Ion exchange is meant the reversible process by which cations and anions are exchanged between solid and liquid phases, and between solid phases if in close contact with each other.
Other terms to be used in this aspect are ion adsorption, meaning an increase in concentration or an accumulation of an ion species on a solid, caused by ion exchange or other reactions; and desorption, which is the opposite process, that is, the replacement or release of an adsorbed ion species.
The soil is a heterogenous, polydisperse system of solid, liquid and gaseous components in various proportions. The solid component of the soil is made up of primary minerals, clay minerals, and hydrous oxides, together with organic matter and living organisms, forming a polyphase system of more or less discrete particles or aggregates.
In this heterogenous system, the soil solution acts as the medium by which chemical reactions between members of the different phases and of the same phase are made possible even when the reactants are not in direct contact. Because of the property of the soil material to bind and exchange cations and anions, many of these reactions are facilitated or made feasible.
Chemical and physical processes more or less intimately connected with ion exchange include weathering of minerals, nutrient absorption by plants, swelling and shrinkage of clay, and leaching of nutrients.
Ion exchange may therefore be considered as the most important of all the processes occurring in a soil.