A COMPARISON OF SOIL ACIDIFICATION AND ALUMINUM UNDER EUCALYPTUS FOREST AND UNIMPROVED PASTURE

A comparison was made of pH (1:5, soil:0.01 M CaCl2) and easily extrac table forms of Al [0.01 M CaCl2 (Al(Ca)); 1 M KCl (Al(K)); and 0.05 M CuCl2 (Al(Cu))] between forest and pasture soils near Bendigo, Victori a. Perennial pasture growth is required in this region to reduce groun dwater recharge as part of the management of dryland salinity, but pas ture growth may be inhibited by Al and Mn toxicity as a consequence of soil acidification. Both forest and pasture soils were found to be ac idic (mean pH of 4.0 and 4.3 respectively) and Al(Ca) concentrations a re sufficient to anticipate toxicity to sensitive species. The forest site was cleared of timber over 50 years ago, and has since acidified by 63 kmol H+ ha-1, which is accounted for by organic anion accumulati on in the forest regrowth. The forest soil has lower concentrations of Al(Ca) for a given pH, and more Al complexed to organic matter (Al(Cu )-Al(K)), even though forest and pasture soils have equal amounts of o rganic carbon. The different Al concentrations in forest and pasture s oils are accounted for by a lag in Al response to acidification, and g reater complexation of Al with organic matter in the forest soil.