Summary

Phosphorus fertiliser inputs are one of the largest variable input costs associated with grain production on SA farms. The P sufficiency index was validated to develop a robust methodology to predict response to phosphorus. An average $40 per hectare increase in partial gross margin was demonstrated on phosphorus responsive soils by using higher rates than recommended by phosphorus replacement strategies. At the same time, P fertiliser investment was minimised by identifying non-responsive sites where rates can be reduced, improving overall fertiliser use efficiency further. Long-term management strategy trials demonstrated that low soil phosphorus availability is difficult to overcome in a single season.

Background

Previous SAGIT project TC219 ‘Improved phosphorus prescription maps – beyond replacement P’, showed that the optimal phosphorus rates varied widely across soil types in trial paddocks, and the variation could be predicted using existing spatial datasets. The P sufficiency index (pHnNDVI) was developed in TC219 and uses a combination of soil pH maps and historical satellite NDVI data to estimate how responsive a site will be to applied phosphorus fertiliser. This project aims to further validate phosphorus responses based on spatial data, expand testing into a broader range of production environments and soil types and determine best practice long-term management of sites that are highly responsive to phosphorus.

Research Aims

The core objective of the project was to increase the profitability derived from phosphorus fertiliser application in a variable landscape by:

• increasing phosphorus fertiliser use efficiency by better understanding the spatial variability in phosphorus availability and response
• formulating longer term phosphorus management strategies on highly phosphorus responsive alkaline soils.

In The Field

Over the three years of this project, 28 phosphorus rate trials were conducted to investigate phosphorus response and evaluate the P sufficiency index across a broader range of environments in the Mid North, Upper North and Yorke Peninsula (YP). Three long-term trials were initiated in 2021.

NDVI, leaf tissue testing and grain harvest measurements were undertaken to further validate the P sufficiency index.

In combination with SAGIT project TC219, a total of 49 trials were monitored which provided a robust database for assessing the accuracy of the P sufficiency index methodology for predicting response to phosphorus.

Long-term phosphorus management strategies were also evaluated on highly phosphorus responsive soils at Hart, Spalding and Crystal Brook.

One-off high rates of phosphorus were compared to repeated high rate applications and alternative phosphorus sources such as chicken litter.

Results

Across the 49 trials, the optimal P rate to maximise partial gross margins ranged from 0 to 47 kilograms of phosphorus per hectare depending on soil type.

The area of predicted high response to phosphorus ranged from 12 to 48 per cent across all trial paddocks evaluated.

The P sufficiency index provided a robust prediction of likely response to phosphorus fertiliser in the Mid North and northern YP and has a number of advantages over alternative methods for generating phosphorus fertiliser paddock prescription maps. The P sufficiency index provided a slightly improved correlation with optimum phosphorus rate to the DGT-P soil test value at approximately half the cost (approx. $12 per hectare less than grid testing DGT-P). The P sufficiency index also uses Veris pH and Sentinel 2 satellite imagery which is higher resolution data compared to the 1-hectare grid method that can miss small scale variability and poorly define boundaries of changing soil types.

In long-term phosphorus management strategy trials on phosphorus responsive soils, applying the district practice of 15kg of phosphorus per hectare per year in three consecutive seasons never delivered the highest partial gross margin (PGM) in any scenario (MAP prices of $500-$1500 per tonne). When MAP was budgeted at $1500 per tonne, chicken litter recorded the highest PGM at the Crystal Brook and Hart sites, with freight and spreading costs accounted for. With high MAP prices, single applications of phosphorus at a high rate (50-90kg P/ha) in the first season followed by two seasons of district practice application recorded higher PGM than repeat applications of higher rates in three consecutive seasons. Both strategies performed better than repeated application of district practice rates. Repeat applications are also expected to have higher reserves and better support ongoing productivity in subsequent seasons.