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 hydnc 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 epiped- 

 ons are technically classified as Oa, Oe, or Oi sur- 

 face layers, and in some cases the terms "mucky" 

 or "peaty" are used as modifiers to the mineral soil 

 texmre term, e.g., mucky loam. 



3) Sulfidic material - When soils emit an odor 

 of rotten eggs, hydrogen sulfide is present. Such 

 odors are only detected in waterlogged soils that are 

 essentially permanently saturated and have sulfidic 

 material within a few inches of the soil surface. 

 Sulfides are produced only in reducing environ- 

 ment. Under saturated conditions, the sulfates in 

 water are biologically reduced to sulfides as the or- 

 ganic materials accumulate. 



4) Aquic or peraquic moisture regime - An aq- 

 uic moisture regime is a reducing one, i.e., it is vir- 

 tually free of dissolved oxygen, because the soil is 

 saturated by ground water or by water of the capil- 

 lary fringe (U.S.D.A. Soil Survey Staff 1975). The 

 soil is considered saturated if water stands in an un- 

 lined borehole at a shallow enough depth that the 

 capillary fringe reaches the soil surface, except in 

 noncapillary pores. Because dissolved oxygen is 

 removed from ground water by respiration of mi- 

 croorganisms, roots, and soil fauna, it is also im- 

 plicit that the soil temperature be above biologic 

 zero (41°F) at some time while the soil is saturated. 

 Soils with peraquic moisture regimes are character- 

 ized by the presence of ground water always at or 

 near the soil surface. Examples include soils of tidal 

 marshes and soils of closed, landlocked depres- 

 sions that are fed by permanent streams. Soils with 

 peraquic moisture regimes are always hydric under 

 natural conditions. Soils with aquic moisture re- 

 gimes are usually hydric, but the NTCHS hydric 

 soil criteria should be verified in the field. 



5) Direct observations of reducing soil condi- 

 tions - Soils saturated for long or very long dura- 

 tion will usually exhibit reducing conditions at the 

 time of saturation. Under such conditions, ions of 

 iron are transformed from a ferric (oxidized) state to 

 a ferrous (reduced) state. This reduced condition 

 can often be detected in the field by use of a colori- 

 metric field test kit When a soil extract changes to a 



pink color upon addition of a-a-dipyridil, ferrous 

 iron is present, which indicates a reducing soil en- 

 vironment at the time of the test. A negative result ^ i 

 (no pink color) only indicates that the soil is not re- 

 duced at this moment; it does not imply that the soil 

 is not reduced during the growing season. Further- 

 more, the test is subject to error due to the rapid 

 change of ferrous iron to ferric iron when the soil 

 is exposed to air and should only be used by exper- 

 ienced technicians. (CAUTION: This test cannot be 

 used in hydric mineral soils having low iron con- 

 tent or in organic soils. Also it does not determine 

 the duration of reduced conditions.) 



6) Gleyed, low chroma, and low chroma/ 

 mottled soils - The colors of various soil compo- 

 nents are often the most diagnostic indicator of hy- 

 dric soils. Colors of these components are strongly 

 influenced by the frequency and duration of soil 

 saturation which leads to reducing soil conditions. 

 Hydric mineral soils will be either gleyed or will 

 have low chroma matrix with or without bright 

 mottles. 



A) Gleyed soils - Gleying (bluish, green- 

 ish, or grayish colors) immediately below the A- 

 horizon is an indication of a markedly reduced soil, 

 and gleyed soils are hydric soils. Gleying can oc- 



cur in both mottled and unmottled soils. Gleyed %. 

 soil conditions can be determined by using the gley 

 page of the "Munsell Soil Color Charts" (Kollmor- 

 gen Corporation 1975). (CAUTION: Gleyed con- 

 ditions normally extend throughout saturated soils. 

 Beware of soils with gray E-honzons due to leach- 

 ing and not to saturation; these latter soils can often 

 be recognized by bright-colored layers below the 

 E-horizon.) 



B) Other low chroma soils and mottled soils 

 (i.e., soils with low matrix chroma and with or 

 without bright mottles) - Hydric mineral soils that 

 are saturated for substantial periods of the growing 

 season, but are unsaturated for some time, com- 

 monly develop mottles. Soils that have brightly 

 colored mottles and a low chroma matrix are indi- 

 cative of a fluctuating water table. Hydric mineral 

 soils usually have one of the following color fea- 

 tures in the horizon immediately below the A- 

 horizon: 



(1) Matrix chroma of 2 or less in 

 mottled soils, or 



(2) Matrix chroma of 1 or less in un- 

 motded soils. | 



14 



