Nitrogen Systems 2025 using NS, NPKS grades vs Polysulphate & Straight N
With the decline in atmospheric deposition the requirement for applications of sulphur in cereals and oil seed rape is well understood. What is less clear is the timing of applications and the total amount of sulphur that is required to optimise nitrogen use efficiency and therefore yield.
Why is sulphur such an important nutrient?
Sulphur is present in all crops and plays an important role in plant metabolism. Sulphur is essential for the formation of plant proteins, amino acids, some vitamins and enzymes. Most compound fertilisers containing sulphur also contain nitrogen, highlighting the close link between these two elements. Sulphur is part of an enzyme required for nitrogen uptake and lack of it can severely hamper nitrogen metabolism. Together with nitrogen, sulphur enables the formations of amino acids needed for protein synthesis. It is found in fatty acids and vitamins and has an important impact on quality of crops. Sulphur is also essentially involved in photosynthesis, overall energy metabolism and carbohydrate production.
Sulphur deficiency in wheat with typical pale chlorosis on newer leaves and stunted growth.
Crops prefer sulphate
Plant roots can take up sulphur only as sulphate ions (SO42-). All elemental soil sulphur must first be mineralised before it becomes available to plants.
Sulphur from mineral fertilisers
Mineral fertiliser contains sulphur as sulphate. Sulphate from fertiliser is immediately available as a nutrient and easily absorbed by plants. Sulphate is highly mobile in the soil and reaches the plant roots quickly. The application of sulphur during an early stage and during intensive plant growth makes it suitable for combination with other fertilisers, especially nitrogen. Applied as elementary sulphur, it needs to be oxidized to sulfate by soil microbes, which takes time. Elementary sulphur also has a strong acidifying effect. Polyhalite as a source of sulphur whilst having around 50% availability in the first couple of weeks after application can take a further 6 to 8 weeks to become fully available to the crop. This compares with traditional Nitrogen Sulphur grades of fertiliser where 100% of the sulphate is released in the first week after application.
Sulphur from organic manures
Manure contains sulphur mainly as organic matter and therefore needs to be mineralised before it can be taken up. A recent study from ADAS found that only 5-10% of the sulphur in cattle manure was available to crops in the spring following an autumn application.
Sulphur leaching
Sulphur behaves similarly to nitrogen in the soil. Sulphate ions, as nitrate ions, are dissolved and very prone to leaching. fertiliser application should therefore be matched to plant growth in order to ensure rapid uptake. Application in the main phase of plant growth is the most efficient strategy. Autumn applications of sulphur are not recommended.
How much sulphur do I need and what difference will it make?
In terms of rates of sulphur then cereals require about 50-60kg SO3 and oilseed rape requires about 80-100kg SO3. However, tissue analysis is a reliable indicator of deficiency in the growing crop and, of course, you will have all the nutrient, as well as sulphur, results back. Guidelines levels for sulphur content in wheat tissue is 0.40% and 0.25% in oilseed rape tissue.
Another analysis that can be carried out is grain analysis; this is a good way to look back on your nutrient strategy and see whether it has worked as intended. In the grain, sulphur results are given as a percentage within the grain but also as the N:S ratio. A ratio lower than 17:1 indicates an adequate sulphur status, with a ratio of 14:1 indicating the optimum. Grain sulphur content should be maintained at 0.12%. Because of the close relationship between nitrogen and sulphur the calculation of the N:S ratio is often used and generally found to be a more meaningful indicator of sulphur deficiency for most crops.
Yield responses to sulphur can vary season to season with 0.2-1.9t/ha seen in recent years across cereals and oilseed rape, but the type of sulphur applied is also important. If applying elemental sulphur it can take a long time (up to 8 weeks) to undergo conversions in the soil before becoming sulphate, which is the form that the plant can take up. Whilst plant leaves can absorb sulphur from the air as sulphur dioxide (now only very low levels) plant roots can only take up sulphates.
So, of all the possible sources – soil reserves, organic manures and mineral fertilisers - only sulphate mineral fertilisers offer a reliable method for preventing deficiency. All others require mineralisation before becoming available to plants, therefore applying a product where the sulphur is already in the sulphate form will give you immediately available sulphur and will also contain nitrate (as ammonium nitrate) because we need both nutrients together.
Sulphur Rates
Some crops need more sulphur than others. Generally the higher the sulphur demand, the higher the sensitivity to deficiency. Oilseed rape has a very high uptake, but most of the sulphur remains in plant residues. In the late 1990s Yara carried out a number of trials investigating the optimum rates of sulphur on different soil types. The results of these trials showed that similar optimum rates were required for crops growing on light soils to those growing on medium and heavy soils, however the response to sulphur was greater on the lighter soils. Yield responses varied from around 0.1 t/ha on the medium to heavy soils up to 0.3 t/ha on the lighter soils. The optimum rate of sulphur for cereals on all soil types is around 50-60 kg/ha SO3 (Figure 14).
Sulphur recommendations
| Crop | Sulphur Recommendation |
| Oilseed Rape | 80-100 kg SO3/Ha |
| Winter Cereals | 50-60 kg SO3/ha |
| Spring Cereals | 30-40 kg SO3/ha |
| Grassland (Silage) | 50 kg SO3/ha per cut |
| Grassland (Grazing) | 15 kg SO3/ha per month |
The actual recommendations for sulphur depend on several factors and should always be checked with a FACTS Qualified Agronomist (FQA) and if manures are applied a full nutrient management plan should be conducted.
Independent work carried out in 2000 looked at the benefits from applying small amounts of sulphur at the nitrogen timings compared with not applying any sulphur or applying it all in the first dose. The results showed a 4% yield increase from the ‘little and often’ approach compared to all the sulphur applied early (Figure 16).
Sulphur does differ from nitrogen when in the plant. Unlike nitrogen the sulphur is not very mobile, hence symptoms show in young leaves first. A strategy that applies sulphur throughout the growing season ensures that a constant supply is readily available as new growth occurs.
Sulphur Timing and Source
Accumulated uptake is only one aspect of sulphur needs. Uptake dynamics are the other important aspect. Crops with a short vegetative period need high amounts of sulphur in a short time. Plants with a longer vegetation cycle have more time to recover sulphur from the soil and are therefore less dependent on external supply. Oilseed rape is specifically demanding with regard to sulphur, due to its short vegetation cycle and high uptake. Sulphur deficiency can therefore cause yield losses of up to 1 or 2 t/ha. Sulphur is required by the plant throughout March, April and May (see below Figure). Multiple applications with nitrogen reduces the risk of sulphur leaching from the soil and helps to alleviate any risk of inadequate supplies of sulphate being available during the main growth phases (mid April-early June). It is for this reason that Yara recommends applying ‘little and often’ at the nitrogen timings.
During the growing season, sulphur requirements for oilseed rape and wheat exceed by far the S supplied by soil. During May, typically half of sulphur needs are missing for wheat and two thirds for oilseed rape.
