The availability of potassium in soils to crops as measured by its release to a calcium-saturated cation-exchange resin

The amounts and rates of release of the different categories of soil K were measured on 21 soils from field experiments in England, Wales and West Germany using a calcium saturated cation exchange resin. The soils contained various amounts of exchangeable, fixed and matrix K; some contained no fixed K at all. These amounts could be qualitatively related to the clay contents of the soils.

Regression analyses of K removed at harvest by spring barley, winter wheat, field beans and sugar beet, grown in the field experiments, were made on the quantities measured by the Ca-resin method and on K exchangeable to M ammonium acetate (EK). The Ca-resin method proved a better predictor of K availability than EK for all four crops. A multiple regression of the amount and rate of release of exchangeable K, and the rate of release of fixed K, as measured by Ca-resin, gave the best correlation with K removed. The contribution of fixed K to the regression increased with the demand of the crop for K and with the length of time the crops had been grown without K fertilizer. For the cereals, fixed K appeared to be either completely available or unavailable depending on whether its rate of release exceeded a critical value characteristic of each crop.

The Ca-resin method is probably the best method available for measuring K reserves. It is, perhaps, too laborious for routine soil analysis, but could be used to characterize the long-term K release properties of a soil.

RESP-09042