Soil acidity in New-South-Wales - Current pH values and estimates of acidification rates

Abstract

An estimate has been made of the mean pH of the surface soils (0-10 or 0-15 cm) of New South Wales by mapping the soil pH (1:2, soil: 0.01 M CaCl2) values of soil samples analysed by the NSW Agriculture & Fisheries soil testing service. Within mapped classes the soil pH values vary around the mean by about 20.4 units for low pH soils (3.8-5.0) to about 20.9 units for high pH soils (>6.0). It is estimated that the areas of surface soils within agricultural holdings in NSW in the pH classes <4.5, 4.51-5 0, 5.01-5 5 and 5.5 1-6.0, are 5 3, 8.4, 5 7 and 5.1 million ha respectively. In general, pH values in the higher rainfall coastal and tablelands areas in the east are below 5.0, with the most acid areas being below 4.25. The latter are usually in the high rainfall zones (>1000 mm) and on low pH buffer capacity soils (sand to sandy loam texture). In the south of the state the area of low pH soils is broader, and extends into lower rainfall zones. Within the mapped pH classes the higher pH buffer capacity clay soils had pH values 0.83 (s.e. 0.6) units higher than the mean, whilst sands and sandy loams had pH values 0.34 (s.e. 0.1) units lower than the mean. Data on the acid addition rates for a number of agricultural systems in NSW and adjacent areas were collated and show net rates of acid addition to the soil profile from near zero to rates of 3-5 kmoles H+ ha-l year-1 over extensive areas. High acid addition rates, of 10-20 kmoles H+ ha-1 year-1, have been measured in some exploitative systems. These acid addition rate values can be used in association with soil pH buffer capacity data to estimate the rate of pH change in the future. At an acid addition rate of 4 kmol H+ ha-1 year-1, the soil pH can decline by one unit, in the surface 30 cm within 30 years for sandy loam soils, and within 120 years for clay soils.