zone in which organic matter is usually being added to the mineral soil. It 

 is also the zone from which both mineral and organic matter are being moved 

 slowly downward. The next major horizon is the B-horizon, often referred to 

 as the subsoil. The B-horizon is the zone of maximum accumulation of mate- 

 rials. It is usually characterized by higher clay content and/or more pro- 

 nounced soil structure development and lower organic matter than the 

 A-horizon. The next major horizon is usually the C-horizon, which consists of 

 unconsolidated parent material that has not been sufficiently weathered to 

 exhibit characteristics of the B-horizon. Clay content and degree of soil 

 structure development in the C-horizon are usually less than in the B-horizon. 

 The lowest major horizon, the R-horizon, consists of consolidated bedrock. In 

 many situations, this horizon occurs at such depths that it has no significant 

 influence on soil characteristics. 

 Influencing factors 



Al. Although all soil-forming factors (climate, parent material, 

 relief, organisms, and time) affect the characteristics of a hydric soil, the 

 overriding influence is the hydrologic regime. The unique characteristics of 

 hydric soils result from the influence of periodic or permanent inundation or 

 soil saturation for sufficient duration to effect anaerobic conditions. Pro- 

 longed anaerobic soil conditions lead to a reducing environment, thereby 

 lowering the soil redox potential. This results in chemical reduction of some 

 soil components (e.g. iron and manganese oxides), which leads to development 

 of soil colors and other physical characteristics that usually are indicative 

 of hydric soils. 

 Classification 



A2. Hydric soils occur in several categories of the current soil clas- 

 sification system, which is published in Soil Taxonomy (USDA-SCS 1975). This 

 classification system is based on physical and chemical properties of soils 

 that can be seen, felt, or measured. Lower taxonomic categories of the system 

 (e.g. soil series and soil phases) remain relatively unchanged from earlier 

 classification systems. 



A3. Hydric soils may be classified into two broad categories: organic 

 and mineral. Organic soils (Histosols) develop under conditions of nearly 

 continuous saturation and/or inundation. All organic soils are hydric soils 

 except Folists, which are freely drained soils occurring on dry slopes where 

 excess litter accumulates over bedrock. Organic hydric soils are commonly 



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