Before ordering your straight nitrogen it is worth considering how much you actually need as it may be you actually need NS or NPKS grades instead.
As usual on farm discussions often revolve around the price of nitrogen but before going ahead and ordering it is crucial to ensure you actually buy what you need.
Make sure you actually buy what your crops need
This may sound obvious, but it is always surprising when next spring comes along, as to the number of discussions that will take place as to how to use this pile of nitrogen that had been purchased, when actually NPKS and NS were also required.
The decision as to what nitrogen product is agronomically correct is easily filtered down. Firstly the question to answer is “Do I need sulphur?” and if “Yes” how am I going to manage it? It is pretty clear, with deposition at extremely low levels, the majority of crops will respond to sulphur with increased yields and improved quality.
There are three other sources of sulphur; soil reserves, organic wastes, and mineralfertiliser. Soil reserves do not build easily. When the organic sulphur mineralises it is often lost via leaching before the crop picks it up, and organic sources of sulphur are unpredictable in their release pattern. This issue has been demonstrated in grassland management where responses to sulphur from mineralfertiliser have been seen even where high levels of manures have been applied.
Sulphur containing mineral fertiliser is the most common method for preventing deficiency. The form of this sulphur is important as data has shown that sulphate sulphur is the most reliable as opposed to elemental sulphur that requires an oxidation process before becoming plant available.
This process takes time and is dependent on prevailing soil and weather conditions. Whilst being similar to nitrogen when in the soil (i.e. mineralisation and mobility), in the plant sulphur, is very different. Nitrogen is easily remobilised by the plant, moving from old leaves and into new as growth and development progresses. Sulphur is the opposite to nitrogen, remaining in the older leaves with deficiency showing in the youngest leaves first.
Sulphur should be applied 'little and often' using N+S fertilisers
In view of this the correct strategy for managing sulphur is to apply multiple, smaller, applications coincidental with the splits of nitrogen. Applying smaller applications in line with crop demand through March, April and May ensures maximum return from the investment.
Trials have shown the yield increase from this approach to be approximately 4% higher than single, large applications early in the season at approximately 0.38 t/ha in wheat and 0.5 t/ha in oilseed. Single, large applications should also be avoided as they create other negative, nutrient interactions, solving one agronomy challenge but creating another. The aim is to maintain a balanced nutrient management strategy that meets the crop demand throughout the growing season. A classic example here is the interaction with molybdenum causing deficiency and a subsequent yield penalty of approximately 0.4.
The next question to address is “What is the rate of sulphur required?” This has been a topic of conversation over recent years with some advice suggesting high rates of application are correct. Trials have been conducted to try and validate such recommendations. The results of these have not confirmed this and therefore current advice would be to target 45 - 50 kg SO3/ ha in cereal crops and 50-75 kg SO3/ha in oilseed. Again ensure this is applied in two applications bringing improved nutrient use efficiency. At these rates we will avoid the negative nutrient interactions which occur as we go towards 100 kg SO3/ha and above.
What return on investment (ROI) can you expect from taking the correct approach of multiple applications, and supplying the optimum rates? In cereal crop trials yield increases of 0.3 t/ha have been observed, whilst in oilseed this has been higher at 0.5 t/ha. Using a granulated nitrate plus sulphate product will result in an ROI of between £2 and £10 return for every £1 invested.
When phosphate and potash are required use an NPKS fertiliser
Now of course the yield benefit from sulphur will only be expressed if this is the limiting factor. If other nutrients are an issue they need addressing as well. In early spring (February, March) when soils are cold and wet Phosphate and Potassium are typically at their lowest point of availability. This is where a ‘Spring Starter’fertiliser should be considered to promote early spring growth of roots and shoots. This fresh spring growth of roots enables the plant to explore the soils phosphate reserves whilst at the same time building above ground biomass that is essential for high yield attainment. This adds a further complication into the product selection, but is simple to resolve.
Rather than using a granulated nitrate plus sulphur product then all that is required is to switch the first spring application to an NPKS Complex Compoundfertiliser (CCF). The target should be to apply 35 – 50 kg P2O5 and K2O / ha along with 50 kg N/ha and 20-25 kg SO3/ha . This nutrient combination is ideal to give arable crops the best possible start in the spring as they look to recover root and shoot growth which has often been lost over the winter period. Investigations into the benefit of this approach have been on-going for many years, across a range of sites / countries and a consistent yield increase that can be expected is a further 0.25 – 0.3 t/ha, above the expected sulphur response.
The following compound fertilisers are recommended for all crops and supply a combination of nitrogen and sulphur (N+S) in appropriate ratios to allow the ideal timing for application of both nutrients.