Influence of clay minerals and organic matter on effects of sodicity on soils

Abstract

This review shows that the permeability of soils decrease because exchangeable sodium causes aluminosilicate clay minerals to swell and disperse in water. Among common soil minerals, only smectites with a high percentage of exchangeable Na show extensive swelling. Illites often remain dispersed in solutions of high electrical conductivity, partly because the shapes of their particles prevent strong cohesion. Smectites are often highly mobile, consistent with their small particle sizes. The dispersion of kaolinites varies with solution pH since a significant proportion of their charge can be variable. The sensitivity of soils to pH may be more strongly influenced by other, variably charged components than by the dominant aluminosilicates. Of these, metal oxides, hydroxides and oxyhydroxides can restrain the dispersion of aluminosilicate clay minerals. Organic matter can aid the dispersion of sodic soils while suppressing swelling. It can also be mobilized in a soluble form and probably also in a colloidal form under sodic conditions. Exchangeable Mg enhances dispersion from sodicity, but apparently to a greater extent in illitic soils than in smectitic soils. Exchangeable Al tends to prevent dispersion but the influence of clay mineralogy on its effect is apparently untested. The effect of sodicity upon soils is amplified by fineness of texture and by mechanical disturbance. Studies of soil fabric should increase the current poor understanding of the macroscopic effects of sodicity in terms of interparticle interactions of the pure components.