Soil acidification under subterranean clover (Trifolium subterraneum L.) pastures in north-eastern Victoria

Eleven paired soil samples to 60 cm depth were collected from grazing properties in north-eastern Victoria. Soils were naturally acid and most were red or yellow podsolics. At each site unfertilised soils (unimproved) and soils which had received regular superphosphate applications (improved) were sampled from either side of a fenceline. The percentage of organic carbon was higher on improved sites but pH was usually lower. Using pH and pH buffering capacity data, the rate of soil acidification under improved pasture, relative to unimproved pasture, was estimated. The improved pastures, on average, required 39 kg CaCO3ka.year to balance the net acid accumulated. The estimated rates of acidification are much lower than those reported previously for similar environments and soil types in New South Wales. There was a relationship between initial profile pH and net acid addition, lower measured net acid addition being associated with low initial soil pH. This paper demonstrates the need for both standardisation of soil pH buffer capacity measurements, and more direct assessment of the role of soil mineral dissolution processes in buffering the pH of strongly acid soils. Until such data exists it will be very difficult to provide convincing information to primary producers regarding the long term alkali input requirements needed for sustainable farming systems.