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In cropping soils, optimising soil structure means minimising soil structure degradation and emphasising natural processes of structure formation.  Man-made soil structure degradation (compaction and shear stresses) has been shown to reduce green boll numbers in cotton up to 50% and cotton lint by 35%.

The Australian Cotton Industry is now, actively minimising the problem of soil structure degradation and providing the basis for optimising soil structure with the adoption of "retained-bed" systems.  In this form of controlled traffic, tractor and equipment wheels are restricted to specific furrows and the beds are only lightly cultivated.  Beds have been retained for up to seven years, which differs dramatically from practices of the 1960s and 1970s where beds were removed after harvest, the soil cultivated and the beds reformed for the next season.

Heavy, cracking clays (Vertisols) account for 84% of the irrigated cotton area.  The nature of these soils ensures that a retained bed system minimises structure degradation and allows natural processes of structure formation to be optimised.  The high water holding capacity of these soils causes them to remain in a plastic (mouldable) state for long periods of time.  However, the presence of swell-shrink clay minerals ensures cracking on drying, and with repeated wet/dry cycles the maintenance or re-formation of suitable soil structure for cropping.  Non-cracking soils (Alfisols) can also benefit with the retained bed system. Reducing mechanical disturbance increases organic matter (from break crops) and increases biological activity, leading to less surface crusting and an increase in the number of continuous macropores from the surface to depth.

Monitoring physical improvements in soil structure under cotton continues at the field, glasshouse and laboratory levels.  Measures include aeration, shrinkage, strength, water infiltration and image analysis. Studies cover several cotton regions and different bed systems and soils.  In new research the potential risk of structure degradation in cropping soils and the potential of these soils for self repair are being investigated.  These classifications will be based on the soil's plastic limit, inherent properties that affect workability and repair potential (cations, clay content, etc.) and the dynamic forces of irrigation and weather.

Conclusions

Almost two decades ago soil structure degradation was recognised as a significant potential threat to the productivity of Australian cotton.  The problem has been intensively researched, leading to practical management strategies to not only prevent and control structure degradation but also to optimise the soil physical environment for reduced-cost cotton production.  Retained beds and minimum tillage of dry soil together with rotation crops will ensure optimum soil structure conditions that maximise natural structure formation and maintenance with minimum effort.  Structure degradation can not be eradicated, but current practices ensure its control and prevents it from being a problem.

Future research will continue to emphasise the optimisation of natural structure formation. However, research currently underway in the Department of Primary Industries (Queensland), is aiming to forecast the potential risk of structure degradation in soils and their potential for self repair.  It is important to know if some soils are more prone to degradation than others, so they can be managed with maximum care.  Alternatively, soils with a high self-repair potential can be cropped more frequently as structure regeneration will be achieved through wet/dry cycles especially with rotation crops.  Additionally, determination of the inherent physical/chemical properties that lead to greater strength or resilience may lead to the use of soil ameliorants, akin to fertiliser inputs to optimise chemical response.

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