Minimising phosphorus losses

When phosphate fertiliser is managed well, recently applied fertiliser generally comprises less than 10% of total phosphorus losses to the environment, a recent review has revealed.

The Fertiliser Association of New Zealand commissioned a review of the factors affecting phosphorus (P) exports soon after mineral fertilisers are applied to pasture-based grazing systems in Australia and New Zealand. The findings were encouraging, with evidence showing that while it's possible for fertiliser to comprise 80% of total losses, well-informed nutrient management reduces losses dramatically.

"The important thing isn't what's possible but what's probable," says the lead researcher, David Nash.

Intensive land use has expanded in New Zealand. Despite this, semi-quantitative evidence suggests that good management practice, combined with increasing awareness of P and the critical source areas, has seen stream water P concentrations decrease in the last ten years.

"New Zealand's National Policy Statement for Freshwater Management has likely made a difference. Having the policy reinforced with voluntary processes and regulation to help farmers optimise what they do is essential. Ultimately, I believe farmers care about the environment and want to do what's best. Financially it's in their interests too as wasting phosphorus would be like pouring money down the drain."

David says that widespread adoption in New Zealand of the 4R Nutrient Stewardship principles that are part of good nutrient management practice have been effective - that is, applying the right source of nutrients at the right rate, at the right time and in the right place. For instance, the review showed that when appropriate P mitigation strategies were applied to 14 catchments in New Zealand, P exports were estimated to be halved with minimal impact on farm profitability.

On well-managed farms, practices are adopted that address a range of soil, fertiliser, effluent management and irrigation issues.

"The answer isn't to limit fertiliser but to make sure that applications aren't any higher than you need - and to choose timing, rates and formulations that are going to minimise their impact."

The paper also looked at selecting fertiliser compounds for specific types of farming systems and effective ways of adapting farming systems.

"Each farming system needs to be looked at in terms of how the water is moving on each individual farm, what the soil properties are, and so on. It's not just a matter of looking at solubility, which is the issue everybody seems to focus on."

Most of the phosphorus is gone from the fertiliser granule within 24 hours when fertiliser is applied, David explains. "If you get it away from the surface and into the soil where the plants can use it, it doesn't get picked up by the water that tends to run across the surface. You want it to go past the 'squelchy layer' because if it's not there it doesn't get picked up and transported offsite unless you've got sub-surface drainage of some kind.

"When you actually look at what farms do and mathematically investigate it, the effects of recently applied fertiliser were really very small. The primary effect was from what had been put on over the years (regardless of solubility), if it has raised the fertility of the soil."

Read the review