Prediction of cation availabity from soil equilibration with strontium

Most potassium (K) soil testing methods are based on the exchangeable amount. According to diffusion models, concentration in the soil solution, buffer power, and the diffusion coefficient control K transport to plant roots. The objective of this paper is to present the chemical principles of the strontium soil test to determine potassium and other cations availability. The method consists of a single equilibration of the soil with 0.004M Sr(NO 3 ) 2 . Monovalent cations K equivalence and divalent cations Ca equivalenoe equations are proposed to express the activity of monovalent and divalent cations as a single-ion, yielding multielement selectivity coefficients. Gapon equation was used to describe cation exchange reactions. Exchangeable amounts of major soil cations were calculated with good accuracy on the basis of estimated selectivity coefficients and of the equilibrium solution composition. The procedure presented may be useful in predicting K diffusion parameters on a routine basis