Soils are one of the three parameters used by wetland scientists when assessing a site's character. One of the first things a wetland scientist will review, even before venturing into the field, is the county soil survey. Soil surveys are developed by the National Resource Conservation Service (NRCS) and they can be reviewed in County Soil Conservation District Offices, libraries, and other public offices. Soil surveys describe the character of the county and its soils, and details each soil found in the county, describing both its physical attributes and appearances, as well as any limitations that soil might impose for different types of development or uses. These include limitations that result from a high water table or poor drainage (wetland hydrology). The back of the soil survey contains aerial photographs of the county with the soil types mapped over the aerial photographs. This information, when paired with the New York State (or county) "hydric soils list," helps the wetland scientist locate areas where wetland soils probably occur.
Wetland soils, also known as hydric soils, are soils which are saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part that favor the growth and regeneration of hydrophytic vegetation ((USDA Soil Conservation Service 1985, as amended by the NTCHS in December 1986). This definition connects the hydric soil to the hydrology under which that soil developed and, unless drained, will continue to exist, as well as to the effect that hydrology has on the biological community in and supported by the soil layer. Hydrology is part of the three parameter approach (hydrology, soils, vegetation) used to identify wetlands.
As part of the national effort to prepare a manual to define wetlands, the National Technical Committee For Hydric Soils (NTCHS) developed a list called "Hydric Soils of the United States." This list was created by looking at a variety of methods used to identify and classify soils, determining which criteria would cause a soil to be hydric, (i.e., to have the features identified in the hydric soil definition), and then screening the national soils database for those soils which fit the criteria below:
1. All histosols (organic soils) except folists (an organic soil developed in forests). Histosols, such as peats and mucks, develop a thick (generally greater than 32 inches) organic layer under continuous soil saturation; the lack of oxygen that results from soil saturation causes the organic material to build up in the soil surface, rather than decay. There is not enough oxygen to allow the organic matter to decompose. This is why muck farms lose their organic topsoil when the soils are drained and exposed to air. The presence of organic soils or soils with a histic epipedon, which is an organic surface layer 8 to 16 inches thick over a mineral subsurface, are indicators of hydric soils. Information about whether a soil is a histisol can be found in the description of the soil in the soil survey. Look for words such as a fibrist (peat soil), hemist (part peat, part muck), and saprist (muck). These words may also be found in the soil taxonomic classification table.
2. Soils which have an aquic moisture regime. This means that the presence of water in the soil is one, if not the primary, overwhelming characteristic of the soil. Typically, the soil is poorly drained or very poorly drained and has a water table between 12 and 18 inches below the surface for a week or more during the growing season.
3. Soils that are ponded for 7 to more than 30 days during the growing season.
4. Soils that are flooded for 7 to more than 30 days during the growing season.
Information about a soil's hydrological characteristics can be found in the description of the soil, as well as in a table, in the soil survey, which details the hydrological and drainage characteristics of all soils in the county. A simple tool to look at in the survey is the "capability class" of the soil, which identifies issues related to agricultural production. The letter "w" is an indication of wetness, and is preceded by a numeral from 1 to 8. A capability classification from 1w to 3w is generally not a wetland soil, whereas a capability classification from 4w to 8w is a soil with significant wetness, typically sufficient to cause it to be hydric.
From this screening, the national hydric soils list was developed and, from that list, state and county lists were also prepared to more accurately describe local conditions.
Soils With the Potential for Wetland Inclusions
"Soils with the potential for hydric inclusions" is a phrase which often causes misunderstanding. Because of the scale at which soil survey maps are prepared, small pockets of different soil may not be mapped separately from larger units of another soil. During the process of preparing the soil survey, soil scientists identify which soil types are likely to be included as pockets in larger soil mapping units, based on their associations with each other in the topographic landscape. When the larger soil unit being mapped is an upland soil, but often contains pockets of wetland soils, the upland soil is said to have the "potential for hydric inclusions" (PHI). The New York State Hydric Soils List has a list of "Soils with the Potential for Hydric Inclusions" attached to it. Identifying a soil type or an area mapped as having the potential for hydric inclusions does not mean that it is a wetland soil. It just means there could be pockets of wetland soil scattered through the area and it needs to be looked at more carefully than upland soils without the potential for hydric inclusions.
The wetland scientist looks at the soil survey map to identify areas where hydric soils are mapped and where soils with the potential for hydric inclusions are mapped. This helps in assessing site conditions, and determining where a wetland boundary, based on soils, is likely to be established.
Field Indicators of Hydric Soils
The work conducted by the NTCHS was used by the U.S. Army Corps of Engineers in developing criteria for determining whether hydric soils, one of the three parameters for identifying wetlands, are present. In descending order of reliability, field indicators of hydric soils include:
1. Organic soils. A thick layer (32 inches) of peat or muck which is saturated for 30 consecutive days during the growing season.
2. Histic epipedon. An 8 to 16 inch organic layer over mineral soil, which is saturated for 30 consecutive days during the growing season..
3. Sulfidic material. The classic smell of rotten eggs indicates that all minerals, including sulfur (which is most difficult to reduce), have been reduced to hydrogen sulfide.
4. Aquic or peraquic moisture regime. The soil exists in an anaerobic condition because it is continuously saturated.
5. Reducing soil conditions.
6. Soil colors. A Munsell Soil Color book, a book of standardized color chips that are compared to the soil at 10 inches or immediately below the A horizon, is used to determine the color of the soil. Gleyed soils (soils which are gray, or bluish to greenish gray, due to the reduced state of mineral components, especially iron, in the soil), or grayish soils with mottles (spots of contrasting color) are indications that the soil exists in an anaerobic state.
7. Soils on the National, State or County hydric soils list. If the wetland or soil scientist can identify the soil series in which they are working, and that soil series is listed on the hydric soils list, then the soil is a wetland soil. This takes expertise and experience in soil identification.
8. Iron or manganese concretions. These are local concentrations of chemical compounds which have developed near the surface of the soil. A concretion looks like a nodule of rust, and is not hard like a pebble or rock. It is often gray or dull orange, and can usually be broken apart with a penknife.
Sandy soils, due to their lack of color development, can pose a special identification problem for wetland scientists. In sandy soils, hydric soils are identified by the presence of a thick organic surface layer, similar to a histic epipedon, and the streaking of the subsurface horizons by the organic matter. In addition, in the area of the water table, a spodic horizon or a hard pan develops where the organic material has passed through the sand above the water table and, then, cements the sand particles at the water table. Typically, in a hydric sandy soil, the gray sand color continues below the thick spodic horizon. In comparison, in an upland sandy soil the spodic horizon is much thinner and poorly developed, and the area below that horizon is a colored sandy layer, rather than gray.
Submitted by: Barbara B. Beall, PWS