POTASSIUM RESERVES IN A SANDY CLAY SOIL FROM THE SAXMUNDHAM EXPERIMENT: KINETICS AND EQUILIBRIUM THERMODYNAMICS

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Abstract
Summary: Potassium‐calcium exchange equilibria in, and the kinetics of K release from, soil from the Nil and PK treatments of the Saxmundham Experiment, Rotation 1, and the Ga, Ca + K and K saturated panicle size fractions of the soils were investigated.The free energy and enthalpy ot exchange showed K preference in all the solids. Selectivity for K decreased with increasing particle size and pre‐treatment with K salts. A comparison of the ‘differential enthalpy of exchange: per cent K saturation’ relationship for the whole soil with those of the particle size fractions suggests that soil dispersion during particle size separation also decreased K selectivity significantly. When related to the mineralogical composition of the soil, the differential enthalpy data suggest that maximum K. selectivity is associated with a vermiculite/smectite component of inter‐stratified minerals in the soil.Isotopic exchange using 4sCa on the decalcified solids showed a measurable rate of exchange for the Ca forms of the 3 or hydroxy‐aluminium polymers), which are removed or rendered porous during treatment with dilute KCl solution, so that isotopic exchange with Ca is then much more rapid.The kinetics of K extraction with a Ca saturated resin, interpreted on the basis of a three‐compartment model, suggest that sorbed K and K released by ‘fast’ and slow processes, representing sites of low and high K selectivity, were associated with minera‐logically distinct phases in the various particle size fractions.