ACIDIFICATION RATES IN THE CENTRAL WHEAT-BELT OF WESTERN-AUSTRALIA .2. ON A DEEP YELLOW SAND

The rate and mechanisms of acidification were determined on a sandy du plex soil (depth of sand 30-45 cm) under a cereal-annual pasture rotat ion in Western Australia. We also evaluated the effect of rotation (in tensity of cropping) on relative acidification of a sandy duplex soil. Rate of acidification was based on a linear regression analysis betwe en soil pH and years since clearing. Sites were sampled to a depth of 50 cm in 10-cm increments and measurements included soil pH, pH buffer ing capacity, and bulk density. The effect of different rotations on t he acidification rate was determined by soil sampling a rotation exper iment which had been established for 25 years. Sampling and measuremen ts were similar to the regression analysis. From regression, the rate of acidification for the profile was 0.15 kmol H+/ha.year, requiring 7 .7 kg CaCO3 to neutralise. Most of the acidification could be accounte d for by removal of alkaline products. Acidification was occurring to a depth of 30 cm, the acidification rate decreasing with depth. In the surface 20 cm the pH decline was 0.005-0.006 units/year. In the rotat ion experiment, the rate of acidification relative to continuous wheat without fertiliser nitrogen (N) ranged from 0.35 kmol H+/ha.year (17. 5 kg CaCO3) for continuous wheat with fertiliser N to 0.92 kmol H+/ha. year (45.8 kg CaCO3) for continuous pasture. Between these rates was 1 year pasture-1 year cereal (0.41 kmol H+/ha.year, 20.5 kg CaCO3) and 2 years pasture-1 year cereal (0.82 kmol H+/ha.year, 41.2 kg CaCO3). A cidification was occurring to 60 cm depth in all rotations, mostly due to nitrate leaching, removal of alkaline products, and build-up of or ganic matter.