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, che mical, 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 qualit y indicators monitored were compaction (bulk density and air-filled po rosity), strength (cone resistance), plastic limit, exchangeable catio ns, nitrate-N, pH, organic C, development of AM, and incidence of cott on root diseases. In comparison with continuous cotton, long-fallow co tton had lower soil strength, and lower plastic limit. Exchangeable Ca and Mg Ir ere higher with continuous cotton only in 1994. Higher nitr ate-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 exc hangeable Mg occurred with both treatments. During the cotton phase, s ubsoil nitrate-N and incidence of black root rot were lower with long- fallow cotton. Uptake of nitrogen by continuous cotton may have been r educed by greater severity of black root rot. Vegetative and reproduct ive growth, water extraction, and cotton lint yields in long-fallow co tton plots were higher than those in continuous cotton plots. AM devel opment was similar with continuous cotton and long-fallow cotton. Comp ared with long-fallow cotton, the lower lint yield in continuous cotto n was thought to be due to the interactive effects of declining nutrie nt availability, higher soil strength, and greater severity of black r oot rot causing decreases in nutrient and water uptake.