The process of replacing one ion that is attached to a charged surface with another. A very important type of ion exchange is the exchange of cations bound to soil particles. Soil clay minerals and organic matter both have negative surface charges that bind cations. In a fertile soil the predominant exchangeable cations are Ca²⁺, Mg²⁺ and K⁺. In acid soils Al³⁺ and H⁺ are also important exchangeable ions. When materials containing cations are added to soil, cations leaching through the soil are retarded by cation exchange.