Simulating the impact of acidifying farming systems on Australian soils

The Hunter Valley is a productive farming and pastoral area in eastern Australia. Land use ranges from viticulture and dairying to extensive grazing and forestry. Soil acidity may become a problem if the soil pH is reduced to critical levels by acid accumulated when nutrient cycles are open (especially N, C and S). This study uses a model to forecast pH changes due to acids derived from land use. Soil samples from a survey in the Hunter Valley were analysed for pH (1:5 soil:0.01 M CaCl2), effective cation exchange capacity (cmol(+)/kg) and pH buffer capacity (pHBC, kmol(+) ha(-1) 0.1 m(-1) pH(-1)). Data from three contrasting soil profiles were used in SPAM (Soil Profile Acidification Model) to forecast soil pH after 25 and 50 years. Results indicate the pH of a soil under native forest is stable. Only small amounts of acid are generated by this land use and the pH is buffered by both cation exchange reactions and aluminium dissolution. However, the acid generated from an intensive pastoral system (2.5 kmol(+) ha(-1) year(-1)) is sufficient to strongly acidify a well-buffered soil (pHBC 0-10 cm = 38 kmol(+) ha(-1) 0.1 m(-1) pH(-1)) to 50 cm depth. A less intensive pastoral system (0.33 kmol(+) ha(-1) year(-1)) was estimated to acidify a soil with below average buffer capacity (pHBC 0-10 cm = 9 kmol(+) ha(-1) 0.1 m(-1) pH(-1)) to 50 cm depth in 50 years. Verification of these results and the usefulness of SPAM are discussed.