Research update - phosphorus run-off

Ballance has a significant research programme that aims to increase our understanding of the use and fate of nutrients in soil, so that we can help our customers farm more effectively – at least as far as fertiliser use is concerned.

For some years now we have been conducting studies into the potential of different types of phosphate fertiliser to resist loss to the environment, specifically through surface run-off to waterways.

In the first series of these studies, we applied different phosphate fertilisers to paddocks, then cut out turfs of soil and examined the effect of simulated rainfall on the phosphorus loss from these samples. As we expected, there was greater loss of phosphorus where soluble phosphate had been applied (as superten) than where a less soluble form (serpentine super) had been used, although this depended on the timing of the rainfall event following fertiliser application. By combining the data from this experiment with historical rainfall data for the region, we were able to model the potential loss under a worst-case scenario, which showed that less than 1% of the applied phosphorus would be lost for an application of superten at 30 kg P/ha, and that considerably less would be lost for an equivalent application of serpentine super.

Recently, we have taken this work a step further, carrying out field trials in both the North and South Islands, examining both pasture production and phosphorus loss in actual field conditions, using different phosphate fertilisers.

Although it’s early days yet (the trial still has another two years to run), the results to date are encouraging.

The trials showed that after one year, there was no significant difference in yield between pasture treated with superten or serpentine super. Both fertilisers performed better than the control, though differences were not statistically significant. This was not unexpected, as the Olsen P of the soil was around 30 at both sites, so we were unlikely to see a response to phosphate fertiliser, certainly in the first year. As you might expect, the slower rate of release of phosphate from serpentine super meant that the Olsen P of plots treated with this product did not increase as much as those treated with superten (see Figure 1). In control plots, with no fertiliser applied, Olsen P declined over the 12-month period.

Figure 1: Change in Olsen P in a 12-month period at test sites treated with no fertiliser (control), with superten or with Serpentine Super. Fertilisers were applied at a rate of 50 kg P/ha.

The relative loss of phosphorus from the plots mirrored the pattern what we had seen in the earlier trial, with more phosphorus being lost in surface run-off when superten had been applied, compared to serpentine super (see Figure 2). What is interesting to note is that at the North Island site, there was no significant difference between any of the treatments, even though the land had a 10% slope, which would have promoted run-off far more than the 2% slope at the South Island site. This highlights several factors that play an important role in phosphorus loss.

Figure 2: Levels of filterable reactive phosphorus (FRP) at test sites treated with no fertiliser (control), with superten or with Serpentine Super. Results are flow-weighted means, to account for the variation in run-off between sites and plots.

First, the amount and timing of rainfall after application is a significant factor. At the North Island site, annual surface run-off was significantly lower than at the South Island site and this lower volume of run-off would have affected phosphorus loss. Second, the phosphate retention of the soil at the North Island site was nearly twice that of the South Island site. This greater capacity to bind phosphorus would have reduced the potential for loss through run-off.

Research like this serves a number of purposes. It helps us to determine how much phosphorus might be lost through run-off in real-life situations; it shows how this is affected by the type of fertiliser used; it gives us targets to aim for when we are developing new fertiliser products; and it contributes to the overall understanding of the effect of soil type on phosphorus run-off. Ultimately, this type of research will enable us to better determine exactly what farming conditions would benefit most from the use of low-solubility phosphorus fertilisers, and to advise customers about appropriate products and practices for their specific farming situation.

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