SummaryThe method of determining the activity ratio(AR) described by Beckett in Part II of this series was applied to a range of moderately to intensely acid Nigerian soils. With the highly acid soils it was necessary to add aluminium ions to the solution to obtain equilibrium. A method for obtaining the true equilibrium solution composition for K, (Ca±Mg), and A1 is described, and a series of typical results given. It was necessary to add benzene to prevent biological activity during sedimentation at 33° C. Altering the total concentration of the solution did not affect the activity ratio. A minority of soils showed evidence of transfer of potassium to non‐exchangeable forms during equilibrium. The slopes of the (exchangeable K)‐AR curves in a single soil were dependent on the amounts of soil Ca±Mg present. In comparisons between several soils, exchange capacity also had an effect. The Gapon equation was approximately correct for these soils over limited