Sulphur has often been overlooked in the past, with crop requirements met from atmospheric deposition. With this no longer the case, Mark Tucker, Yara’s UK Agronomy and Business Development Manager, offers 10 ’need to know’ facts to bear in mind when considering optimum sulphur and nitrogen management.
1. Sulphur is no longer considered a secondary nutrient!
Sulphur is needed by all crops and plays a vital role in the formation of plant proteins, amino acids, some vitamins and enzymes. In fact, with annual sulphur deposition now less than 15kg SO3/ha/year, many leading agronomists feel that sulphur should be viewed as a major nutrient – and the second most important nutrient after nitrogen.
2. Sulphur and nitrogen go together
Plants need sulphur to be able to use nitrogen efficiently. Together, nitrogen and sulphur are vital building blocks for protein, so N and S should be applied at the same time. The nutrient uptake per ton of yield is typically in a ratio of 5:1 for brassicas, and 10:1 for cereals. A granulated nitrate plus sulphate product can give a Return on Investment (ROI) of between £2 and £10 for every £1 invested.
3. UK soils are sulphur deficient
For many years it has been shown that UK soils are becoming more sulphur deficient. Analysed by Lancrop Laboratories from August to October 2018, 98% of tested soils growing wheat were sulphur deficient and 94% of soils growing OSR. Rather than NPK, think NPK and S!
4. Crops only want sulphates
Whilst plant leaves can absorb sulphur from the air as sulphur dioxide (now a minor contribution) plant roots can only take up sulphates. So, of all the possible sources – soil reserves, organic manures and mineral fertilizers – only sulphate mineral fertilizers offer a reliable method for preventing deficiency. All others require mineralisation before becoming available to plants, whereas sulphate reaches the plant roots quickly and is easily absorbed.
5. Sulphur deficiency affects yield and quality
Unlike nitrogen, sulphur is immobile in plants and remains in the older leaves. This means that a sulphur deficiency will show as a yellowing of new leaves. In cereals the number of grain sites and the size of grain is reduced, with lower grain protein also resulting. Whilst symptoms appear too late for effective treatment, the majority of crops will respond to the timely application of sulphur with both increased yields and improved quality.
6. Apply sulphur ‘little and often’
Because it is mobile in the soil BUT less mobile in plants, sulphur should be applied ‘little and often’ using multiple, smaller applications through March, April and May, coinciding with the splits of nitrogen. This ensures the sulphur will be available when needed during periods of rapid growth and reduces the risk of leaching. Recommended application rates are 45-50kg SO3/ha in cereal crops and 50-75 kg SO3/ha in oilseed.
7. Too much is wasted
There’s no benefit in applying too much sulphur all at once as it may not be available when the crop needs it. Trials have shown the yield increase from the ‘little and often’ 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.
8. Nitrate Fertilisers can halve ammonia emissions
Farmers are under great pressure to reduce ammonia emissions by 8% of the 2005 baseline by 2020. Fertiliser application accounts for 23% of all agricultural ammonia emissions so will rightly attract attention.
Nitrate based fertilisers have one of the lowest ammonia emissions of all nitrogen sources, emitting half that of urea with an inhibitor – ensuring not only a quality crop, but helping UK agriculture comply with environmental targets.
9. Straight N – a poor buying decision
However attractive the price, before buying, always ask yourself ‘how much straight nitrogen do I need?’ As a guide, 50-75% should be in the form of a nitrate plus sulphate fertilizer and 25% will be in the form of an NPKS Complex Compound Fertilizer – a ‘spring starter’ containing phosphate and potassium that will promote early spring growth of roots and shoots.
10. Know what you need
Take time to assess your requirements. For sulphur, tissue analysis is a more reliable indicator of deficiency than soil analysis which can change quickly due to plant uptake. Generally, sulphur levels should exceed 0.3% of dry matter for most crops and 0.45% for oilseed rape. Actual recommendations should always be checked with a FACTS Qualified Agronomist.