The release of potassium from four soil clays containing illites of different degrees of degradation was found to be independent of the solution of silicic acid or aluminium from the clay minerals. The lattice potassium is released into solution by a displacement reaction. The displacement of potassium by calcium ions is dependent on the prior release of one lattice potassium by one hydrogen ion, presumably because the entry of the hydrogen ion makes the potassium at neighbouring sites in the lattice accessible to other cations. The reaction may be written as 7Ks+ + H+ + 3Ca2+ 7K+ + 3CaS2+, where the subscript S refers to the solid (clay) phase. At pH values above pH 11, potassium is released by breakdown of the mineral structure by the action of hydroxyl ions.