INFLUENCE OF CLAY-MINERALS AND ORGANIC-MATTER ON EFFECTS OF SODICITY ON SOILS

This review shows that the permeability of soils decrease because exch angeable sodium causes aluminosilicate clay minerals to swell and disp erse in water. Among common soil minerals, only smectites with a high percentage of exchangeable Na show extensive swelling. Illites often r emain dispersed in solutions of high electrical conductivity, partly b ecause the shapes of their particles prevent strong cohesion. Smectite s are often highly mobile, consistent with their small particle sizes. The dispersion of kaolinites varies with solution pH since a signific ant proportion of their charge can be variable. The sensitivity of soi ls to pH may be more strongly influenced by other, variably charged co mponents than by the dominant aluminosilicates. Of these, metal oxides , hydroxides and oxyhydroxides can restrain the dispersion of aluminos ilicate clay minerals. Organic matter can aid the dispersion of sodic soils while suppressing swelling. It can also be mobilized in a solubl e form and probably also in a colloidal form under sodic conditions. E xchangeable 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. Stu dies of soil fabric should increase the current poor understanding of the macroscopic effects of sodicity in terms of interparticle interact ions of the pure components.