Cation activity ratios in the soil solution and availability of potassium to rice in a submerged soil

Abstract

SUMMARYThe activity ratios of potassium in dynamic equilibrium with a submerged soil planted with rice were determined fortnightly for 10 weeks. The simple activity ratio, aK(aCa + aMg)-1, and the modified activity ratio to include the activity functions of Fe2+ and Mn2+ as aK/(aCa+ aMg)½ + B(aFe)½ + C(aMn)½, gave linear relationships when plotted against the change in exchangeable K content of the soil. The modified activity ratio of potassium correlated better with potassium concentration (r = 0·91) in and potassium uptake (r = 0·84) by rice than the simple activity ratio (r = 0·86 and 0·81 respectively).Submergence for 6 weeks almost doubled the potential buffering capacity of potassium, PBCK, showing that the submerged soil sustained a low level of potassium in the soil solution for a long time.A negative intercept of the plot of equilibrium labile K, Kel, on the ‘quantity’ axis, against the modified activity ratio showed that there is some non-labile K which equilibrates with the soil solution during growth. The value of this intercept (3·40 mequiv/kg soil) is a measure of non-labile K.