Nitrogen management in vegetable production

New Zealand was late to the action of management of nitrogen in water, writes Dan Bloomer of LandWISE.

Awareness that nitrogen from agriculture can have unwanted impacts on ecosystems is not new.

While nitrogen management regulation is headline New Zealand news, we are perhaps somewhat behind our trading partners. The United States Clean Water Act of the 1970s addressed high nitrate levels in Chesapeake Bay, the result of increasing population combined with agricultural expansion1.

Denmark introduced action plans to reduce nitrate leaching in 19852, nitrogen caps in 1994, and by 2003 had reduced discharges by about 48%. The Dutch government has worked to reduce nitrogen emissions since the 1990s. Animal agriculture impact drove these regulations.

The European Union has strict nitrate directives and countries have been referred to the European Court of Justice for poor implementation3. Artificial nitrogen use and excessive leaching are increasingly viewed as critical risks. Appearing in releases from trade ministries and the financial sector, it is extending beyond improving water quality to reducing greenhouse gas emissions4.

In NZ, the overarching legislation is Te Mana o te Wai (the power of water) which recognises the vital importance of water. Policies for managing nitrogen are set in the National Policy Statement for Freshwater Management 20205. Implementation is through regional councils, which are required to monitor every freshwater management unit, maintain or improve nitrogen levels, set desired outcomes and respond to any deterioration. Plans are to be in place before 2027. A nitrogen cap for pastures was introduced in NZ in 20216, 27 years after Denmark.

What has all this got to do with vegetable production?

Vegetable growing can be a high nitrogen input system. While nitrogen output in crop sold can also be high, there is a significant risk to freshwater quality if nitrogen is not well managed. We need to get our rates and timing right, and make sure equipment is applying our target quantities. The trouble is we don’t necessarily know the appropriate rates for all crops, and we may not be budgeting correctly.

We need sound science to understand and justify our applications, and if necessary, manage any losses.

What is being done?

Many activities aim to address the knowledge gaps and provide growers with best management practice BMP advice based on sound research.

The most up-to-date industry guidance is the 2019 Horticulture New Zealand publication, “Nutrient Management for Vegetable Crops in New Zealand” by Reid and Morton7.

Based on reviews of earlier research, it brought together information for many crops in a standard format, helping growers select rates based on standard soil tests and predicted yields. However, it couldn’t include all the crops grown and doesn’t necessarily include tests that measure all the nitrogen available in the soil. Up-to-date data based on present growing practices and yields is needed.

The Californian Nitrate Quick Test8 calibrated for NZ by Plant & Food Research is a useful tool, especially when used with the Foundation for Arable Research (FAR) online tool to convert solution concentration to kg nitrate per hectare9. It helps determine the amount of nitrate present before application and when checking levels post-harvest.

From 2018 to 2021, LandWISE worked with vegetable growers in Gisborne and Levin through the Ministry for Primary Industries (MPI) Sustainable Food and Fibre Future Proofing Vegetable Production project. This brought existing tools together to address nitrogen rate prescription and application, retention in the root zone, and mitigation of any losses.

We discovered growers lacked a suitable nutrient budgeting template, so created simple tools for nitrogen and phosphorous, available as paper or PDF downloads or as an online tool for nitrogen10. The budget is based on Reid’s and Morton’s recommended guidelines. We used the FertSpread11 tool to calibrate fertiliser spreaders and developed a new module for placement machines. And we used the Irrig8 bucket test method12 to check the right depths of irrigation were being evenly applied.

Track all the N

Research projects track all the nitrogen in the system, not just nitrate in the standard top 15cm. The whole root depth (and more), mineral nitrogen and potentially available nitrogen. Nitrogen in roots, shoots, export crops and residues should all be considered.

Sustainable Vegetable Systems (SVS) supported by MPI, Potatoes NZ, Horticulture NZ and Vegetable Research & Innovation aims to secure NZ growers’ social and regulatory license to grow vegetables for domestic and export markets. Aiming for a sustainable horticulture industry and a healthy environment, SVS is conducting numerous controlled field trials and intensively monitoring soil and plant nitrogen so modelling tools can be developed. Additionally, SVS promotes collaboration and knowledge sharing across the horticulture industry, empowering growers to better manage nitrogen13.

SVS surveys found growers and agronomists use a variety of tools and practices to gain a better understanding of what is happening in the soil, and the amount of nutrients required for their crops. They use several methods to assess nitrogen and other nutrients in their growing system.

Noting tools requiring a lot of effort can be difficult to work with and off-putting, they want quick results to fit the pace of their decision-making needs: “No grower wants to wait two weeks to get a soil test result back for nitrogen”. SVS growers express a range of views about the NQT.

Nitrate quick test

Some strongly recommend it, and like the speed and ability to choose relevant root depths for sampling. Others note the effort required to get representative soil samples, the time for clay soil solutions to settle for reading, and difficulty in wet periods.

Nitrate sampling in wet soil is difficult regardless of the test used. Matthew Norris, the Plant & Food scientist who did much of the work calibrating the NQT for NZ soils, says getting a relevant result is tricky when soil is wet because the sample is very diluted, and it is almost impossible if the soil is beyond field capacity.

LandWISE has two projects investigating nitrogen management, one for asparagus crops14 and one for process vegetable crops15. With only about 25kg of nitrogen exported in five tonnes of spears, asparagus is a low N crop, and some growers apply no fertiliser at all to mature crops. In line with other research, we found about 80% of nitrogen in the system was in the massive storage roots, and they are what feeds the spear production. Any additional fertiliser should be added at the start of fern growth in summer, reloading the roots for the next spring.

Process crops have a range of nitrogen needs, depending on crop type and highly driven by yield which can be difficult to predict. After our first season collecting data from tomato, green bean, beetroot and sweetcorn crops, the biggest question we have is, “What should we be measuring?” Generally, when we look pre-planting at the amount of nitrate in the top 15cm of soil, we find there is a deficit and fertiliser is indicated. But if we sample the whole root depth, often at least 50 cm or more depending on crop and soil, and include the potentially available nitrogen present, we find no extra fertiliser would be recommended. Our rate trials so far are showing no statistically different yields with different nitrogen rates. In several paddocks the average yields were slightly higher in plots getting more nitrogen, but in one crop the plots receiving no extra N yielded more.

In the next year or so, we anticipate guidelines for vegetable growers will be updated. Expect recommendations based on improved modelling based on up-to-date data relevant to varieties, yields and growing practices. And expect a greater market demand and regulatory requirements to justify every kilogram of synthetic fertiliser applied! Hopefully the demands are informed by the data.

References

1. nitrogen-in-the-chesapeake-bay-a-retrospective.pdf (umces.edu)
2. mst.dk/trade/agriculture/nitrates-directive/implementation-in-denmark
3. The Dutch Nitrogen Crisis – FAIRR
4. Denmark presents plan to reduce nitrogen emissions in farming (thelocal.dk)
5. FS30A-Managing-nitrogen-factsheet-final.pdf (environment.govt.nz)
6. Nitrogen Cap (N Cap) | Hawke’s Bay Regional Council (hbrc.govt.nz)
7. co.nz/mdocs-posts/nutrient-management-for-vegetable-crops-in-nz/
8. ucdavis.edu/pdf/fertilization/fertilization_UsingthePre-SidedressingSoilNitrateQuickTesttoGuideNFertilizerManagement.pdf
9. far-qttool.shinyapps.io/shinyapp/
10. landwise.org.nz/
11. fertspread.nz/
12. pagebloomer.co.nz/resources/irrigation-calibration/irrig8lite/
13. co.nz/casestudy/sustainable-vegetable-systems-svs/
14. org.nz/projects/nitrogen-for-asparagus-crops/
15. org.nz/projects/nitrogen-best-practice-for-process-crops/