Change in soil pH, manganese and aluminium under subterranean clover pasture

Changes in soil pH, manganese and aluminium as a result of long periods under subterranean clover pasture were examined in soils formed on granite, basalt and sedimentary rocks near Goulburn, New South Wales. Decreases in the pH of yellow duplex soils formed on granite, sedimentary rocks and basalt had occurred to depths of 60, 40 and 30 cm, respectively. The smaller depth of acidification in the latter two soils is considered to be due to their shallower A horizons over well buffered, clay B horizons. Under the oldest pastures (55 years) the decreases exceeded one pH unit throughout the entire sampled depth (60 cm). In some soils, under old improved pastures, calcium chloride-extractable manganese had increased to more than 20 ppm throughout the 60 cm profile and to greater than 50 ppm in the surface 10 cm. These levels are considered toxic to sensitive plant species and the highest levels may be toxic to subterranean clover. The amounts of extractable manganese in soils appear to be determined by both pH and the amounts of reactive manganese. In general, the amounts of total and reactive manganese were appreciably higher in the soils of basaltic origin. Substantial increases in extractable and exchangeable aluminium had also accompanied the decrease in pH and, in the surface 10 cm, were greatest in the soils formed on sedimentary parent materials. In many of the soils under old improved pastures, exchangeable aluminium, as a percentage of the effective cation exchange capacity, now exceeds 12%, especially in the 5-10 cm layer, and is probably harmful to sensitive species. Increases in exchangeable aluminium also occurred below the surface 10 cm and, in the granitic soils under the oldest pastures, exchangeable aluminium accounted for 30-50% of the effective cation exchange capacity throughout the 5-50 cm soil depth. The adverse changes in pH, manganese and aluminium observed in this study can be expected to continue under many improved pastures and to generate soil conditions unsuitable for many agricultural plants. The use of lime to arrest or reverse these changes seems inevitable.