Reducing nitrogen loss

Canterbury's history with nitrate

The story of increasing nitrate levels in Canterbury’s fresh water can be traced right back to the start of farming on the plains more than 150 years ago. It’s taken time for nitrate levels to rise – and it will take time for those levels to fall again.

But it’s the rapid intensification of land use over the last 40 years that has led to the worrying concentrations we see today.

Canterbury’s surface water is strongly connected to groundwater, and affects both water quality and quantity. Adding to the challenge of managing our land and water is the lag effect where nutrients and contaminants like nitrogen can take many years to move into groundwater. This means that we need to make changes today for improvements to be seen in the future.

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Working to reduce nitrogen loss

Nitrogen loss reductions have been written into the Canterbury Land and Water Regional Plan – and the associated plan changes – in a way that gives farmers time to make improvements on farm over time.

For example, under Plan Change 1, more than 200 farms in the Selwyn Waihora catchment with nitrogen losses greater than 15kg of nitrogen per hectare per year have farming land use consent conditions that require specific percentage reductions in nitrogen loss from 1 January 2022.

Good Management Practices (GMP) have also been introduced across Canterbury and are supported by the industry for efficient farming that manages environmental impacts – including nitrogen loss to water.

Many farmers have already made good progress to operate at or beyond GMP expectations. We recognise the effort that has gone in to achieve this. The ongoing monitoring under the Farm Environment Plan (FEP) audit programme aims to evaluate where farms are meeting these expectations.

In 2021, we commissioned a review of various actions farmers can take beyond GMP to further reduce nitrogen loss. While many of these options have the potential to reduce nitrogen leaching, the report identified four that stand out as effective and practical options for farmers, offering the best value for money, industry support, and availability.

Planting catch crops after winter grazing on forage crops

Catch crops can serve to mop up excess nitrogen that's been left in the soil, reducing the amount of nitrate that can be leached during one of the highest-risk times of year.

In Canterbury, oats, ryecorn, triticale, wheat and barley can all be effective catch crops.

Catch crops should be sown as soon as practicable after the grazing of winter forage crops. This will allow them to use the nitrogen in the soil and produce a cost-effective spring feed for livestock.

• Plant & Food research has more information on the benefits and best management of catch crops.
• Refer to the Catch Crops for Reduced Nitrate Leaching guidelines produced in collaboration by industry organisations.

Scheduling irrigation

Irrigation should be managed to optimise plant growth while minimising drainage and loss of nutrients to water.

During the spring and autumn shoulder seasons, evapotranspiration rates are lower, and the full point trigger for irrigation can be different than during the main dry season. Soil moisture can be managed slightly lower to allow for as much rainfall capture as possible when rainfall events occur.

This requires monitoring of soil moisture, weather forecasts and crop requirements, and adjusting irrigation targets in a soil water budget to ensure rainfall is captured and does not lead to excess nutrient loss.

• Research funded by Landcare Research and NIWA has examined irrigation management approaches for economic and environmental benefits.
• Irrigation NZ has resources on how to use soil moisture technology and soil water budgets to support crop growth whilst achieving good environmental outcomes.

Sowing plantain in pasture

Plantain has been shown to dilute nitrogen in cow urine, leading to reduced nitrate leaching in grazed soils. It can be adopted into a pastoral system with the appropriate management techniques.

DairyNZ has been leading plantain research and can help with decisions around how and when to plant, and how much plantain to sow. Research and modelling indicate that for a Canterbury farm with 30% plantain, nitrogen leaching can be reduced by 7.5-25%. This only accounts for the effect of plantain on urinary nitrogen. Current and ongoing research indicates that further reductions are possible from the effect of plantain on retaining nitrogen in the soil.

Farmers wanting to use plantain as part of their nutrient management plan will need to use an approved cultivar according to the DairyNZ Plantain Cultivar Evaluation System, and the DairyNZ Assessing Plantain On Farm tool to determine the proportion of plantain in the diet. 

For more information, contact your DairyNZ partner or visit the DairyNZ website.

Conducting soil nitrogen testing

Knowing the amount of potentially mineralizable nitrogen (PMN) in your soil is the key to understanding how much nitrogen fertiliser to apply – informing your nutrient budget and potentially benefiting both your wallet and the environment.

The recent development and adoption of PMN testing means it is available at your soil testing laboratory. Submit soil samples prior to fertiliser application to measure the soil nitrogen availability and match it to crop requirements

Plant & Food Research has been developing the tools and research on soil nitrogen testing and have produced guidelines to support the use of this mitigation strategy on farm.

While this work has focused on arable systems, further research is being undertaken on opportunities for use of nitrogen testing in the pastoral sector.

• See the Plant and Food Research page on soil nitrogen testing and predicting nitrogen supply for more information.