Saturday, 15 July 2006

Coal-Fines as a Potential Amendment for Subsoil Acidity in Western Australia.

Y. Pal1, M. T. F. Wong2, and R. W. Bell1. (1) Murdoch Univ, School of Environmental Science, Murdoch, Australia, (2) CSIRO, Land and Water, Wembley, Australia

Sub-soil acidity leading to aluminium (Al) toxicity is a major problem for the West Australian (WA) wheatbelt and no cost-effective solution has yet been developed to overcome it. Coal-derived organic materials have been shown to increase soil pH, decrease Al saturation and increase plant growth in glasshouse experiments. Coal-fines, available in WA from the Collie Basin, have prospective use as a soil amendment. When treated with alkali such as potassium hydroxide, coal fines have the potential of forming water soluble fulvates. Little is known about the downwards mobility of these reaction products following incorporation in the field. This investigation involved field work to test the benefits of alkali-treated (calcium carbonate plus potassium hydroxide) coal relative to lime application for the amelioration of sub-soil acidity. At three field experimental sites (a grey sand in Brookton, plus a grey and a yellow sand in Mingenew) lime, alkali treated coal-fines, and calcium nitrate treatments were mixed in the top 10 cm soil at rates containing the same calcium as lime added to raise soil pHCaCl2 to 5.5. The experimental design included untreated coal-fines, and control plots with no addition of ameliorants. Soil samples were collected before break-of-season rains in May 2005, before treatment application, and again after the peak of rainfall in September 2005, from surface (0-10 cm) and subsurface depths (10-20, 20-30 and 30-40 cm). Annual rainfall in 2005 was 503 mm at Brookton and 468 mm at Mingenew. At Brookton, both alkali treated coal-fines and lime raised the soil pH in the top 20 cm layer. Treated coal-fines also increased pH by 0.5 unit at 30 cm. This effect was not matched by other treatments. Untreated coal had no effect on soil pH. Al saturation in the control plots at Brookton increased from 0 in the 0-10 cm layer to 16% in the 30-40 cm layer. Only treated coal-fines, lime and calcium nitrate decreased Al saturation in the 10-20 cm layer. Of these treatments, only treated coal further decreased Al saturation in the 20-30 cm layer. Other treatments had no effect on Al saturation. In Mingenew, both treated coal and lime increased soil pH in the top 20 cm layer whereas untreated coal decreased pH slightly at both the grey and yellow sand sites. In Mingenew, Al saturation in the control plots on yellow sand increased from 1% in the 0-10 cm layer to a maximum of 36% in the 20-30 cm layer. Untreated coal and calcium nitrate had no effect on Al saturation. Lime and alkali-treated coal-fines decreased Al saturation in 20-30 cm layer to 30 and 24% respectively. These initial results show that treated coal has a beneficial effect in ameliorating subsoil acidity. Treated coal-fines appeared marginally better than lime in treating subsoil acidity in the first year of measurements but these benefits need to be weighed against the higher costs of applying treated coal-fines. Key words: lime, calcium nitrate, aluminium toxicity.

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