The phosphorus dynamics of dryland cotton production require further elucidation, particularly the role of residual fertiliser P and mycorrhizal infection in yield. Incitec Pivot Ltd., in collaboration with independent researchers, has conducted a long term farming systems experiment at Tulloona , NSW , since 1995. The experiment is a complete randomised block, split plot, design with the treatments being 0 , 10, 20 kg/ha P and 0, 30, 60 , 90, 120 kg/ha N : and all permutations of the above. Nutrients, as urea and MAP, have been applied either pre or at plant for each crop planted, and rotations have included, sorghum , mungbean, wheat, chickpeas, and barley, throughout the duration of the project. The site is responsive to both N and P, with background Colwell P values > 10mg/kg, and with soil N levels varying depending on fallow mineralisation.

For the 2014/15 summer the rotation crop chosen for the site will be raingrown cotton. This will follow rain grown wheat sown June 2013. There has previously been very little previous experimentation investigating performance of raingrown cotton under these scenarios, and in this region. With soil test Colwell P values showing residual soil P levels ranging from 10 mg/kg P where no P has been applied, to 98 mg/kg following annual 20 kg P/ha applications.

This provides an ideal, and unique, scenario to investigate cotton crop performance under a range of residual P values, but also to investigate whether applied fertiliser P at sowing is providing a crop response or if labile P being released into solution is in fact the primary source of P. This experiment will also assist with validation of soil test critical values investigating current soil P test extraction methods such as Colwell P, BSES P, and less commonly used on Grey Vertisols DGT P. Core sampling throughout the trial will also allow quantification of the effect of P status on mycorrhizal colonisation of cotton following a 12 month fallow. the full report is available.