SummaryA model (based on the short‐ and long‐term Q/I relations of K in a soil, on graphs of K‐uptake by crop against post‐uptake K‐activity ratios, and the exhaustion value of K‐activity ratios below which K can no longer be taken up) is presented by means of which to estimate the amount of labile K in a soil, and to compare the contributions of labile and fixed K to plant uptake. The application of the model was partially explored in a field experiment. Relative values for the magnitudes of the pools of labile and fixed K developed by 4 years’build‐up treatments were calculated on the basis of the model. Their relative contributions to the K taken up by a ryegrass test crop in the following year were calculated from the soil Q/I relations, and the measured uptakes of K. While the data were incomplete and so could not provide conclusive evidence for the model, the values produced were sensible and appare