Acidification problems of duplex soils used for crop-pasture rotations

Abstract

The acidification of duplex soils used for crop-pasture rotations has been reported widely in Australia in the winter dominant rainfall regions. At some locations induced soil acidity limits crop and pasture yield. The rate of soil acidification is affected by soil properties, agricultural management and rainfall. Rates of acid addition of 0.6-6 kmol H+/ha.year have been measured from long term crop pasture rotation experiments; these rates are comparable with values reported from pastoral studies in higher rainfall areas. Components of both the carbon and nitrogen cycles contribute to this acid addition, with loss of nitrate nitrogen below the rooting depth of these predominantly annual plant systems likely to be the main cause of acidification. Lime application has been recommended as a means of correcting acidification and improving crop and pasture yield. There is little information on the longevity of any beneficial effects of lime, the movement of lime in the soil and re-acidification of the soil in crop-pasture systems. A long term experimental site with rotation, deep tillage and lime treatments has been soil sampled throughout a 9-year period for changes in soil pH and aluminium. Soil pH decreased with increasing time after lime application. At lower lime rates (0.5-1.0 t/ha) there was no difference in pH or exchangeable A1 after 9 years, compared with the unlimed soil. At the higher lime rates there was downward movement of the neutralising effect of lime with time, as well as acidification of the soil. However, the yield responses obtained with all of the lime rates were maintained 9 years after 1 application of lime, even though the soil was strongly acid according to the measures used. Strategies for countering soil acidificaton may require an initial application of lime if acidity factors are restricting yield. Management systems which increase the permeability of the B horizon of duplex soils and which promote plant growth and a deep root system are essential for countering acidity in a croppasture rotation.