A Soil Test for Aluminium Toxicity in Acidic Subsoils of Yellow Earths in Western Australia

Many of the yellow earths in the Western Australian wheatbelt have naturally acidic subsoils which can reduce the yield of wheat grown on them. Current methods of assessing soil acidity cannot identify which soils have subsoil acidity severe enough to restrict wheat yields. We conducted 53 field experiments at 34 sites in 5 regions over 3 years to determine the relationship between yield of wheat and several different indices for identifying subsoils with toxic concentrations of aluminium, AI. Initially, we identified that the concentration of aluminium, [AI], in the soil solution and in 1 : 5 0·005 M KCI extracts of soil from the 15-25 cm layer was responsible for the majority of the decrease in wheat yield. The concentration of Al in a 1 : 5 0·005 M KCI extract in the 15-25 cm layer was well correlated with grain yield of wheat grown on yellow earth soils in the Merredin region, provided the soils had similar fertilizer treatments. The ratio [AI]: [Na] in a 1 : 5 0·005 M KCI extract was a better predictor than [AI] alone of grain yield of wheat grown on yellow earths in different regions and with different fertilizer practices. The three seasons had little effect on the correlation between yield and different soil indices. The correlations determined were strongly affected by regional differences, which were probably due to differing water supply and availability. The choice of toxicity index depended on the uniformity of fertilizer management practices within a region and it appeared that both ionic strength and calcium were important mitigating factors.