Soil acidification under clover pasture

Abstract

Long-term changes in some chemical properties of yellow podzolic soils under subterranean clover pastures were studied near Binda, New South Wales. The rate of decrease in pH of the surface 10 cm of soil was greatest in the early years following pasture establishment, and decreased with time. Over 50 years the decrease in pH was approximately one unit, resulting in current soil pH values of about 5.0. The increased acidity had led to increases in CaCI,-extractable manganese and exchangeable aluminium. Although the build-up of soil organic matter under a 32- year-old subterranean clover pasture was largely confined to the surface 10 cm, the decrease in pH extended to a depth in excess of 30 cm. Total soil nitrogen increased gradually throughout the 50-year period, reflecting increases in the soil organic matter. There were associated increases in cation exchange capacity and titratable acidity. Total soil phosphorus also increased with time and was correlated with the decrease in pH. This reflected the accumulation of organic and inorganic residues from phosphate fertilizers, but it seems unlikely that the acidity of the superphosphate contributed directly to the increase in soil acidity although it is essential for the establishment and maintenance of the legume, the main source of the increase in soil organic matter. It seems likely that the area of pasture soils adversely affected by acidity problems will increase in the future.