The capacity of the soil to hold on to positively charged ions in the soil is called the cation exchange capacity (CEC). Examples of cations include calcium (Ca2+), magnesium (Mg2+), and potassium (K+), sodium (Na+). According to the fact sheet, sandy soils low in organic matter have a very low CEC (less than 3 cmolc/kg) while heavier clay soils or soils high in organic matter generally have a much higher CEC (greater than 20 cmolc/kg). The study goes on to say, “The higher the CEC the more clay or organic matter present in the soil. This usually means that high CEC (clay) soils have a greater water holding capacity than low CEC (sandy) soils.” This implies a direct relation between high cation exchange capacity and increased moisture retention of soil. Typical estimates of CEC of different soils is given below:
CEC (cmolc/kg soil)
CEC 25! Fine-textured soils developed from clayey lake sediments and medium- to fine-textured soils developed from lake sediments
CEC 20 Medium- to fine-textured soils developed from calcareous glacial
till, medium-textured to moderately fine-textured soils developed from slightly calcareous glacial till mixed with shale, and medium-textured soils developed in recent alluvium.
CEC 18 Moderately coarse textured soil developed from glacial outwash
and recent alluvium and medium textured acid soil developed on
CEC 16 Coarse- to medium-textured soils formed from glacial till or glacial outwash.
CEC 12 Coarse- to very coarse-textured soils formed from gravelly or sandy glacial outwash or glacial lake beach ridges or deltas.
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*Elemite, a 100% montmorillonite clay based soil amendment is used by organic farmers in lieu of fertilizer to increase CEC in their soil. Elemite also promotes increase microbial activity in soil.