All aqueous species, inorganic and organic, that exist as cations in soil solutions may participate in cation-exchange reactions. Cations that participate in exchange reactions remain hydrated and form outer-sphere surface complexes or reside in the diffuse-ion swarm of the solid-solution interface. Outer-sphere complexation occurs because the Lewis base surface functional group does not have the base strength necessary to dislodge waters of hydration (which are also Lewis bases) from the primary hydration sphere of the metal cation or ionized organic moiety. Most commonly, the surface functional groups associated with cation exchange are the ditrigonal siloxane cavity, found in the interstitial regions of phyllosilicates, and the deprotonated carboxylic and phenolic moieties of soil organic matter (SOM). The inorganic functional groups on hydrous metal oxides may also participate in ion exchange reactions, particularly in highly weathered soils. However, the contribution of hydrous metal oxides to the soil’s capacity to retain exchangeable ions is relatively minor.