Assessment of Soil Potassium Reserves Available to Plant Roots

Summary Successive extraction with Ca‐saturated sulphonated resin is used to remove potassium from soils. The kinetics of K‐release are analysed mathematícally to define three simultaneous rate processes by the amounts and rate‐functions associated with them. The diffusion coefficient (3 × 10 ‐19 cm 2 sec ‐1 ) calculated for the slowest process (suggested to be from the mineral matrix) is of the same magnitude as that for illitic clay minerals. The effect of K manuring on these parameters is evaluated from measurements on soils from the Nil and K‐fertilised plots of the Rothamsted Classical Experiments. Based on this analysis, a procedure is suggested for deriving from a smaller number of observations the amount of K a soil would release in a given time, and the contribution of each process to this amount is demonstrated.