Relative significance of electrolyte and cation exchange effects when gypsum Is applied to a sodic clay soil

Abstract

The hydraulic conductivity of disturbed samples of a sodic clay taken from a field experiment at Benerembah, in the Riverina of New South Wales, was increased markedly by the addition of gypsum (at 12.5 t ha-1). This initial response is considered to result mainly from an electrolyte effect rather than from cation exchange. After three seasons of irrigation of cotton, essentially all the gypsum had dissolved and the response had declined very considerably. Residual cation exchange accounted for an increase of about 10% in exchangeable calcium, and a decrease of 20 % and 5 % in exchangeable sodium and magnesium contents respectively, in the top 30-40 cm. The exchange involved 40-50% of the calcium applied and represented, in terms of exchangeable sodium lost, a replacement efficiency of some 20-30%. Below 60 cm the situation was reversed with an apparent loss of exchangeable calcium and an increase in exchangeable sodium. Leaching of soluble calcium, magnesium and sodium chlorides, somewhat greater than that in normally tilled soil, occurred especially from between 40 and 120 cm in the gypsum treated-deep tilled soil. Soluble sulphate, on the other hand, was retained in the profile above 130 cm, in amounts about equal to that applied as gypsum. The soluble chlorides accumulated below 130 cm, including apparently the unaccounted applied calcium and exchangeable calcium lost from depths below 60 cm. Deep tillage alone enhanced leaching of soluble calcium and sodium in the upper 80 cm to a similar degree to that when combined with gypsum treatment.