is less than 6.0
in/h in any layer within 20 in, or
Soils that are frequently ponded for long duration or very long duration
during the growing season, or
Soils that are frequently flooded for long duration or very long duration
during the growing season.
GLOSSARY OF TERMS USED IN DEFINING HYDRIC SOILS
a situation in which molecular oxygen is virtually absent from the
artificial hydric soil:
a soil that meets the definition of a hydric soil as a result
of an artificially induced hydrologic regime and did not meet the definition
before the artificial measures were applied.
a condition in which ground or surface water has been removed by
a condition in which the soil surface is temporarily covered with
flowing water from any source, such as streams overflowing their banks,
runoff from adjacent or surrounding slopes, inflow from the high tides,
or any combination of sources.
frequently flooded, ponded, saturated:
a frequency class in which flooding, ponding, or saturation is likely
to occur often under usual weather conditions (more than 50 percent chance
in any year, or more than 50 times in 100 years).
the portion of the year when soil temperatures are above biologic
zero at 50 cm (19.7"). The following growing season months are assumed
for each of the soil temperature regimes of Soil Taxonomy:
plant life growing in water or on a substrate that is at least periodically
deficient in oxygen as a result of excessive water content.
a duration class in which innundation for a single event ranges from 7
days to 1 month.
the ease with which water passes through a bulk mass of soil or a layer
of soil. In the Map Unit Interpretation Record (MUIR) database, permeability
is expressed as the number of inches per hour that water moves downward
through the saturated soil.
a subdivision of a soil series based on features that affect its use and
management (e.g. slope, surface texture, stoniness, and thickness).
a condition in which water stands in a closed depression. The water is
removed only by percolation, evaporation, or transpiration.
water is removed from the soil so slowly that the soil is saturated periodically
during the growing season or remains wet for long periods.
a condition in which all voids (pores) between soil particles are filled
a group of soils having horizons similar in differentiating characteristics
and arrangements in the soil profile, except for texture of the surface
somewhat poorly drained:
water is removed slowly enough that the soil is wet for significant periods
during the growing season.
very long duration:
a duration class in which innundation for a single event is greater than
very poorly drained:
water is removed from the soil so slowly that free water remains at or
on the surface during most of the growing season.
the upper surface of ground water where the water is at atmospheric pressure.
In the Map Unit Interpretation Record (MUIR) database, entries are made
for the zone of saturation at the highest average depth during the wettest
season. It is at least six inches thick and persists in the soil for more
than a few weeks. In other databases, saturation, as defined in Soil
Taxonomy (Soil Survey Staff. 1999), is used to identify conditions
that refer to water table in Criteria 2.
Environmental Laboratory. 1987. Corps of Engineers Wetland Delineation
Manual, Technical Report Y-87-1, US Army Engineer Waterways Experiment
Station, Vicksburg, Miss.
Soil Conservation Service. 1994. National Food Security Act
Manual. Title 180. USDA Soil Conservation Service, Washington, D.C.
Soil Survey Staff. 1999. Soil Taxonomy: A Basic System of Soil
Classification for Making and Interpreting Soil Surveys. USDA Natural
Resources Conservation Service, Agric. Hdbk. 436, U.S. Government Printing
Office, Washington, D.C. 869 pp.
Soil Survey Staff. 1994. National Soil Survey Handbook.
USDA Soil Conservation Service, Washington, D.C.