Results from this project have shown that:

  • The Diffusive gradients in this films test for measuring plant available phosphorous provides a far more accurate indication of likely yield response to applied P fertiliser than the commonly used Colwell, Olsen and Bray2 P tests.
  • The majority of sandy soils that were involved in the research trials had very high starting soil P concentrations, likely owing high applications of P fertiliser in the past.
    • These sites only require between 30 to 40 kg of P applied, based on removal rates; standard industry applications were found to be 66 to 110 kg/ha.
    • Reducing rates can lead to cost savings >$200 per hectare based on current fertiliser prices, with no significant negative impact on yield.
    • Given the very low affinity of sandy soils to tie up P fertiliser, there is an increased risk that phosphorous could be lost to the environment through leaching and erosion from these sites.
  • Potatoes grown on soil types with a high affinity to tie up P fertiliser, such as those on Kangaroo Island, will suffer significant yield penalties if no P fertiliser is applied, despite often having soil P concentrations (as measured by the Colwell, Olsen and Bray methods) that exceeded previously determined critical values.
  • Further research will help to refine the critical DGT values that have been determined, especially if more sites with a high affinity to tie up P are included.

In addition, evaluation of project extension activities found that:

  • 43% of advisors who responded suspected they were recommending too much P fertiliser; 43% neither agreed nor disagreed
  • 78% agreed or strongly agreed that their P recommendations would change as a result
  • 100% agreed or strongly agreed that they would use the DGT-P test in the future
  • 100% agreed or strongly agreed that their knowledge of soil P had increased as a result of participating in the extension activities
  • 100% agreed or strongly agreed that they would like to learn more about soil chemistry and fertility

Similarly, 66% of participating potato producers suspected they were applying too much P fertiliser and 73% agreed their P fertiliser rates would change as a result of the project; 100% indicated they will use the DGT-P test in the future. Additionally, 100% of producers agreed that the on-farm research was beneficial and all agreed to participate in future research activities.

This project was funded and delivered by:





With the proud support of:




Members of the SA Potato Industry



Project Partners

Potatoes South Australia PIRSA RSSA National Landcare Programme The University of Adelaide premiumgreen-logo