Ion Exchange Property of Soils

Ion exchange is a reversible process. Soil has capacity to exchange its ions due to its colloidal property. The water existing in the immediate vicinity of clay and humus particles which are both colloidal in nature is considered to be water associated with exchangeable cations. The ion exchange reactions may be cation exchange or anion exchange reactions. The cations adsorbed on the surface are subject to exchange with cations held in the soil solution. For example, a calcium ion held on the colloidal surface is subject to exchange with two H⁺ ions in the soil solution. 

Soil - Ca²⁺ + 2H^{+   →} Soil H⁺_H+   +  Ca²⁺