A comparison of soil acidification and aluminum under Eucalyptus forest and unimproved pasture

Abstract

A comparison was made of pH (1:5, soil: 0.01 M CaCl2) and easily extractable forms of Al [0.01 M CaCl2 (Alca); 1 M KCl (AlK); and 0.05 M CuCl2 (AlCu)] between forest and pasture soils near Bendigo, Victoria. Perennial pasture growth is required in this region to reduce groundwater recharge as part of the management of dryland salinity, but pasture growth may be inhibited by Al and Mn toxicity as a consequence of soil acidification. Both forest and pasture soils were found to be acidic (mean pH of 4.0 and 4.3 respectively) and AlCa concentrations are 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 accumulation in the forest regrowth. The forest soil has lower concentrations of AlCa, for a given pH, and more Al complexed to organic matter (Alcu-AlK), even though forest and pasture soils have equal amounts of organic carbon. The different A1 concentrations in forest and pasture soils are accounted for by a lag in A1 response to acidification, and greater complexation of Al with organic matter in the forest soil.