Overview of Aquic Conditions and Hydric Soils
- M. J. Vepraskas and
- S. W. Sprecher
The rules of U.S. soil taxonomy and wetland delineation are the two principal systems used in the USA to classify soils that are saturated and chemically reduced. The purpose of this chapter is to describe the major reactions that occur in soils classified by both types of systems, and to compare and contrast the systems themselves. Oxidation-reduction (redox) reactions must occur in saturated soils for the soils to have aquic conditions or for them to be hydric soils. The reactions begin when organic matter is oxidized during bacterial respiration and electrons and protons are produced for the reduction process. Reduction of four elements (O, Mn, Fe, or S) is responsible for the creation of virtually all soil indicators (e.g., redoximorphic features) that show that redox reactions have occurred. In U.S. soil taxonomy, soils that are seasonally saturated and reduced have aquic conditions. The term reduced is defined to mean that Fe(II) must be in solution at some time. Soils with aquic conditions must have redoximorphic features, or one of approximately 10 other types of indicators within 50 cm of the surface. Hydric soils are used to identify jurisdictional wetlands. These are wetlands that may not be filled-in unless a permit is issued by an agency of the U.S. Government. Hydric soils formed under conditions of inundation or saturation that lasted long enough during the growing season to develop anaerobic conditions in the upper part of the soil (within 30 cm of the surface). Major differences between hydric soils and soils with aquic conditions are the depths at which saturation and reduction must occur, the timing of the saturation and reduction, and the principle user groups.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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