Cation exchange capacity and reactions

In soils with a neutral reaction, the metal cations calcium, magnesium, potassium and sodium are electrostatically attracted to soil particles, described as an electric double layer (EDL) of positively charged cations and negatively charged surfaces. In soils dominated by permanently charged clay minerals, cations in the EDL are rapidly and reversibly exchanged with cations in the bulk solution, as described by selectivity constants. In soils low in clay minerals, the major source of negative charge is soil organic matter and variably charged metal oxides. In acidic organic and forest soils, cation exchange between aluminum and hydrogen ions is an important pH buffering process.