Testing the reliability of phosphorus recommendations: Preliminary results from the uPtake project

One of 32 trials in South West WA conducted to assess phosphorus recommendations

Preliminary results from two years of pasture trials are supporting the use of an existing national model as the basis for making phosphorus fertiliser recommendations in South West WA.

The national model was developed by the “Better fertiliser decisions (BFD) for grazed pastures” project, a four-year project that collated results of more than 250 pasture-fertiliser experiments from around Australia. Advisors use the model to interpret soil test results and predict the yield response for different nutrients.

However, with many of these experiments conducted decades ago in other states using older pasture varieties, there is some concern that the model is considered less trustworthy for contemporary pastures or WA soils, so is not always being used. This may explain why phosphorus levels are typically higher than required for pasture growth in the South-West of Western Australia, increasing the risk of phosphorus loss from farms and reducing both profitability and water quality in estuaries.

To address these concerns, a collaboration of stakeholders led by the Department of Water and Environmental Regulation came together in 2019 to launch the “uPtake” project to test the suitability of the national model in South-West WA.

The project’s research leader is David Weaver, Principal Research Scientist at the Department of Primary Industries and Regional Development (DPIRD). With 37 years’ of experience in nutrient management and research, Mr Weaver developed a rigorous process to test the national BFD model on varying soil types throughout the South-West. That process was recently explained at a fertiliser industry meeting in Busselton.

“The first thing we need to do is compare apples to apples. Given that all of the BFD trials are framed in terms of “relative yield”, we need to do the same,” Mr Weaver said.

“Relative yield is determined by running a phosphorus-rate trial on a site with a known level of soil phosphorus (measured with the Colwell extraction method) and the soil’s Phosphorus Buffering Index (PBI), which affects how much of the phosphorus will be available to plants.

“In these trials, different rates of phosphorus are applied in replicated plots, with pasture growth cut, dried and weighed. The pasture biomass in plots with nil phosphorus is compared to the highest yielding phosphorus rate to determine relative yield, which is the percentage of the maximum yield achieved with the pre-existing level of soil phosphorus for the measured PBI.”

For example, in Figure 1, the conceptual trial had a relative yield of 60 percent because the nil-phosphorus rate yielded 60 percent relative to the yield of the maximum rate. So for this conceptual trial with a specific soil phosphorus and PBI, a significant increase in yield could be achieved by applying phosphorus.

Figure 1

Figure 1. Determining the relative yield for a trial site – the percentage yield achieved by the plots where no P (phosphorus) was applied compared to the maximum yield.

“When we run the same trial on soil within the same PBI group (a pre-determined PBI range) but with a different level of pre-existing phosphorus, we achieve a different relative yield. We then plot all the relative yield and pre-exising phosphorus levels for each PBI group to develop a response calibration curve (curve of best fit, Figure 2).”

Figure 2

Figure 2. Response calibration to phosphorus (Colwell P).

More points (or trials) increases our confidence that the calibration curve can be used to estimate the relative yield from a soil test result and predict whether an application of phosphorus will significantly increase yield.

“From this calibration we can determine a critical Colwell phosphorus value (indicated by the question mark in Figure 2) to achieve 95% of relative yield.”

A relative yield of 95% is often considered the maximum yield to target because, although higher yields may be achieved, the cost of the phosphorus can outweigh the gain in yield. However, for less intensive farming systems (e.g. set stocked or under-stocked farms), or when fertiliser price is high and commodity prices are low, a relative yield of 80 or 90 percent may be more economical.

Critical soil test values for a range of relative yields and PBIs have been published by DPIRD. More recently a paper published on findings from the BFD project provides equations that can be used to determine critical soil test values for phosphorus, potassium and sulphur. The equations from the recently published paper have been incorporated into an online nutrient calculator to simplify the estimation of soil nutrient status and capital nutrient requirements to achieve a target relative yield.

In higher PBI soils, more soil phosphorus is required to achieve the same relative yield because more is bound to the soil, making it less available to plants. This means that a calibration curve must be developed for each soil PBI group.

“So, to compare relative yields to the BFD project, we need to run trials on soils with different levels of Colwell phosphorus within each of the eight PBI groups.”

Table 1. Eight PBI categories targeted by the trials with a rating of the category’s phosphorus holding capacity. Note that there was insufficient data to derive a response relationship for PBI above 840 in the BFD project.

The spread of sites captured so far can be seen in Figure 3, which demonstrates how it has been difficult to achieve a good coverage of sites within each PBI range. This figure will inform site selection for 2021.

Figure 3

Figure 3: The coverage of soil PBI and phosphorus (P) fertility trialled in the uPtake project in 2019 and 2020. Continuing sites held trials over both years.

Figure 3 converts the level of soil phosphorus into a fertility index (Y-axis), which is determined by dividing the trial site’s measured soil phosphorus by the amount required to achieve a relative yield of 95%. This representation of soil phosphorus helps to visualise sites that are expected to respond to phosphorus application (with an index less than 1), and sites that are unlikely to respond (where the index is greater than 1).

Figure 4. How to calculate P (phosphorus) fertility index using the site’s measured Colwell P, and the target Colwell P required to achieve 95 percent relative yield, found in Table 1 of this link.

Mr Weaver demonstrated how preliminary results from 32 trials over two years align with BFD data by seeing how well the fertility index, which is based on BFD model, predicted trial results (Figure 5).

“Based on the BFD model, we would expect sites that responded to phosphorus in our trials to have a fertility index below 1, and this was the case except for one outlier. We would also expect non-responsive sites to have a fertility index above 1. However, some of these were below 1.”

Figure 5

Figure 5. Responsiveness of different fertility indexes to P application recorded in the trial

Mr Weaver concluded that results to date are consistent with, and within margins of error compared to, the BFD critical values for phosphorus.

“Aiming for a phosphorus fertility index of 1 is not only sufficient for productivity and profitability, but it will also be good for water quality.”

However, the trials have also demonstrated that in terms of dry matter yield, nitrogen, potassium and sulfur are often much more important than phosphorus.

“Some treatments in the trials were combined with what we called basal nutrients (nitrogen, potassium, sulfur and trace elements) to ensure they were not deficient, while other treatments only received phosphorus. We often saw a significant response to these basal treatments, including when there was no response to phosphorus.

“What this shows is that the trials are also consistent with Liebig’s law of the minimum, which says that if some nutrients such as phosphorus are adequate, but another nutrient is low, plant growth will be limited by the more deficient nutrient.

“So, soil testing, and in some cases tissue testing, is critical to identify your most limiting nutrient.”

The uPtake project is jointly funded through Royalties for Region’s Regional Estuaries Initiative and the Australian Government’s National Landcare Program. For more information, visit https://estuaries.dwer.wa.gov.au/uptake/

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