Cation-exchange capacity

In soil science, cation-exchange capacity or CEC is the number of exchangeable cations per dry weight that a soil is capable of holding, at a given pH value, and available for exchange with the soil water solution. CEC is used as a measure of soil fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination. It is expressed as milliequivalent of hydrogen per 100 g of dry soil (meq+/100g), or the SI unit centi-mol per kg (cmol+/kg). The numeric values are the same in any system of units.

Clay and humus have electrostatic surface charges that attract and hold ions. The holding capacity of clay varies with the type of clay. Humus has a CEC that is two to three times that of the best clay.

One way to increase the CEC of a soil is to enhance the formation of humus.

In general, the higher the CEC, the higher the fertility of that soil.

The CEC is the number of positive charges (cations) that a representative sample of soil can hold. It is usually described as the number of hydrogen ions (H⁺) necessary to fill the soil cation holding sites per 100 grams of dry soil. Alternatively, an equivalent amount of another cation (Al³⁺ or Ca²⁺) can be used in the measure. In soil science, an equivalent is defined by the number of charges in terms of a given number of hydrogen ions. As hydrogen ions have only one positive charge (H⁺), this makes calculations relatively simple. An equivalent of Al³⁺ that could be held would amount to one third as many of those ions, and Ca²⁺ would have half as many ions.

Translation from meq/100g to an applicable unit, for instance lbs/acre of available nutrients, can calculated, but must consider the atomic weight, the ion's valence, and using a reasonable estimate of soil depth and density. Mengel gives the following correspondence for 1 meq/100g:

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