Effect of long-fallow on soil quality and cotton lint yield in an irrigated, self-mulching, grey Vertosol in the central-west of New South Wales

Reduced crop growth rates ('long-fallow disorder') can be a feature of long-fallow; cotton (cotton, alternating with a bare fallow, is sown every other year). This is usually attributed to decreased development of arbuscular mycorrhiza (AM), although associated soil physical, chemical, and biological properties are very rarely reported. A study was conducted from 1993 to 1997 in a grey, self-mulching Vertosol in the central-west of New South Wales to characterise soil properties under long-fallow cotton with a view to identifying soil factors other than AM that could contribute to cotton growth rate reductions. Soil quality indicators monitored were compaction (bulk density and air-filled porosity), strength (cone resistance), plastic limit, exchangeable cations, nitrate-N, pH, organic C, development of AM, and incidence of cotton root diseases. In comparison with continuous cotton, long-fallow cotton had lower soil strength, and lower plastic limit. Exchangeable Ca and Mg Ir ere higher with continuous cotton only in 1994. Higher nitrate-N was also observed with long-fallow during the first fallow phase of the experiment. Long-fallow did not have any significant effect on soil organic carbon. However, a net decline in soil organic C and exchangeable Mg occurred with both treatments. During the cotton phase, subsoil nitrate-N and incidence of black root rot were lower with long-fallow cotton. Uptake of nitrogen by continuous cotton may have been reduced by greater severity of black root rot. Vegetative and reproductive growth, water extraction, and cotton lint yields in long-fallow cotton plots were higher than those in continuous cotton plots. AM development was similar with continuous cotton and long-fallow cotton. Compared with long-fallow cotton, the lower lint yield in continuous cotton was thought to be due to the interactive effects of declining nutrient availability, higher soil strength, and greater severity of black root rot causing decreases in nutrient and water uptake.