Soil
Soil Modifiers
Soil is one of the most important physical components of wetlands. Through its depth, mineral composition, organic matter content, moisture regime, temperature regime, and chemistry, it exercises a strong influence over the types of plants that live on its surface and the kinds of organisms that dwell within it. In addition, the nature of soil in a wetland, particularly the thickness of organic soil, is of critical importance to engineers planning construction of highways or buildings. For these and other reasons, it is essential that soil be considered in the classification of wetlands.
As noted in Section 2.2, we have placed the boundary between wetlands and deepwater habitats in the Riverine and Lacustrine Systems at a depth of 2.5 m (8.2 ft) below low water because this represents the approximate limit of soil as defined in Soil Taxonomy (Soil Survey Staff 1999) and the approximate maximum depth to which emergent plants normally grow (Welch 1952, Zhadin and Gerd 1963, Sculthorpe 1967). Thus, according to our definitions, inland wetlands may have soil, but inland deepwater habitats do not. All Palustrine waters are less than 2.5 m deep; therefore, potentially all Palustrine habitats have soil. In the Marine and Estuarine Systems, the deep limit of soil lies at a depth of 2.5 m below extreme low water; however, we separate wetlands from deepwater habitats precisely at the extreme low water mark in those Systems. So, according to our definitions, Marine and Estuarine wetlands may have soil, and deepwater habitats also may have soil out to the 2.5-m depth limit.
The most basic distinction in soil classification in the U.S. is between mineral soil and organic soil (Soil Survey Staff 1999). In general“a soil is classified as an organic soil (Histosols) if more than half of the upper 80 cm (32 inches) of the soil is organic or if organic soil material of any thickness rests on rock or on fragmental material having interstices filled with organic matter.” Soil that does not meet this criterion is considered mineral soil. Organic soil material is soil material that contains at least 12-18 percent organic carbon by weight, the required amount depending on the clay content in the mineral fraction (Soil Survey Staff 1999). See Appendix E for additional details on the differences between mineral and organic soils.
The U.S. soil classification is hierarchical and permits the description of soils at several levels of detail. For example, suborders of the order Histosols are recognized according to the degree of decomposition of the organic matter. In the WCS, we use the Soil Modifiers Organic and Mineral, based on the criteria presented above. If a more detailed soil classification is desired, the latest edition of Keys to Soil Taxonomy (Soil Survey Staff 2010) should be used.
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