According to the U.S. Soil Conservation Service, Soil Survey Staff (1975:1-2), soil is limited to terrestrial situations and shallow waters; however, "areas are not considered to have soil if the surface is permanently covered by water deep enough that only floating plants are present ...." Since emergent plants do not grow beyond a depth of about 2 m in inland waters, the waterward limit of soil is virtually equivalent to the waterward limit of wetland, according to our definition. Wetlands can then be regarded as having soil in most cases, whereas deepwater habitats are never considered to have soil.

The most basic distinction in soil classification in the United States is between mineral soil and organic soil (U.S. Soil Conservation Service, Soil Survey Staff 1975). The Soil Conservation Service recognizes nine orders of mineral soils and one order of organic soils (Histosols) in its taxonomy. Their classification is hierarchical and permits the description of soils at several levels of detail. For example, suborders of Histosols are recognized according to the degree of decomposition of the organic matter.

We use the Modifiers mineral and organic in this classification. Mineral soils and organic soils are differentiated on the basis of specific criteria that are enumerated in soil taxonomy (U.S. Soil Conservation Service, Soil Survey Staff 1975:13-14, 65). These criteria are restated in our Appendix D for ready reference. If a more detailed classification is desired, the U.S. Soil Conservation Service classification system should be used.