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 were 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.
