Cation-exchange capacity is defined as the amount of positive charge that can be exchanged per mass of soil, usually measured in cmol/kg. Some texts use the older, equivalent units me/100g or meq/100g. CEC is measured in moles of electric charge, so a cation-exchange capacity of 10 cmol/kg could hold 10 cmol of Na cations (with 1 unit of charge per cation) per kilogram of soil, but only 5 cmol Ca (2 units of charge per cation).
Cation-exchange capacity arises from various negative charges on soil particle surfaces, especially those of clay minerals and soil organic matter. Phyllosilicate clays consist of layered sheets of aluminium and silicon oxides. The replacement of aluminium or silicon atoms by other elements with lower charge (e.g. Al replaced by Mg) can give the clay structure a net negative charge. This charge does not involve deprotonation and is therefore pH-independent, and called permanent charge. In addition, the edges of these sheets expose many acidic hydroxyl groups that are deprotonated to leave negative charges at the pH levels in many soils. Organic matter also makes a very significant contribution to cation exchange, due to its large number of charged functional groups. CEC is typically higher near the soil surface, where organic matter content is highest, and declines with depth. The CEC of organic matter is highly pH-dependent.
Cations are adsorbed to soil surfaces by the electrostatic interaction between their positive charge and the negative charge of the surface, but they retain a shell of water molecules and do not form direct chemical bonds with the surface. Exchangeable cations thus form part of the diffuse layer above the charged surface. The binding is relatively weak, and a cation can easily be displaced from the surface by other cations from the surrounding solution.