Abstract

Soil acidity has been of major concern in Australia since European settlement. Acidification processes have been accelerated due to agricultural activities such as N fertiliser application and leguminous N-fixation in farm rotations. In this paper, we measured the acid buffering capacity (pHBC) of Vertosols, soils used predominantly for growing cotton in northern New South Wales. The pHBC values were used to calculate decrease in soil pH assuming net acid input due to agricultural practices. We combined the acidification results with geostatistics to spatially simulate the decline in soil pH of surface soils over time. The results indicate that it would take 10–417 years for soil pH to decrease by 1 unit on an assumed acid input of 5 kmol H⁺/ha.year. Soil pH will drop by 1 unit within 100 years for 90% of the soils and within 15 years for 10% of the soils. This reflects the variability of the pHBC for the studied soils. In 50 years from present, most of the eastern and north-western parts of the study region may become highly acidic with soil pH declining to 5.5. There may be a potential threat to sustainable agriculture from acidification in the region, although more work needs to be done to corroborate the counter-effects of water fluxes and carbonate dissolution. Sensitivity analysis indicates that even at low levels of acid input, some areas in the study region may experience significant decline in soil pH in the surface layer.