Sulphur (S) is an essential plant nutrient, with important roles in crop productivity, quality, and nitrogen (N) use efficiency.
In plants, S and N play a synergistically central role in the synthesis of proteins by being a constituent of the amino acids cysteine (Cys) and methionine (Met) and S limitation affects N use efficiency.
Sulphur is involved in enzymes that allow the conversion of nitrate to amino acids and then to protein. If S is lacking, nitrates may accumulate in plant tissue, possibly to levels toxic to grazing animals.
Increased demand for sulphur
Recent research which focuses on S is relatively scarce compared to N and phosphorus (P), particularly for grasslands. Across Europe, industrial emissions of sulphur dioxide and other S sources in the atmosphere have declined steeply, leading to lower concentrations of plant available S forms in rainfall over the past decades.
The levels of sulphur deposition on agricultural land are predicted to decrease by 70-90% by the end of the century and crop deficiencies in S are likely to become more prevalent.
We are in a new era of relatively low S deposition compared to the past. As a consequence, balancing S-N fertilisation can improve N use efficiency, minimising nutrient loss to the environment and increasing plant productivity.
New research on sulphur
New research examining the soil, plant and environmental loss response to S fertilisation on a range of temperate grassland soils was recently completed at the Teagasc Johnstown Castle research centre.
The soil lysimeter facilities were used to capture data across multiple soil types and years.
Yield response to S fertilisation was found to be related to soil texture, with yield responses of 31-51% (up to 4t/ha extra DM) in the sandy loam soils compared to as little as 4% (0.5t/ha extra DM) in the finest textured soil.
Large reductions in nitrate-N (NO3--N) leaching of 45-51% were observed in the sandy loams in response to S fertilisation with N fertilisation.
The combination of S and N application on S-deficient soil significantly decreased NO3-N leaching (a major threat to water quality) while improving NUE (up to 61kg/ha N extra plant uptake) and grass productivity.
Sulphur fertilisation strategies
Different strategies for S application were examined in the study. Results showed that S availability from slurry applied at a rate of 22m3/ha (2,000gal/ac) was insufficient to meet the S requirement on a responsive soil.
However, applying a mineral fertiliser S top-up with the cattle slurry significantly reduced NO3-N leaching.
Sulphur application to these grassland soils brought NO3-N concentrations in leachate below the EU maximum allowable nitrate concentration for drinking water.
Finally, the effects of different N and S fertilisation rates on NUE, leaching, and yield were examined using a sandy loam soil. Higher N application rates required higher S application rates to optimise NUE and plant productivity while also reducing NO3-N leaching.
Overall, results suggest that, in temperate grassland soils where S and N are co-limiting, S and N behave synergistically in terms of yield, N and S uptake and N leaching responses.
Including sulphur in fertilizer plans
A higher response to S is most likely on coarser textured soils. From a management perspective, assessing and correcting S deficiency should be considered as part of the fertiliser plan before increasing N rate to boost plant performance; a strategy which can result in higher NUE and lower N losses to the environment. Overall, S is an important nutrient for the efficient production of grass in temperate grassland soils and has significant potential to help environmental sustainability on farms, particularly through reducing the potential NO3-N leaching from well-drained soils.