How to Minimise Phosphorus Loss from Fertilised Pasture

In New Zealand’s soils, phosphorus does a great job at growing plants but unfortunately it does the same thing if it makes it into our water. Research from the Our Land and Water National Science Challenge aims to help farmers choose the right fertiliser for their type of land and apply it strategically, so it stays on the farm and out of waterways.

New Zealand farmers can keep up to 70% more phosphorus in their soil by understanding how nutrients move from land to water on their type of land, and where their Critical Source Areas (CSAs) are located. This knowledge can give farmers the confidence to choose the right fertiliser for their land type and apply it at the right rate, at the right time, and in the right place, so it stays on the farm and out of waterways.

That’s a lot of ‘rights’ for one sentence – four in fact, known as the 4Rs of nutrient management.

In 2014, scientists from the International Plant Nutrition Institute – whose membership is made up of fertiliser companies – published the 4R Nutrient Stewardship guidelines for best management practices for fertiliser use worldwide.

The 4Rs: Apply the Right source of nutrients, at the Right rate, at the Right time, and in the Right place

“Managing fertiliser well isn’t expensive,” says Professor Richard McDowell, Chief Scientist for Our Land and Water, one of 11 National Science Challenges that fund research into the biggest issues facing New Zealand.

“Farmers can halve the amount of phosphorus leaving their farm with minimal impact on farm profitability.”

In New Zealand, the adverse consequences of inappropriate fertiliser use are increasing, making it even more important to follow the 4R guidelines.

“Here, intensive pastoral agriculture has expanded into areas at high risk of losing phosphorus,” says McDowell. “These high-risk areas include land with a moderate to high slope, regular and frequent rainfall, or with soil that has a low capacity to absorb phosphorus.”

Research Backs Up the 4Rs

McDowell is a phosphorus expert and prolific author of phosphorus research. He is co-author of a paper that provides a scientific basis for New Zealand and Australian farmers to choose the right fertiliser for their type of land and apply it strategically, so it stays on the farm rather than draining away as pollution.

The research found that when fertiliser is managed badly, it’s responsible for 30% to 80% of the phosphorus that drains away from a farm. When it’s managed well, that can decrease to less than 10%.

The paper backs up and adds scientific credibility to the 4R concept for Australasian farmers.

“When farmers understand how phosphorus moves from land to water on their type of land, they can be more confident in choosing the right type of fertiliser and optimising the application rate, timing and location,” says Professor McDowell.

In 2014, Professor McDowell investigated the use of phosphorus mitigation strategies in 14 New Zealand catchments, and estimated that phosphorus loss could be halved for less than 2% of farm earnings before interest and tax.

For the most efficient and cost-effective reductions in nutrient loss, he recommends farmers understand where the CSAs are on their farm and target mitigation strategies to those areas.

Research from Our Land and Water has shown that targeting mitigation actions to CSAs is six to seven times more cost-effective than an untargeted approach, and that on-farm mitigations are working to decrease concentrations of phosphorus in streams and rivers.

“Focusing on critical-source-area management within farm environment plans is helping to reduce phosphorus losses from land to water nationally,” says Professor McDowell.

Having a farm environment plan that is measurable and linked to catchment objectives is crucial, he says, so that landowners have the information they need, to know when and where to apply mitigations and make informed decisions about suitable land use.

Science-Backed Advice for Application

When and Where to Apply?

• Apply annual fertilisers when the probability of surface run-off or leaching is lowest.
• Always avoid applying phosphorus- based fertiliser when rain is forecast within the next 14 days.
• Run-off can also occur in dry times of year when soils are hydrophobic. Soil water repellence is common during mid-summer to autumn on north slopes in the eastern regions of both islands, and in the central plateau.

What to Apply?

• When run-off or leaching is unavoidable (for example, where rainfall is frequent or heavy enough to produce run off or leaching events year-round) consider applying a partially water-soluble phosphorus fertiliser, such as reactive phosphate rock (RPR), Serpentine Super or products coated with a polymer to control the rate of P release.
• Data shows that at a catchment scale, RPR can potentially decrease phosphorus in streams by up to 58% compared with superphosphate. Other products, such as Serpentine Super, can also decrease losses by about 20–30% compared to superphosphate.
• A cautionary note: data shows that products such as RPR only produce the same amount of pasture as superphosphate when soil pH is <6 and rainfall is >800mm.
• In ideal conditions transitioning to RPR from superphosphate, replace a third of the superphosphate with RPR each year: 33% in year one, 66% in year two, and 100% in year three.

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For more information, see:

• Phosphorus Best Practice
• This article originally published in Ground Effect, Autumn 2020
• Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures, Journal of Environmental Quality, August 2019
• Estimating the mitigation of anthropogenic loss of phosphorus in New Zealand grassland catchments, Science of The Total Environment, January 2014
• Why are median phosphorus concentrations improving in New Zealand streams and rivers? Journal of the Royal Society of New Zealand, February 2019