The influence of organic matter extracted from humified clover on the properties of amorphous aluminosilicates. III. Potassium-magnesium exchange selectivity

Abstract

Potassium-magnesium cation exchange selectivity measurements were made on a series of synthetic and amorphous aluminosilicates, allophanic soil clays and hydrous alumina after treatment with aqueous extracts of humified clover. The results were compared with those obtained from similar measurements on samples treated with only the inorganic components of the humified clover extract. For the soil clays and the aluminosilicates of intermediate composition organic treatment increased the preference for Mg2+. With the siliceous aluminosilicates, however, the effect of organic treatment on Mg2+ preference at acid pH values was limited because of specific interaction of K+ with these samples. This occurred despite the large increase in surface negative charge which would be expected to favour the divalent cation. In the case of the siliceous aluminosilicates organic treatment was found to decrease the preference for Mg2+ at neutral and alkaline pH values. This is attributed to the removal of aluminium-hydroxy material providing sites for the specific adsorption of Mg2+. While hydrous alumina specifically adsorbs Mg2+ at neutral and alkaline pH values, it does not adsorb K+. However, the organic matter adsorbed by hydrous alumina during organic treatment provides sites for the electrostatic adsorption of K+ as well as Mg2+.