EFFECT OF CROP-ROTATION AND RESIDUE MANAGEMENT ON PROPERTIES OF CRACKING CLAY SOILS UNDER IRRIGATED COTTON-BASED FARMING SYSTEMS OF NEW-SOUTH-WALES

The effects of planting cereal or leguminous crops in rotation with irrigated cotton (Gossypium hirsutum L.) on the properties of cracking day (swelling) soils in the Macquarie and Namoi Valleys of New South Wales, Australia were evaluated during the summer of 1992-3. The observations were made on commercial farmers' fields. The soil properties evaluated were the particle size distribution, the dispersion index, the plastic limit, the percentage of coarse (particle diameter 212-2000 mum) and fine (particle diameter 53-212 mum) particulate soil organic matter, soil respiration rate, soil reactivity, soil aggregate density, pH, nitrate-N and exchangeable Ca, Mg, K and Na. In general, the planting of rotation crops decreased the dispersion index, plastic limit and soil aggregate density, and increased the amount of coarse particulate organic matter. Planting rotation crops also resulted in significantly higher clay and lower silt contents in the Macquarie Valley, and significantly higher soil respiration in the Namoi Valley. Soil pH, nitrate-N and exchangeable cation concentrations were not significantly affected by planting rotation crops in the Macquarie Valley, whereas exchangeable Na was increased in the Namoi Valley. The retention of crop residues in situ, compared with burning crop residues, decreased the dispersion index, plastic limit and aggregate density, and increased the amount of coarse particulate soil organic matter at all measured depths of the Macquarie Valley. The retention of crop residues in the Namoi Valley decreased the plastic limit and dispersion index only in the 0-50 mm depth range, whereas burning crop residues increased exchangeable K at all depths. In general, planting rotation crops and the retention of crop residues had greater beneficial effects on the soil physical properties in the Macquarie Valley than in the Namoi Valley, and in the topsoil compared with the subsoil. These differences are attributed to a shorter rotation interval in the Namoi Valley, smaller amounts of coarse particulate soil organic matter in the subsoil, and differing soil types in the two valleys. In the Namoi Valley the coarse organic matter produced by leguminous crops appeared to be more effective in promoting structural stability than that from non-leguminous crops, although no such difference was observed in the Macquarie Valley.