Effectively Reducing Nitrogen Surpluses

A comparison of different methods of winter-wheat fertilisation with nitrogen showed that nitrogen surpluses can be significantly reduced by means of site-specific variable-rate nitrogen fertilisation.

What approaches can be used to reduce nitrogen surpluses in the Swiss agricultural sector? This is a topical issue, since nutrient losses of nitrogen and phosphorus are meant to be reduced by 20% by 2030.  This is the aim of the Absenkpfads Nährstoffe (‘Nutrient Reduction Initiative’), passed by the Swiss Federal Council in April 2022.

Needs-based fertilisation using sensors

Using the example of winter-wheat cultivation, Agroscope and ETH Zurich researchers have developed a sensor-based method enabling variable-rate fertilisation of a field, based on the crop requirements. This technical solution was compared with a standard fertilisation strategy.

Nitrogen-surplus reduction and the effect on yields were evaluated, as was the economic efficiency of the two approaches. Experimental data from seven winter-wheat field trials over three years (2018−2020) at the Swiss Future Farm in the Tänikon area (Canton of Thurgau) formed the basis for the assessment.

Variable-rate fertilisation reduced nitrogen surplus by one-third

According to the evaluation, the sensor-based variable-rate fertilisation used 13% less nitrogen fertiliser on average than the standard-rate fertilisation approach and reduced the nitrogen surplus by 32% without adversely affecting yield on five of the seven fields. Nitrogen fertiliser costs were lower on average across all fields for the variable-rate fertilisation than for the standard-rate fertilisation approach.  Variable-rate fertilisation would, however, require additional investments, e.g. for the purchase of sensors and the analysis of satellite data.

Could increased fertiliser prices achieve the same effect?

A further option would be to increase the price of nitrogen fertilisers to bring the economic optimum in line with an ‘ecological optimum’. An N-surplus of 30 kg per ha was adopted as an ‘ecological optimum’. Calculation of the tax is based on a wheat price of 52 centimes per kg and a fertiliser price of 42 centimes per kg (depending on the type and nitrogen content of the fertilisers).

This solution demonstrated that a price increase of up to 5.4 times the 2019 fertiliser price would be necessary to reduce the nitrogen surplus to an environmentally sound level. Although fertiliser prices have risen by more than half in recent years, a tax as high as this hardly seems politically realistic.

Promising technical solutions

The study shows that possible solutions for reducing the nitrogen surplus can be evaluated with the help of production-related, economic and ecological data and that technical solutions such as variable-rate fertilisation have the potential to reduce nitrogen fertiliser use without adversely affecting in-kind and monetary yields.

Nevertheless, it appears that nitrogen demand varied more strongly between the years than between the individual fields in the same year. This makes it difficult to accurately estimate the ecologically optimal fertiliser use at field level. For this, progress is needed in many areas: in our understanding of site-adapted fertilisation, nitrogen mineralisation potential and seasonal fluctuations. Moreover, better sensors and models are needed, as are larger datasets from different fields and sites.

High investments required

In addition, economic potential varies according to farm size, and investing in expensive technologies for small and medium-sized farms is often not worthwhile. In these circumstances, joint use of machinery or using contractors’ services could be more advantageous than purchasing equipment.

Another option would be to financially support technologies of this sort. Such support could be funded inter alia with the proceeds from a tax on mineral fertilisers.