Federal Register / Vol. 56. No. 157 / Wednesday. August 14. 1991 / Proposed Rules 40457 



physical characteristics that are usually 

 indicative of hydric soils. 



Soils are separated into two major 

 types on the basis of material 

 composition: organic soil and mineral 

 soil. In general, soils with at least 16 

 inches of organic material in the upper 

 part of the soil profile and soils with 

 organic material resting on bedrock are 

 considered organic soils [Histosols]. 

 Soils largely composed of sand. silt, 

 and/or clay are mineral soils. For 

 technical definitions, see "Soil 

 Taxonomy". U.S.D.A. Soil Survey Staff 

 1975. 



Organic Soils 



Accumulation of organic matter in 

 most organic soils results from 

 anaerobic soil conditions associated 

 with long periods of submergence or soil 

 saturation during the growing season. 

 These saturated conditions impede 

 aerobic decomposition (oxidation) of the 

 bulk organic materials such as leaves, 

 stems, and roots, and encourage their 

 accumulation over time as peat or muck. 

 Consequently, most organic soils are 

 characterized as very poorly drained 

 soils. Organic soils typically form in 

 waterlogged depressions, and peat or 

 muck deposits may range from about 1.5 

 feet to more than 30 feet deep. Organic 

 soils also develop in low-lying areas 

 along coastal waters where tidal 

 Qooding is frequent. 



Hydric organic soils are subdivided 

 into three groups based on the presence 

 of identifiable plant material: (1) Muck 

 (Saprists) in which two-thirds or more of 

 the material is decomposed and less 

 than one-third of the plant fibers are 

 identifiable; (2) peat (Fibrists) in which 

 less than one-third of the material is 

 decomposed and more than two-thirds 

 of the plant fibers are still identifiable; 

 and (3) mucky peat or peaty muck 

 (Hemists) in which the ratio of 

 decomposed to identifiable plant matter 

 is more nearly even (U.SJ3JL Soil 

 Survey Staff 1975). A fourth group of 

 organic soils (Folists) exists in tropical 

 and boreal mountainous areas where 

 precipitation exceeds the 

 evapotranspiration rate, but these soils 

 are never satiirated for more than a few 

 days after heavy rains and thus do not 

 develop under hydric conditions. All 

 organic soils, with the exception of the 

 Folists, are hydric soils. 



Hydric organic soils can be easily 

 recognized as black-colored muck to 

 dark brown-colored peat. Distinguishing 

 mucks from peats based on the relative 

 degree of decomposition is fairly simple. 

 In mucks (Saprists), almost all of the 

 plant remains have been decomposed 

 beyond recognition. When rubbed, 

 mucks feel greasy and leave hands dirty. 



In contrast, the plant remains in peats 

 (Fibrists) show little decomposition and 

 the original constituent plants can be 

 recognized fairly easily. When the 

 organic matter is rubbed between the 

 fingers, most plants fibers will remain 

 identifiable, leaving hands relatively 

 clean. Between the extremes of mucks 

 and peats, organic soils with partially 

 decomposed plant fibers (Hemists) can 

 be recognized. In peaty mucks up to 

 two-thirds of the plant fibers can be 

 destroyed by rubbing the materials 

 between the fingers, while in mucky 

 peats up to two-thirds of the plant 

 remains are still recognizable after 

 rubbing. 



Hydric Mineral Soils 



When less organic material 

 accimiuJates in soil, the soil is classified 

 as minerd soil. Some mineral soils may 

 have thick organic surface layers (histic 

 epipedons) due to heavy seasonal 

 rainfall or a high water table, yet these 

 soils are still composed largely of 

 mineral matter (Poimamperuma 1972). 

 Mineral soils that are covered with 

 moving (flooded) or standing (ponded) 

 water for significant periods or are 

 saturated for extended periods during 

 the growing season meet the NTCHS 

 criteria for hydric soils and are 

 classified as hydric mineral soils. Soil 

 saturation may result from low-lying 

 topographic position, groundwater 

 seepage, or the presence of a slowly 

 permeable layer (e.g., clay, confining 

 layer, confining bedrock, or hardpan). 



The duration and depth of soil 

 saturation are essential criteria for 

 identifying hydric soils and weUands. 

 Soil morphological featxires are 

 commonly used to indicate long-term 

 soil moisture regimes (Boimia 1983). 



A thick dark surface layer, grayish 

 subsurface and subsoil colors, the 

 presence of orange or reddish brown 

 (iron) and/or dark reddish broyvn or 

 black (manganese) mottles or 

 concretions near the surface, and the 

 wet condition of the soil may help 

 identify the hydric character of many 

 mineral soils. The grayish subsurface 

 and subsoil colors and thick, dark 

 surface layers are the best indicators of 

 ciirrent wetness, since the yellow- or 

 orange-colored motUes are very 

 insoluble and once formed may remain 

 indefinitely as relict motties of former 

 wetness (Diers and Anderson 1984). 



A histic epipedon (organic surface 

 layer) is evidence of a soil meeting the 

 NTCHS criteria. It is an 8 to 16 inch 

 organic layer at or near the surface of a 

 hydric mineral soil that is sat\u-ated with 

 water for 30 consecutive days or more in 

 most years. It contains a minimum of 20 

 percent organic matter when no clay is 



present or a minimum of 30 percent 

 organic matter when clay content is 60 

 percent or greater. Soils with histic 

 epipedons are inundated or saturated 

 for sufficient periods to greatly retard 

 aerobic decomposition of organic 

 matter, and are considered hydric soils. 

 In general, a histic epipedon is a thin 

 surface layer of peat or muck if the soil 

 has not been plowed (U.S.D.A. Soil 

 Survey Staff 1975). Histic epipedons are 

 typically designated as O-horizons (Oa, 

 Oe, or Oi surface layers), and in some 

 cases the terms "mucky" or "peaty" are 

 used as modifiers to the mineral soil 

 texture term, e.g., mucky loam. 



Soil-related Evidence of Significant 

 Saturation 



Identification of some weUands and 

 delineation of the upper boundary in 

 many wetlands is not readily 

 accomplished without a detailed 

 examination of the underlying soil. 

 Colors in the soU are strongly influenced 

 by the frequency and duration of soil 

 saturation which causes reducing 

 conditions. A gleyed layer and a low 

 chroma matrix with high chroma 

 motUes, near the surface are common 

 indicators of hydric soils throughout the 

 county. Other soil markers of significant 

 soil saturation vary regionally. These 

 signs include thick organic siirface 

 layers (^ 8 inches), gleying, and certain 

 types of mottling. 11 significant drEunage 

 or groundwater alteration has taken 

 place, then it is necessary to determine 

 whether the area in question is 

 effectively drained and is now 

 nonweUand or is only partly drained 

 and remains wetiand despite some 

 hydrologic modification. Guidance for 

 determining whether an area is 

 effectively drained is presented in the 

 section on disturbed areas. 



Soils saturated for prolonged periods 

 during the growing season in most years 

 are usually gleyed in the saturated zone. 

 Gleyed layers are predominanUy gray in 

 color and occasionally greenish or 

 bluish gray. In gleyed soils, the 

 distinctive colors result from a process 

 known as gleization. Prolonged 

 saturation of mineral soil converts iron 

 from its oxidized (ferric) form to its 

 reduced (ferrous) state. These reduced 

 compounds may be completely removed 

 from the soil, resulting in gleying 

 (Veneman. et al. 1976). Mineral soils 

 that are always saturated are typically 

 uniformly gleyed throughout the 

 saturated area. Soils gleyed to the 

 surface layer are evidence of wetiand 

 hydrology and anaerobic soil conditions 

 These soils often show evidence of 

 oxidizing conditions only along root 

 channels. Some nonsaturated soils have 



