Sandy soils Loamy soils Clayey soils
Moderately coarse Medium
Sand; loamy sand Sandy loam; fine sandy loam Very fine sandy loam; loam; silt loam; silt
Clay loam; sandy clay loam; silty clay loam
Sandy clay; silty clay; clay
a USCS, Unified Soil Classification System.
Source: Adapted from USEPA, Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008, CERI, Cincinnati, OH, 2002.
application of biosolids or wastewater and in turn depends on surface vegetation as a treatment component, then soil chemistry is a very important factor in the development and future maintenance of the vegetation. The following tests are suggested for each of the major soil types on the site:
• pH, cation exchange capacity (CEC), exchangeable sodium percentage (ESP), and electrical conductivity (EC)
• Plant available nitrogen (N), phosphorus (P), potassium (K), and lime or gypsum requirements for pH adjustment and maintenance
A few standard test procedures are available for chemical analysis of soils (Black, 1965; Jackson, 1958). The interpretation of soil chemical test results can be aided by extension specialists and the use of Table 2.16.
The cation exchange capacity of a soil is a measure of the capacity of negatively charged soil colloids to adsorb cations from the soil solution. This adsorption is not necessarily permanent, because the cations can be replaced by others in the soil solution. These exchanges (except for excess sodium percentage in clay soils) do not significantly alter the structure of the soil colloids. The percentage of the CEC occupied by a particular cation is the percent saturation for that cation. The sum of the exchangeable hydrogen (H), sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg), expressed as a percentage of the total CEC, is the percent base saturation. Optimum ranges for percent base saturation for various crop and soil combinations have been identified. It is important for Ca and Mg to be the dominant cations, rather than Na or K. The cation distribution in the natural soil can be changed easily by the use of soil amendments such as lime or gypsum.
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