The solubility of potassium from soil illites. III. Reactivity towards other ions

Abstract

The potassium equilibrium solubility and the potassium extracted from five illitic soil clays by various solutions have been measured. The solutes were chlorides of inorganic monovalent and divalent ions, of amines, and of heterocyclic nitrogen bases; and in addition other salts of sodium and calcium, several amino acids, and a few non-ionic compounds were used. The amounts of potassium displaced from these clay preparations depended on the size and shape of the displacing cations-generally the smaller the cation the more readily did it release potassium-and this effect continued to control release down to quite low levels of potassium solubility. This confirms the previous view that the potassium left in these clays after extensive treatment with sodium and calcium chloride lies in the interlayer spaces of the illite minerals, and provides a partial verification of the original hypothesis that potassium release from these interlayer spaces is a complex reaction involving hydrogen ions. There is some evidence that the potassium removed comes from a limited number of interlayers which show decreasing accessibility to larger ions. There are no positive indications that chemical attack of the clay mineral structure is responsible for potassium release when the clay is not heated or dried. Amino acids are able to displace potassium with an effectiveness which decreases at greater molecular weights and approaches that of primary amines of similar size in spite of the presence of the negatively charged carboxyl groups. Otherwise the release of potassium depends almost entirely on displacement by other cations and is unaffected by anions or non-ionic compounds.