Birchip Cropping Group – Phosphorus use in the Wimmera Mallee

In 2008, through collaboration between the University of Adelaide and the BCG, a survey was performed in the Wimmera and Mallee region of Victoria with the objective to determine the P-responsiveness of paddocks and compare whether those responses were correctly predicted by either Colwell P or DGT. The survey simply required growers to have a control strip (no fertiliser applied for one seeder width across the paddock) in their paddock by turning off their fertiliser input. Soil samples at sowing, dry matter (in-crop) and grain yield were taken at each site

The survey found that the new DGT method correctly predicted 89% of responses, while the Colwell P method only predicted 74% however, incorrectly predicted a highly deficient paddock. The survey also found that of the relatively small sample size, 35% of paddocks were responsive to applied-P, which highlights the importance of soil testing. With a superior predictive ability, the DGT method has greater potential to assist growers to maximize their production and ensure a financial return from a P application.

A paddock survey was undertaken in 2009 with BCG farmers keen to participate. Each grower volunteered to sow control strips (no P fertiliser) in a particular paddock on their property. Each of these paddocks required an initial soil sample taken from within the control strip and analysed for Colwell P and DGT. The in-crop assessment of dry matter (end of tillering) occurred over a 10 day period. Depending on the farmer’s equipment, grain yield was measured through yield monitors or weigh bins. Grain quality (protein, moisture and screenings) was also measured. The fertiliser response was assessed by comparing adjacent field trial strips.

Both surveys in 2008 and 2009 have demonstrated the potential benefits of DGT as a predictive tool for crop response to applied P and to improve fertiliser decisions. In both years, DGT has outperformed the commonly used Colwell P test. IThe PBI test improves the Colwell P method to a degree where a predicted success rate can be achieved.

Utilising DGT, PBI and combining information from replicated field trials the amounts of P fertiliser required to maximise yields can be estimated. Despite these findings, DGT will not be commercially available until 2012. Subsequently, using Colwell P and PBI together is currently the best tool available. Given Colwell P and PBI can account for 50 – 60% of responses, it indicates that decisions should not be made on these values alone. Paddock history and P-balances would further improve the decision process and allow fertiliser rates to be refined.

Local consultants and agribusiness were informed about the findings at the field days and presentation at the BCG Trials Review day. Information regarding the project was also presented at the BCG Grains and Research Expo (2nd July). The work has increased the links with agribusiness further as providing greater confidence in the current commercially available testing methods and prominsing results from the DGT test.

The project was very successful in demonstrating greater confidence in the results of soil tests (e.g. Colwell P), but also showed the new P test DGT performed exceptionally well. The impact for growers is that the survey findings has given all participants the greater confidence in soil tests for predicting P responses and knowing when P-rates can be modified.