For depletions of soil exchangeable potassium of between 15 and 50%, potassium concentrations in ryegrass and strawberry leaf were linearly related to pAROK√BCOwhere AROKis the equilibrium potassium activity ratio and BCOis the slope, measured at AROKof the curve relating the quantity of labile potassium and the activity ratio. Potassium supplying power as measured by pAROK√BCOcould also be expressed as a function of the equilibrium activity ratio and the mineral component of the cation exchange capacity of the soil.