Charged ions that are adsorbed on to sites, oppositely charged, on the surface of the adsorption complex of the soil (mainly clay and humus colloids). Exchangeable ions can replace each other on this surface, and are also available to plants as nutrients. Although cations (e.g. calcium and magnesium) are the most common, exchanging at negatively charged sites, some anions (e.g. sulphate and phosphate) do exchange at positively charged sites. See anion-exchange capacity; and cation-exchange capacity.
Subjects: Earth Sciences and Geography — Plant Sciences and Forestry.