SUMMARYA model based on measurements of potassium release from spherical aggregates (Heming&Rowell, 1985, Part I) has been used to predict release from larger aggregates, for longer times, drier soils and varying diffusion coefficients. The model has been adapted to predict supply to a single root from a soil cylinder and compared with measured uptake by onions. A ‘supply index’ shows likely restrictions in the availability of exchangeable K and in the rate of supply of non‐exchangeable K resulting from diffusion limitations over a 4 week period for (i) aggregates covered with a dense roots system, and (ii) roots growing at known densities. For this period diffusion alone should not limit exchangeable K uptake in the field if the root density is at least 3.5 cm/cm3: at 0.9 cm/cm3about 50% may be unavailable. The supply rate of non‐exchangeable K increases with root density and may be important for high‐demanding crops in dr