SUMMARYUptake of K was measured using a single onion root technique with four soil cylinder diameters, and measuring changes in exchangeable K in the soil. Baldwin's model fitted the measured uptake when only exchangeable K was taken up, but that of Heming&Rowell allowed predictions of uptake of both exchangeable and non‐exchangeable K (Knes) after fitting the necessary parameters. Release of Knexoccurred in both 3 mm and 6 mm cells (root densities of 14cm cm”−3and 3.5 cm cm−3) in 10 days. Predictions were made for longer times and differing soil water contents: after four weeks at a water content midway between field capacity and wilting point the contribution of Knexto total uptake was significant (up to 60%) for plants with high root de