exchangeablecation

An exchangeable cation is a positively charged ion that is held on the surface of soil colloids, such as clay minerals and organic matter, by electrostatic forces and can be displaced by other cations present in the soil solution. These cations reside on the cation exchange complex, a negative charge produced by isomorphous substitution in clays and by functional groups in organic matter.

Most soils contain a mix of exchangeable cations, including calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na+), ammonium (NH4+), hydrogen (H+), and aluminum (Al3+). The total capacity to hold exchangeable cations is called the cation exchange capacity (CEC), expressed in cmolc(+)/kg. Soils high in clay and/or organic matter typically have higher CEC and can retain more nutrients.

Exchangeable bases (Ca2+, Mg2+, K+, Na+) occupy a portion of the CEC, while exchangeable acidity is formed by H+ and Al3+ on the exchange complex. Base saturation is the percentage of CEC occupied by exchangeable bases; higher base saturation often correlates with greater fertility. Liming or supplying specific cations can shift the balance by increasing pH and reducing exchangeable Al3+ in acidic soils.

Measurement of exchangeable cations usually involves displacing them with a neutral salt solution such as 1 M ammonium acetate at pH 7, followed by analysis of the released cations. The concept is central to soil fertility management, fertilizer planning, and assessing sodicity via the sodium adsorption ratio (SAR).