Assessment of potassium in soils

Potassium in soils from selected plots of the Broadbalk Experiment at Rothamsted, and Rotation I experiment at Saxmundham, U.K. was measured by five methods: the boiling nitric acid extractions of Haylock and Maclean, electro‐ultrafiltration, release to Ca‐saturated ion exchange resin, and a new procedure using extraction by HCl under reflux. All. the methods showed clearly the Gifferences between the two soil types, and between the differently fertilized plots on the basis of amounts, and 1n some cases rates of release, of different categories of soil K (exchangeable, fixed and native). The quantity of exchangeable K measured by each method, except that by Ca‐resin, gave significant correlations with the K balance of the soils, and yield and/or K offtake of winter wheat grown on the plots. The new HCl ‐reflux method gave the closest correlations. The amount and rate of release of fixed (available but not exchangeable) K estimated by HCl‐reflux was also correlated with yield and offtake.