Hydric Soils Technical Note 1
Proper use of Hydric Soil Terminology.
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Several terms are frequently used to describe hydric soil delineation methodology. These are: Hydric Soil Definition, Hydric Soil Criteria, Hydric Soil Lists, Hydric Soil Indicators, and, lastly, hydric soils. According to the deliberations of the National Technical Committee for Hydric Soils (NTCHS), each of these terms has a specific meaning and use. All hydric soils must satisfy requirements of the Hydric Soil Definition. Hydric Soil Criteria are used to generate Hydric Soil Lists. Hydric Soil Lists contain a listing of soils that have a probability of being hydric. Hydric Soil Indicators are primarily morphological indicators used for field identification of hydric soils. Hydric Soil Criteria and Hydric Soil Lists are primarily used as offsite assessment tools. A hydric soil is a soil that meets the Hydric Soil Definition; presence of one (or more) of the Hydric Soil Indicators is evidence that the definition has been met.
The Hydric Soil Definition (Federal Register, July 13, 1994) is: "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." This definition replaced the older 1991 version and accomplished two things. First, a soil that is artificially drained or protected (ditches, levees, etc.) is a hydric soil if the soil in its undisturbed state meets the definition of a hydric soil. Estimated soil properties for manipulated soils are based on best professional estimates of the properties thought to exist before manipulation. Second, the link between the definition and criteria was removed.
The main purpose for the criteria is to create Hydric Soil Lists. According to the NTCHS, Criteria 1, 3, and 4 can be used to document the presence of a hydric soil; however, proof that anaerobiosis exists must also be obtained. Either data or best professional judgement may be used to prove anaerobiosis. Criterion 2 cannot be used to document the presence of a hydric soil; Hydric Soils Indicators are used to document the presence of a hydric soil for saturated soils.
Hydric Soil Lists
Hydric Soil Lists are created by comparing estimated soil properties with the Hydric Soil Criteria. Most commonly the interpretive rating of whether or not a soil has a probability of being hydric is obtained by comparing the estimated soil properties found in a published soil survey with the Hydric Soil Criteria. If any portion of the range of estimated properties for a soil is within the criteria that soil appears on Hydric Soils Lists. For example, if a soil with a permeability of less than 6 in./h. has an estimated water table of 1.0 to 2.0 ft. during any portion of the growing season, that soil would be on the hydric soil list, although most of the range in estimated water table (> 1.0 ft.) is outside the criteria.
Thus, the presence of a soil on a hydric soil list does not mean that the soil is in fact hydric. This is only an interpretive rating. Just as with all interpretations based on information in a published soil survey or other sources of estimated soil properties, hydric soil interpretations are confirmed by on-site investigations.
Hydric Soil Indicators
Field indicators of hydric soils are routinely used in the field to confirm the presence or absence of a hydric soil. The publication "Field Indicators of Hydric Soils in the United States" (USDA, NRCS, 1996, as revised; see Hydric Soil Tech. Note 2) is the current guide that should be applied to identify and delineate hydric soils. The NTCHS is responsible for revising and maintaining the Hydric Soil Indicators. The list of Hydric Soil Indicators is not static. Changes are anticipated as new knowledge of morphological, physical, chemical, and mineralogical soil properties accumulates. Revisions and additions will continue as we continue to gain a better understanding of the relationships between the development of recognizable soil properties and anaerobic soil conditions. Comments regarding field observations of hydric soil conditions that cannot be documented using the presently recognized Hydric Soil Indicators are welcome; however, any modifications must be approved by the NTCHS. To properly use the Indicators, a basic knowledge of soils, soil landscape relationships, and soil survey procedures is necessary. Many of the Hydric Soil Indicators are landform specific. Professional soil or wetland scientists familiar with local conditions are best equipped to make an on-site hydric soil determination. The indicators are designed to be regionally specific. Each indicator states the Land Resource Regions (LRRs) or the Major Land Resource Areas (MLRAs) in which it can be used. The geographic extent of LRRs and MLRAs is defined in USDA Ag. Handbook 296 (USDA, SCS, 1981).
The Indicators are used to identify the hydric soil component of wetlands; however, there are some hydric soils that lack one of the currently listed indicators. Therefore, the lack of an indicator does not exclude the soil from being classed as hydric. Such soils should be investigated in detail and their characteristic morphologies documented. This documentation will serve as a basis for establishing additional Hydric Soil Indicators that are not presently recognized. The approved indicators should be tested for use in LRRs other than those listed. Also, 16 indicators for testing have been identified for specific LRRs. Users are encouraged to test these and submit any additional morphological properties they believe are indicative of hydric soils. See Hydric Soils Tech. Note 3 (Protocol for Requesting NTCHS Changes to "Field Indicators of Hydric Soils in the United States").
All hydric soils must satisfy requirements of the definition. Criteria are used to generate lists. Lists contain a listing of soils that have a probability of being hydric. Indicators are primarily morphological indicators used for field identification of hydric soils. Criteria and lists are primarily used as offsite assessment tools. A hydric soil is a soil that meets the definition; presence of one (or more) of the indicators is evidence that the definition has been met.