Variability of acidity in agricultural soils-the impact of timber burning at land clearing

摘要

Patchiness in the growth of barrel medic, Medicago truncatula, in the central west of New South Wales, near Condobolin, has been associated with variability of soil acidity. There is evidence of the effect of timber burning on soil properties and it is possible that such burning of timber stacks and windrows on land recently cleared for agriculture may add alkali and contribute to such pasture growth. Of the three sites used, two were already in a 20-year farming system and one was constructed in a recently cleared paddock. Soil sampled at the 0-0.10, 0.10-0.20, and 0.20-0.30m soil depths indicated significant increases in soil pH(Ca) and extractable Ca to a soil depth of at least 0.20m for up to 20 years after timber had been burnt. The effect of the timber burn on soil chemistry was due to the conversion of alkali oxides to either hydroxides or carbonates. This addition of alkali moved relatively rapidly through the soil profile in a low-rainfall farming system. Additional alkali was found in the soil, mainly at the 0-0.10m depth, in the form of free lime. Using assumed rates of soil acidification, the burn effect could persist for up to 1227 years before reverting to the pre-burn soil pH. It was also found that timber burning at the sites contributed to the spatial variability of soil acidity.