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The yield of cotton lint in Australia has increased greatly over the last 15 years. This improvement is due partly to the development of soil management systems that are based on the objective measurement of soil structure in individual fields. Average yields now exceed those of other major producers elsewhere in the world. Grey swelling clays (Vertisols) dominate, but hard-setting red duplex soils with non-swelling surfaces (Alfisols) are also important for cotton in some areas. On these soils mechanical compaction and instability in water are soil structural problems that can cause major yield declines if managed incorrectly. Most Australian cotton is grown on ridges and is furrow irrigated.
Available methods for overcoming natural and man-induced soil structural problems include shrinkage crack formation (created by drought-stressed rotation crops; particularly useful for Vertisols), biopore formation and organic matter accumulation (due to decomposing roots, and soil fauna such as earthworms and ants; particularly useful for the topsoil of Alfisols), low draft deep tillage, and the use of gypsum and lime. Extra water and nitrogen fertiliser can be used to obtain high yields on degraded soils, but such an approach tends to be inefficient and may cause off-site pollution. Where the soil has favourable conditions for cotton root growth and water movement, ‘controlled traffic – reduced tillage’ systems are recommended to minimise costs.
In the future it is necessary to provide soil structure assessment procedures that are more objective and many of the procedures for improving and living with degraded soils need to be refined. Also, better farm machinery should be developed for controlled traffic systems under cotton; vital are engineering and soil mechanics inputs to optimise axle loadings, and tyre and tool dimensions and configurations, on soils with different water contents and pre-stress conditions.