The relationship between the exchangeable potassium content of a soil and the activity ratio, aK/√(aCa+Mg), of the solution with which it is in ‘instantaneous equilibrium’ is a characteristic of the soil which is independent of the soil: solution ratio or the total electrolyte concentration (up to O·06 M) of the solution. It is unaffected by the transfer of substantial amounts of potassium ions to or from the fixed state. Hence instantaneous equilibrium curves provide a basis against which to measure the changes in exchangeable potassium content due to fixation and release. This procedure has the important advantages that all measurements are made in the solution phase and that the solutions in contact with the soil approximate to the natural soil solution.

The technique has been employed successfully in studying the fixation and release of potassium by several widely different soils.

The results confirm that fixation or release are determined by the extent to which the soil contains more or less exchangeable potassium than its equilibrium percentage potassium saturation. Seemingly the equilibrium percentage potassium saturation, and hence the equilibrium activity ratio of potassium in the soil solution aK/√(aCa+Mg), is regulated by the amounts or proportions of potassium in the fixed state. The amounts or proportions of potassium in the fixed state may be increased by repeated applications of potassium fertilizers and reduced by continued cropping.