Spatial Phosphorus Management at Cleve: Relocation can beat more fertiliser
Background
Recent work led by Trengove Consulting and funded by SAGIT has examined how phosphorus (P) requirements vary across paddocks, mainly in the Mid North and Yorke Peninsula broadacre cropping regions of South Australia. More recently, this work has expanded to Eyre Peninsula, with trials run during 2024–2025. The results show that crop responses to applied P are most likely in parts of the paddock with alkaline soils, calcium carbonate, higher PBI values (>80), low DGT P, and historically low NDVI compared with the rest of the paddock. In the past, variable-rate P programs based on simple replacement calculations may have reduced P inputs in these lower-performing areas, which can worsen P deficiency if it is not correctly diagnosed. In contrast, other areas of the same paddock can build P reserves where soils have a lower capacity to bind applied P (PBI <70).
A major outcome of this work has been the development of a pHnNDVI index. This index has helped improve variable-rate P decisions and increase partial gross margins across farms. In simple terms, areas that are likely to respond to higher P inputs tend to have a high pHnNDVI value. These areas usually combine high soil pH in CaCl2 (7.5–8.0) with low normalised NDVI (<0.8). Areas with neutral to acidic soils and consistently high normalised NDVI often have stronger P reserves, creating an opportunity to reduce P inputs without compromising production.
Example of how pHnNDVI works within the same paddock:
Phosphorus responsive area = 7.8 (pH in CalCl2)/ 0.7 (low normalised NDVI) = 11.1
Phosphorus non-responsive area = 6 (pH in CalCl2)/ 1.3 (high normalised NDVI) = 4.6
The project is now expanding and validating this approach in new regions. The index is practical because it only needs two data layers: a soil pH map and a series of NDVI images across seasons. At a minimum, the zones generated by the index can help growers and advisors target soil sampling where it is most likely to add value.
Spatial P responses in the Cleve area
The 2026 trial is testing spatial approaches to refine P inputs in a paddock just west of Cleve. Using previous NDVI imagery and a grid soil pH map, three P response trials have been established in low and high pHnNDVI zones (Table 1, Figure 1). At each location, several P rates have been applied, ranging from 0 to 90 kg P/ha. Wheat was sown on 29 April 2026. These trials will help determine the P rate needed to maximise grain yield and the rate that delivers the best partial gross margin under current grain and fertiliser prices. All sites emerged evenly within 5–7 days following 6–10 mm of rainfall.
Early observations
This paddock is a good example of how soil type can drive residual available P levels. The high pH, low NDVI areas in the South and East sites are strongly P deficient. These soils have high PBI values, meaning they bind more of the P applied in previous and current fertiliser applications. As a result, residual P levels are low, as shown by very low Colwell P and DGT P results. In contrast, the North site has lower soil pH and roughly half the PBI. This soil has a lower capacity to bind P, which has helped maintain high residual P levels and strong early crop growth, reflected in the NDVI imagery.
Early crop performance and response to P inputs have matched the predictions from pH and NDVI mapping, supported by pre-sowing soil testing. The North site has shown no visible response to applied P during the season (Figure 2), which is consistent with its higher residual P levels. In contrast, wheat at the South and East sites has responded strongly to applied P (Figure 3), with responses observed up to 50 kg P/ha, equivalent to 227 kg MAP/ha.
Watch this space
The project team will keep growers and advisors updated as the trials progress through the season, including grain yield results and the economic analysis needed to guide future P decisions.
This trial is part of a larger program of work being delivered on Eyre Peninsula with trials similar trials conducted at Tumby Bay and Mt Damper in the project “Sustainable phosphorous strategies for Eyre Peninsula” as part of the SA Discovery Farms program, funded by the Australian Government’s Future Drought Fund. AIR EP is leading the project, with co-investment by GRDC and SAGIT, aimed at improving P fertiliser applications in variable paddocks/landscapes and refining long-term management strategies on highly P responsive soils. The overall aim is to increase profitability from P fertiliser applications and determine sustainable P fertiliser strategies for EP. Project delivery partners are SARDI Minnipa Ag Centre, Trengove Consulting, Agronomy Solutions, Nutrien Cleve Ag Services and EPAG Research.
Acknowledgments
Thanks to Harris Agricultural for hosting the trial site. Thanks to Nutrien Cleve Agricultural Services for paddock selection and trial implementation.
