AL TOXICITY OF WHEAT GROWN IN ACIDIC SUBSOILS IN RELATION TO SOIL SOLUTION PROPERTIES AND EXCHANGEABLE CATIONS

Toxic concentrations of soluble Al in the subsoil decrease the yield o f wheat grown on many yellow earths in the eastern wheatbelt of Wester n Australia. In our previous research (Carr et al. 1991), we observed variable plant response to high concentrations of soluble Al in subsoi ls of yellow earths in different regions of the wheatbelt. Environment al conditions (e.g. water supply) and/or an unidentified soil mitigati ng factor may have contributed to the variable plant response to solub le Al in some of the regions studied. We collected ten soils from four regions of the eastern wheatbelt of Western Australia. m a glasshouse experiment using these soils, we studied the effect of soil solution and KCl extract properties on wheat growth under uniform environmental conditions. The concentration of Al in a 0.005 M KCl extract was able to explain 97% of the variation in root fresh weight of wheat grown i n the 10 soils, even though the soil solution properties were found to differ markedly between regions. For example, 97% of the variation in root fresh weight (RFW) was explained by the total [Al] in soil solut ion extracted from soils in one region (Merredin). In comparison, 58% of the variation in RFW was explained by the total [Al] in the soil so lution extracted from soils collected from all four regions studied. I onic strength differences and possibly [SO4] were the major chemical p roperties that differed between Merredin and the other regions studied . These chemical differences presumably altered the toxic proportion o f Al in the soil solution, and hence, the plant response in some regio ns. The effect of ionic strength on toxic Al appeared to be simulated by extraction of the soil with 0.005 M KCl.