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 Wag ga Wagga, NSW. The lime was applied to the surface of the direct drill ed-soil without any incorporation, but in the case of the cultivated s oils, the lime was incorporated into the top 10 cm by scarifying. Afte r liming, an initial temporary reduction in macroaggregate (>2 mm) sta bility 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 de crease in structural stability was attributed to lime-induced increase s in biological decomposition and the resulting soil organic carbon lo sses. Subsequent samplings did not detect any difference in either mac ro-or micro- (<50 mu m) aggregate stability of this soil as a result o f lime treatment. In contrast, for the 2 cultivated soils which had lo wer initial structural stability and organic carbon levels, a decline in stability was not observed. Instead, significant increases in macro aggregate and microaggregate stability were detected 1.5 years after l ime application. By the end of 3 years, macroaggregate stability of th e limed cultivated soils approached that of the direct-drilled soil. T he improvement in structural stability extended to 7.5 cm depth 3 year s after lime application. Wet-sieving experiments using prolonged peri ods of shaking indicated enhanced stability of the water-stable aggreg ates of the limed cultivated soils but not the direct-drilled soils.