Effect of lime (CaCO3) application on soil structural stability of a red earth

Changes in soil structural stability as a result of lime application (1.5 t/ha) were monitored over 3 years in a red earth with contrasting initial pH, organic carbon, and structural stability conditions at Wagga Wagga, NSW. The lime was applied to the surface of the direct drilled-soil without any incorporation, but in the case of the cultivated soils, the lime was incorporated into the top 10 cm by scarifying. After liming, an initial temporary reduction in macroaggregate (>2 mm) stability was detected in the immediate surface (0-2.5 cm) of the direct-drilled soil where the highest increases in pH, losses in soil organic carbon, and increases in microbial biomass were also observed. The decrease in structural stability was attributed to lime-induced increases in biological decomposition and the resulting soil organic carbon losses. Subsequent samplings did not detect any difference in either macro-or micro- (<50 mu m) aggregate stability of this soil as a result of lime treatment. In contrast, for the 2 cultivated soils which had lower initial structural stability and organic carbon levels, a decline in stability was not observed. Instead, significant increases in macroaggregate and microaggregate stability were detected 1.5 years after lime application. By the end of 3 years, macroaggregate stability of the limed cultivated soils approached that of the direct-drilled soil. The improvement in structural stability extended to 7.5 cm depth 3 years after lime application. Wet-sieving experiments using prolonged periods of shaking indicated enhanced stability of the water-stable aggregates of the limed cultivated soils but not the direct-drilled soils.