Original Articles

Sodicity phenomena in red sesquioxic clay subsoils


Abstract

Monitoring for many years of the soils of an effluent disposal site in the eastern Transvaal has shown that the disposal of alkaline and saline effluents on red sesquioxic clays has not appreciably affected soil permeability but has brought about high ESP levels (< 40%) and moderate salinity in the lower subsoil horizons. The soil in these horizons remains permeable even after complete removal of electrolytes by leaching. It was found that although ESP increased with depth, the pH of the saturated pastes remained unusually low and that within the soil profile, pH decreased with Increasing ESP. Furthermore, in spite of moderate salinity in the lower subsoils, the exchange complex did not appear to be fully base saturated when CEC was determined by the ammonium acetate method (pH 7) in spite of soil pH being close on 7. In an attempt to find a reason for these apparent anomalies, five subsoil samples with ESP's ranging from 13 to 45 were selected from two profiles and leached with successive small quantities of deionized water. It was found that this removed not only the normal free water-soluble salts but also significant amounts of apparently adsorbed sodium, thus reducing ESP and further lowering base saturation. This was accompanied by only a small rise in soil pH, by about 0,3 to approximately 7,1.

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