SummaryThe Q/I relations of labile soil K relate its availability or intensity (I) to the amount (Q) present. The activity ratio aK/√(aCa±Mg) has already been proposed as a measure of the intensity factor (part I). The ability of a soil to maintain the activity ratio against depletion by plant roots is governed partly by the character of the pool of labile K and also by the rate of release of fixed K, and by the diffusion and transport of K ions in the soil solution. We may isolate the Potential Buffering Capacity of the pool of labile K by determining theQ/Irelation over short periods. Suchimmediate Q/I relationshave been determined for a number of soils. They are all linear over the range of activity ratios commonly encountered in agricultural soils and approximate to familiar ion‐exchange equations. At low values of the activity ratio theQ/Irelations are all curved and asymptotic to the Q‐axis.The effects of drying, pH, the addition of lime or K fertilizers, and changes in temperature on the form of theQ/Irelations are reported.Oven‐drying a soil, particularly if repeated, appears to cause proportionately more K than Ca to be fixed in the fine clay