Hydric Soils of the United States

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)

Introduction

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 National 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 the database.

In addition, the wording of criteria 1 & 2 were changed in 2000 to incorporate recent changes in Soil Taxonomy ( Soil Survey Staff, 1999).

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 and tested.

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 wetland.

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 hydric 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

anaerobic:: a situation in which molecular oxygen is virtually absent from the environment.
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.
drained:: a condition in which ground or surface water has been removed by artificial means.
flooded:: 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).
growing season:: 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:
hydrophytic vegetation:: plant life growing in water or on a substrate that is at least periodically deficient in oxygen as a result of excessive water content.
long duration:: a duration class in which innundation for a single event ranges from 7 days to 1 month.
permeability:: 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.
phase, soil:: a subdivision of a soil series based on features that affect its use and management (e.g. slope, surface texture, stoniness, and thickness).
ponded:: a condition in which water stands in a closed depression. The water is removed only by percolation, evaporation, or transpiration.
poorly drained:: water is removed from the soil so slowly that the soil is saturated periodically during the growing season or remains wet for long periods.
saturated:: a condition in which all voids (pores) between soil particles are filled with water.
soil series:: a group of soils having horizons similar in differentiating characteristics and arrangements in the soil profile, except for texture of the surface layer.
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 1 month.
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.
^*water table:: 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.

Literature Cited

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.