AMELIORATION OF STRUCTURALLY UNSTABLE GREY SOILS IN THE NORTHWESTERN WHEAT BELT OF NEW-SOUTH-WALES

Methods of amelioration of structurally unstable grey clays were studied in field experiments conducted at three sites in the Gwydir Valley in northern New South Wales. A set of ameliorative treatments was established at each site in 1973, and a second set of treatments was established in 1974. Plots were sown with wheat each year until 1977 to study residual effects. In the first series of experiments, gypsum at 12.5 t ha-1 greatly increased wheat yields at all sites, with residual effects still strongly evident in 1977. The yield increases were commonly associated with reduced surface soil crust strength, increased wheat establishment, and increased stored soil water at sowing. Potassium sulphate at 3.75 t ha-1 and organic matter (chopped hay at 12 t ha-1) gave initial yield increases, but residual effects did not persist. Neither polyvinyl alcohol (200 kg ha-1) nor deep ploughing (to 25 cm) was successful as an ameliorant. In the second series of experiments, gypsum at 2.5 t ha-1 consistently increased wheat yields, with residual effects still evident in 1977, although yield increases were much less than those obtained with 12.5 t ha-1 gypsum in the first series of experiments. A crop of oats ploughed under was not a successful ameliorant, but calcium carbonate (5 t ha-1) showed some potential. © 1979 ASEG.