Potassium reserves in a sandy clay soil from the Saxmundham experiment - kinetics and equilibrium thermodynamics

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 of 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 <0.2 and 0.2–2 /μm fractions. This is attributed to traces of blocking materials (CaCO3 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 mineralogically distinct phases in the various particle size fractions.

RESP-07943