Agronomy Advice
February 22, 2023

Maximise Nitrogen Availability with a Robust Sulphur Strategy

Farmers have two reasons to minimise nitrogen losses – maximise the amount of nitrogen absorbed by growing crops (including grassland) and protect the environment from nitrogen leaching, which can often contaminate water courses.

fields from a recent Nitrogen trial
fields from a recent Nitrogen trial

The link between sulphur and nitrogen is well known, but very few grassland studies have been done. However, a 2021 Teagasc study on a sandy loam soil concluded that optimising sulphur nutrition is important in delivering agronomic benefits as well as acting as a nitrate leaching mitigation strategy.

Both nitrogen and sulphur are required in the formation of amino acids cysteine and methionine which are important building blocks of protein. A lack of sulphur will therefore have a detrimental effect on plant growth and stress tolerance. It also has a negative impact on nitrogen use efficiency. In the past, sulphate would be deposited in our soils through rain, but over time, as we’ve cleaned up our environment, the levels of sulphate have significantly reduced leaving most soils deficient.

Teagasc undertook an experiment to test the hypotheses that adopting the right sulphur application strategy will increase grass yield, optimise sulphur and nitrogen uptake and decrease nitrogen leaching.

Teagasc sulphur application experiment 

The experiment consisted of six treatments with five applications of each treatment: 

  1. Zero nitrogen, zero sulphur control treatment
  2. Nitrogen only (CAN, 27%N) 
  3. Nitrogen plus sulphur (CAN+S, 26.6%N, 12.5%S03) – distributed N and S application across the season with seven splits 
  4. Nitrogen plus sulphur (CAN + Ammonium Sulphate AS, 21%N, 60%S03) CAN was delivered throughout the season, but AS delivered all the sulphur in one early spring application. 
  5. Slurry plus CAN (slurry replaced the inorganic fertiliser for one spring application but was supplemented with CAN for all other split applications) 
  6. Slurry plus CAN+S (slurry replaced the inorganic fertiliser for one spring application but was supplemented with CAN+S for all other split applications)

Sulphur experiment results 

Unsurprisingly, nitrogen fertilisation increased yields when compared to the zero-N control but adding sulphur as well increased yield further and improved nitrogen uptake in the crop whilst also reducing nitrogen leaching. On average, the application of 115 kg SO3/ha increased yield by 2609 kg DM ha over the course of the study.

Applying nitrogen and sulphur together also increased apparent fertiliser nitrogen recovery (AFNR) from 39% to 49%. AFNR is a metric used to describe how much of the applied N was taken up by the crop. 

All three treatments containing sulphur resulted in increased plant nitrogen which can also be linked with lower levels of nitrogen leaching. Adding mineral sulphur to the slurry application again significantly improved yields suggesting that the 22kg/ha of available sulphur present in a single application of slurry alone is insufficient. 

Nitrate leaching losses per ha for the N only plots was 48.2kg NO3-N, whilst the N + S treatments was 26kg NO3-N which was only 2kg greater than the Zero N control treatment. The nitrate leaching losses for the slurry treatments were very surprising. The N + slurry treatment had losses of 82.8kg NO3-N, in contrast to the N + S + slurry treatment which had losses of 33kg NO3-N.

Sulphur Fertiliser Application Recommendations

Ultimately, applying sulphur along with your mineral nitrogen fertiliser is a win-win for both improved grassland productivity, but also in protecting the environment. 

At a time when nitrogen fertiliser is so expensive, it’s important to make it work harder to see the best return possible on your investment. By investing in a nitrogen fertiliser that also contains sulphur you’ll improve nitrogen use efficiency and lose less nitrogen to the surrounding environment. 

But beware, most fertilisers contain sulphate which is prone to leaching, which is why when applying all your sulphur in one spring dressing is not sufficient. By applying sulphur little and often throughout the growing season, you’ll ensure that your growing grass has a plentiful supply.