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 ir rigated cotton (Gossypium hirsutum L.) on the properties of cracking d ay (swelling) soils in the Macquarie and Namoi Valleys of New South Wa les, Australia were evaluated during the summer of 1992-3. The observa tions were made on commercial farmers' fields. The soil properties eva luated were the particle size distribution, the dispersion index, the plastic limit, the percentage of coarse (particle diameter 212-2000 mu m) and fine (particle diameter 53-212 mum) particulate soil organic ma tter, soil respiration rate, soil reactivity, soil aggregate density, pH, nitrate-N and exchangeable Ca, Mg, K and Na. In general, the plant ing of rotation crops decreased the dispersion index, plastic limit an d soil aggregate density, and increased the amount of coarse particula te organic matter. Planting rotation crops also resulted in significan tly higher clay and lower silt contents in the Macquarie Valley, and s ignificantly higher soil respiration in the Namoi Valley. Soil pH, nit rate-N and exchangeable cation concentrations were not significantly a ffected by planting rotation crops in the Macquarie Valley, whereas ex changeable 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 t he amount of coarse particulate soil organic matter at all measured de pths of the Macquarie Valley. The retention of crop residues in the Na moi Valley decreased the plastic limit and dispersion index only in th e 0-50 mm depth range, whereas burning crop residues increased exchang eable K at all depths. In general, planting rotation crops and the ret ention of crop residues had greater beneficial effects on the soil phy sical properties in the Macquarie Valley than in the Namoi Valley, and in the topsoil compared with the subsoil. These differences are attri buted to a shorter rotation interval in the Namoi Valley, smaller amou nts of coarse particulate soil organic matter in the subsoil, and diff ering soil types in the two valleys. In the Namoi Valley the coarse or ganic matter produced by leguminous crops appeared to be more effectiv e in promoting structural stability than that from non-leguminous crop s, although no such difference was observed in the Macquarie Valley.