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What is cation exchange capacity?

Cations

An ion is an atom or molecule that has lost or gained one or more electrons, giving it respectively either a positive (cation) or negative (anion) electric charge. The common cations are hydrogen (H⁺), sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), ammonium (NH₄⁺) and aluminium (Al³⁺). Others occur as well.

CEC

The cation exchange capacity (CEC) of a soil is its capacity to exchange cations between the soil particles and the soil solution (the water in the soil). It is determined by the negative electric charge on the surface of soil particles. This charge attracts the cations, restricting them from leaching away.

The phenomenon is called cation exchange because it is possible to exchange or displace one cation for another. For example, excessive use of ammonium fertiliser can displace calcium, leading to poor pH buffering and soil acidification. As plants take up nutrients from the soil, they exchange them for hydrogen, slowly acidifying the soil.

CEC is used as a measure of fertility and nutrient retention capacity, and indicates a soil’s capacity to protect groundwater from nutrient contamination. It also indicates a soil’s structural resilience, or shrink–swell potential: a low value (<20 meq/100 g) indicates that a soil will not recover from compaction, whereas a high value (>40 meq/100 g) indicates that soil structure will recover naturally.

Organic matter and most clays (except kaolinite, for example) have a high CEC, whereas sands have almost no CEC. Therefore, clays and organic matter retain applied nutrients much better than sandy soils, which tend to lose them quickly. A reasonable CEC for a sandy soil would be 5 meq/100 g. A reasonable value for a clay loam would be 30 meq/100 g. In contrast, humus has a typical value of 250 meq/100 g. For plant production, the preferred value is >10 meq/100 g.

Measuring CEC

Two standard methods are commonly used to measure the CEC of a soil sample. Both require the use of an atomic absorption spectrophotometer. This is why it is necessary to ask a laboratory to measure the CEC of your soil.

Expressing CEC

The quantity of cations that a clay mineral can hold on its negatively charged surface is commonly expressed in milliequivalents (meq) per 100 g. (CEC is also expressed as cmol(+)/kg, or centimoles of charge per kilogram. This is identical to meq/100 g.) But this masks variations in the proportions of individual cations. A better way to express CEC is to show each cation as a percentage of the total CEC. The following table gives an idea.

Normal ranges of exchangeable cations in soil, expressed as % of total CEC