AbstractThe theory on colloids with a pH‐dependent surface potential indicates that the anion exchange capacity of soils containing hematite or similar oxides depends on pH, electrolyte concentration and valency of counterions.These relationships were tested for the salinity‐ and pH‐range of importance to agriculture, using a percolation technique. MgSO4was chosen as impregnating salt to minimize the overall error caused by salt exclusion.The assumptions on the anion exchange behaviour proved to be correct; moreover they applied also to a considerable fraction of the cation exchange. The results were formulated in terms of multiple linear regression equations relating the exchange capacities to pH and the square root of the electrolyte concentration.Consequently, the adsorption behaviour of ferruginous soils can not be characterized by the determination of CEC and AEC at one level of pH‐ and salt concentration in solution.Due to the inefficiency of the ethanol washing step, two standard methods overestimated CEC by a factor