Hydric Soil Determination and Delineation by: Wade Hurt, USDA, NRCS, NSSC/University of Florida, Gainesville, FL and Victor Carlisle, University of Florida, Gainesville, Florida

Introduction Although it is impossible to delineate hydric soils without actually being on-site, this lecture provides some basic operating procedures; it explains where to look on the landscape and where to look in a soil profile to determine if an indicator is met. It enforces that pedon description forms should be completed on both sides of hydric soil boundaries. Much of the material presented in this lecture comes from Hurt and Carlisle

Landform Recognition A landscape is the land surface that an eye can comprehend in a single view (Tuttle, 1975; Soil Survey Staff, 1993). Most frequently it is a collection of landforms. Landforms are physical, recognizable forms or features on the earth's surface that have characteristic shapes produced by natural processes. Hydric soils occur on landforms (US Department of Agriculture, 1993a) that include backswamps, bogs, depressions, estuaries, fens, interdunes, marshes, flats, flood plains, muskegs, oxbows, playas, pocosins, potholes, seep slopes, sloughs, and swamps (next slide). One of the most important factors in hydric soil determination and delineation is landform recognition.

Hydric Soil Landforms Hydric soils develop because unoxygenated water saturates the soil or collects on the soil surface. A concave surface frequently augmented by slower percolating subsurface soil horizons allows this process to occur. Hydric soil indicators normally begin to appear at this concave slope break and continue throughout the extent of the wetland although concavity may not exist throughout the wetland (next slide).

Idealized landscape depicting uplands and the hydric soil landforms pocosin, flat, depression, back swamp, swamp, pot hole, and seep slope. Note that each hydric soil area begins at a slightly concave slope break, although not all of each hydric soil area express concavity through the landform (flat and seep slope). Vertical scale is exaggerated.

Delineating Hydric Soils Wetland delineators need to become very familiar with the landscapes and hydrology of their area in order to recognize the often very subtle slope break. They need to anticipate where inundated or saturated soils are likely to occur. Water is the driving force behind the development of hydric soils (wetlands) and hydrology of the landscape must be understood prior to making hydric soil determinations and delineating wetlands.

Delineating Hydric Soils vs. Delineating Wetlands What, if any, are the differences between delineating wetlands and delineating hydric soils? –Delineated wetlands must meet the three criteria of hydric soils, wetland vegetation, and wetland hydrology. –Delineated hydric soils need meet only the one criterion of hydric soils (hydrology and/or vegetation may be altered or removed altogether). –Delineated hydric soil include wetlands and areas capable of being restored to wetlands.

Getting Started Soil Science and its Application to Hydric Soils Soil Science and its Application to Hydric SoilsTo document a hydric soil first remove all loose leaf matter, bark, and any other material to expose the surface (picture in first slide). Dig a hole approximately 20 to 30 cm in diameter to expose the soil profile (nest slide). Then describe the soil profile to the depth necessary using the procedures outlined in the lecture Soil Science and its Application to Hydric Soils Soil Science and its Application to Hydric Soils.

Describe the Hydric Soil Use the completed soil description to specify which, if any, of the hydric soil indicators (Hurt, et al. 2002) have been identified (next slide). When describing the soil profile pay close attention to depths, texture, color of matrix and contrasting areas. Note in the remarks section if contrasting areas are due to redox processes (redox) or not (non-redox).

Describe the Nonhydric Soil Use the completed soil description to specify that a hydric soil is indicator is not met. When describing the soil profile pay close attention to depths, texture, color of matrix and contrasting areas. Note in the remarks section if contrasting areas are due to redox processes (redox) or not (non-redox).

Depth of Examination Deep examination of soil may be required where hydric soil indicators are not easily seen. It is always recommended that soils be excavated and described as deep as necessary to make reliable interpretations. For example, examination to less than 50 cm may suffice in soils with surface horizons of organic material or mucky mineral material because these shallow organic accumulations mostly occur only in hydric soils. Conversely, depth of excavation will often be greater than 50 cm in Mollisols because the upper horizons of these soils, due to the masking effect of organic material, often contain no visible redoximorphic features. At many sites making exploratory observations to 1 m or more is necessary. These observations should be made with the intent of documenting and understanding the variability in soil properties and hydrologic relationships on the site.

Depth from which to Measure? Depths used in making hydric soil determinations are measured from the very top of the material upon which standing – nationwide when applying indicators A1, A2 (Soil Survey Staff. 1999), and A3 and – in LRRs F, G, H, and M if the material beneath any mucky peat and/or peat is sandy. In LRR R depths used in making hydric soil determinations are measured from the top of the mineral surface (underneath any and all fibric, hemic, and/or sapric material) except for application of A1, A2, and A3. In the remaining LRRs for all soil materials and in LRRs F, G, H, and M if the material beneath any mucky peat and/or peat is loamy/clayey, depths used in making hydric soil determinations are measured from the muck or mineral surface (underneath any fibric and/or hemic material) except for application of A1, A2, and A3.

Depth from which to Measure The soil to the right has hemic soil material (texture of mucky peat) approximately 9 cm thick directly underlain by sandy mineral soil material. For hydric soil indicator application, the depth from which to measure would be the very surface in LRRs F, G, H, and M and from the sandy mineral surface in the remaining LRRs. If the soil material was loamy/clayey the depth from which to measure would be the mineral surface in all LRRs.

Delineating The process of delineating hydric soil boundaries on undisturbed landscapes is really rather simple in concept but can be difficult in practice. Where the landscape is undisturbed, the upland boundary of hydric soils is at a landform change. That change is usually a convex/concave slope break. Hydric soils occur at the concave slope change and soils that are nonhydric occur at the convex slope change (left arrow in photo). The right arrow denotes a constructed slope break.

Delineating The slope break may be very subtle or hidden with vegetation but it will be there. The two arrows denote two convex/concave slope breaks on the landscape (note the convex to concave shape of the PVC pipe at the left arrow). The site is being monitored to determine which, if either is the hydric soil convex/concave slope break. Often the boundary delineates a very intricate pattern of extremely small areas of hydric soils and soils that are nonhydric.

Delineating The easiest way to delineate hydric soils is to begin on the upland side of a wetland and traverse toward the wetland looking for concave slope breaks. Not all concave slope breaks delineate hydric soils; however, the hydric/nonhydric boundary of undisturbed soils will usually be at a concave slope break (see the section on Disturbed Soils for an explanation of how to delineate these soils). By traversing once or twice the hydric soil boundary can frequently be located. Once the boundary is located using vegetation is most expeditious or, where vegetation is absent, the landform change (convex to concave slope break) to assist in completing the delineation. Most often, if natural vegetation is present, one or two species can be correlated to the hydric soil boundary and thereby used to provide the key to a correct delineation.

Delineating: Example In the flatwoods and associated landform areas of the southeastern United States (LRRs T and U), most often the nonhydric soils have the shrub saw palmetto (Serenoa repens L.) which disappears near the hydric soil boundary to be replaced by other shrubs such as gallberry (Ilex glabra L.) and fetterbush (Lyonia lucida L.) in LRR T or by herbaceous plants such as blue maidencane (Amphicarpum mulenbergianum L.) in LRR U.

These are neighboring pedons at the convex to concave slope break. Note that a stripped matrix occurs within 15 cm at the extreme right (hydric) and below 15 cm at the extreme left (nonhydric).

Indicator Development Understanding that the field indicators are indicators known to identify hydric soils is important. They were developed by observing soil pedons both inside and outside ecological wetlands. Pedons inside the line were described; descriptions of pedons outside the line were not deemed necessary. For example, S7 (Dark Surface) requires a layer 10 cm or more thick starting within the upper 15 cm of the soil surface with a matrix value 3 or less and chroma 1 or less with at least 70% of the visible soil particles covered, coated, or masked with organic material and the matrix color of the layer immediately below the dark layer must have chroma 2 or less.

Indicator Development This does not mean that the pedons outside the hydric soil boundary had all requirements of this indicator except thickness of the dark surface. It means that, because of the concave slope break, pedons outside the line are normally very dissimilar to pedons inside the line(next slide). Normally, neighboring pedons outside the line have a surface layer that has a salt and pepper appearance and is more of a 50/50 mixture of soil material covered, coated, or masked with organic material (pepper) and soil material not covered, coated, or masked (salt).

These two soil profiles were taken from areas about 2 m apart. The left profile is hydric with a very obvious S7 (Dark Surface) indicator. The soil on the right is nonhydric and has a salt and pepper surface layer.

Summary Hydric soils most often occur starting at a slope break that is changing from convex to concave. Depths from which to measure vary with the soil material present and vary according to the LRR in which the decision is being made. A pedon description form should be completed from soil profiles on both sides of the delineation boundary paying close attention to depth, texture, color of matrix and contrasting areas while noting if the contrasting areas are formed from redox processes or not and indicating which, if any, hydric soil indicator is met. To adequately defend delineations, the two pedons described should be less than 2 maters apart.

Literature Cited Hurt, G. W. and V.W. Carlisle Delineating hydric soils. In Wetland Soils: Their Genesis, Morphology, Hydrology, Landscapes and Classification. J.L. Richardson and M.J. Vepraskas, (Eds.) Lewis Publishers, Boca Raton, FL. Hurt, G.W., P.M. Whited, and R.F. Pringle ( Eds.) Field indicators of hydric soils in the United States (Version 5.0), USDA, NRCS, Fort Worth, TX. Soil Survey Staff National Soil Survey Handbook. USDA, Soil Conservation Service, US Govt. Printing Off., Washington, DC. Soil Survey Staff Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA Agricultural Handbook 436. US Govt. Printing Off., Washington, DC. Tuttle, S.D., Landforms and landscapes. Wm. C. Brown Company, Dubuque, IA.