cation exchange is a process by which cations (positively charged ions) in solution in a soil exchange with other cations that are held on negatively charged clay and organic matter. These cations are called exchangeable cations and the amount held per kilogram of soil defines the cation exchange capacity. Cation exchange can result in potentially toxic concentrations of exchangeable aluminium in very acid soils (soil ph less than 5), whereas exchange of sodium for calcium cations in soils with high concentrations of soluble salts can lead to very high pH soils (greater than pH 9) when the salts are leached out. See also ion exchange.
