Phosphorus (P) fertiliser is one of the largest input costs in grain production, making it increasingly important for growers to target applications where they will deliver the greatest return.
This project validated a practical methodology that combines soil pH and historic crop NDVI data (pHnNDVI) to identify where crops are most likely to respond to phosphorus fertiliser. It also continued long-term phosphorus management trials to better understand the residual value of phosphorus fertiliser and identify strategies that improve profitability while maintaining soil fertility over time.
Project description:
Led by Trengove Consulting, this project expanded previous phosphorus research by validating the pHnNDVI methodology across new environments on Yorke Peninsula, Eyre Peninsula and the South East, while continuing long-term phosphorus management trials established in the Mid North.
Single-season trials investigated phosphorus responses across paddocks with differing soil types and production zones, helping determine where phosphorus applications are most likely to generate an economic return. At the same time, long-term trials at Crystal Brook, Spalding and Hart continued to compare repeated phosphorus applications, residual phosphorus strategies and alternative management approaches to better understand their impacts on crop performance, profitability and soil fertility over multiple seasons. Together, these complementary research streams provide growers with practical tools to manage both within-paddock variability and long-term phosphorus investment.
Key achievements and results:
The project expanded validation of the pHnNDVI methodology into new production environments, increasing the phosphorus response database to 73 trial sites across South Australia. Results demonstrated that the methodology can successfully identify areas with low, moderate and high phosphorus responsiveness in many environments, allowing growers to better target fertiliser applications where they are most likely to improve profitability while reducing unnecessary inputs in less responsive areas.
The research also highlighted important regional differences. At Malinong and Tumby Bay, the methodology accurately predicted differences in phosphorus responsiveness across paddock zones, supporting more targeted fertiliser decisions. In contrast, results from Minnipa showed that responses were often uneconomic despite predicted phosphorus responsiveness, demonstrating that low-yielding environments require different management approaches and further refinement of the methodology.
Long-term phosphorus management trials continued to improve understanding of how different fertiliser strategies perform over time. Results confirmed that both repeated phosphorus applications and strategic residual phosphorus investments can sustain crop productivity, although the most profitable strategy depends on fertiliser prices, soil type and production potential. This provides growers with greater confidence when balancing short-term input costs against maintaining long-term soil fertility.
Project findings were widely shared through GRDC Regional Updates, crop walks, field days and adviser presentations across the Mid North, Eyre Peninsula, South East and Mallee. The work has been well received by growers and advisers, with increasing adoption of the pHnNDVI approach already occurring. One adviser reported that five clients had committed to using the methodology across approximately 8,000 hectares, demonstrating early industry confidence in the approach.
Further information: