Effect of tillage and crop rotation on the surface stability and chemical properties of a red-brown earth (Alfisol) under wheat

Xeralfs with predominantly sandy, or sandy loam surface textures are widespread throughout the Mediterranean climatic region of the southern Australian wheatbelt. The surface structure of these soils is prone to deterioration with frequent cropping and cultivation. Recognition of this problem has led to interest in conservation tillage as one method for reducing soil degradation. To test the efficacy of this system and to understand the role of crop rotation, information is needed on how different tillage and rotation practices affect the surface stability and chemical properties of these soils. Changes in the surface stability and chemical properties of a red-brown earth (Xeralf) arising after 5 years in a tillage and rotation trial were measured. The stability of air-dried earthworm casts was also measured because of growing interest in their role in soil structure development. Organic carbon levels were significantly higher with direct drilling, compared to conventional cultivation, but were not significantly affected by crop rotation. Aggregate stability was significantly affected by the percentage of organic carbon. Earthworm casts contained relatively low levels of organic carbon and were among the most unstable aggregates tested. Aggregate disruption arising from the escape of entrapped compressed air was found to have less effect on aggregates from direct drilling than on aggregates from conventional cultivation, or eathworm casts. In most cases, the release of air resulted in the breakdown of aggregates > 1 mm to smaller aggregates > 250 μm. As expected for a surface soil of low clay content, the cation exchange capacity (CEC) was also strongly dependent upon the percentage of organic carbon and in consequence, was higher with direct drilling than with conventional cultivation. CEC was also significantly greater under the wheat-lupin rotation than either the wheat-pasture rotation or continuous wheat. Soil pH declined in all tillage and rotation treatments relative to the pH under the original untilled continuous pasture. The smallest decrease occurred in the wheat-lupin rotation and direct drilling likewise produced only a minor drop in pH. Direct drilling also significantly increased the percentage of total soil nitrogen. The results indicate that soil degradation can be effectively reduced under crop management systems that maximise surface cover with residues and reduce the rate of decomposition of organic matter by direct drilling