Sulphur and nitrogen use efficiency

June 2022

It is well understood that sulphur is an important nutrient for plant growth and development. Due to the central role of both sulphur and nitrogen in the synthesis of proteins, the supplies of these nutrients in plants are highly inter-related. Studies have shown that one nutrient will accumulate in plants when the other is deficient, and when this deficiency is corrected the accumulated nutrient is then used in protein synthesis. Therefore, a shortage in the availability of sulphur will reduce the efficiency of nitrogen use.

With the availability of nitrogen sulphur grades likely to be limited for next season, this brings into focus the strategy for ensuring arable and grass crops receive enough sulphur. Where ammonium sulphate-based products have been used previously, but are not available or limited, alternative sulphur options will need to be considered. The options for this include:

• Solid organic materials, such as farmyard manure and biosolids. These contain significant amounts of sulphur, but are slow release, because the organic sulphur must be mineralised to sulphate so the plant can take it up. This takes several months and warmth, so they need to be applied in the autumn, despite a small amount potentially being lost through leaching of the sulphate over the winter.
• Liquid organic materials, such as slurries, and poultry manure. These supply available sulphur much more quickly and should be applied early in the year because autumn applications are very susceptible to leaching of the sulphate over the winter. The PDA leaflet on Organic Materials has a table showing the crop-available sulphur of various materials, according to the season of application. This is the sulphur you can expect to be supplied to the crop in the growing season and can therefore be included in the nutrient plan for the crop.
• Elemental sulphur fertilisers. These need to convert to the sulphate form to become crop available and therefore need more time to be effective and must be applied well in advance, such as the previous autumn. There is an inevitable risk that a significant amount of the sulphur is mineralised to sulphate in the autumn, and this may be lost by leaching, over the winter.
• These are potash/magnesium/sulphur fertilisers such as Polysulphate and PotashpluS. Polyhalite is a complex sulphate molecule, with progressive release characteristics that can be applied straight or incorporated in compound fertilisers.