SUMMARYMeasurements and predictions of uptake of non‐exchangeable K have been made for three soils. Exchangeable K is important in controlling the extent to which non‐exchangeable K is used; rate of diffusion of exchangeable K seems less important. Rate of release of non‐exchangeable K did not vary much between soils, but critical release concentrations in solution varied from 33 to 85 μM with little obvious effect on the extent of use of non‐exchan