Written by James Warne of Soil First Farming
In the years from 1979 to 2016 there was a 97% drop in sulphur dioxide emissions to air in the UK. That’s a reduction of 6.32 million tonnes over 37 years. While the UK has made an impressive shift to clean up our atmosphere this has resulted in a net drop of sulphur deposition to land. The downside is that what once arrived free of charge has now become a commodity that you have to pay for. But it seems not everyone is convinced by the need to apply sulphur. The most recent British Society of Fertiliser Usage census (2018) reveals that 25% of winter OSR and 30% of winter wheat does not receive additional sulphur.
Sulphur is one of the 5 or so bulk elements found in all living organisms. It is taken up as the sulphate anion SO42- . Translocation is mainly by the xylem with very little phloem movement, making it relatively immobile within the plant. Consequently sulphur deficiency symptoms are usually expressed on new growth. Sulphur is a constituent of some amino acids, principally cysteine and methionine. Both of these amino acids are the essential building blocks of proteins and as such many proteins contain sulphur, as do protein enzymes which regulate activities such as photosynthesis and nitrogen fixation. Sulphur is also an essential ingredient of aromatic oils such as those found in brassicas. Legumes also require larger amounts of sulphur due to higher protein levels.
How plants absorb Sulphur
The plant has access to three main sources of organic sulphur; organic matter, soil minerals and gaseous sulphur. Sulphur within organic matter tends to be strongly bonded to carbon. Biological activity breaks these bonds slowly over time into soluble forms such as sulphate. Mineral forms of sulphur are more plentiful but again rely on biological activity to make them into plant available forms. As described above gaseous forms in the atmosphere have decreased rapidly in recent years and so the amount of available sulphur is decreasing, plant can however absorb small amounts of sulphur foliarly.
Sulphur can be easily lost from soils. Leaching is the main sulphur loss pathway. Being negatively charged in the sulphate form, sulphate is usually accompanied by an associated cation such as calcium or magnesium. This process increases in-line with increasing amounts of excess winter rainfall. Under anaerobic soil conditions sulphur maybe lost through volatilization in sulphide compounds.
Lighter textured soils tend to have lower reserves of organic sulphur and therefore tend to show deficiency symptoms first. Whilst heavier textured soils and those with higher organinc matter content will usually have greater sulphur reserves.
What does this mean for you…?
As sulphur is an essential constituent of proteins, sulphur deficiency results in an inhibition of protein synthesis. The sulphur containing amino acids cysteine and methionine which are essential building blocks of protein are deficient and thus proteins cannot be synthesized. Sulphur deficiency tends to reduce shoot growth rather than root growth. Most apply early nitrogen to encourage tillering, or at least to encourage tiller survival, especially in later drilled crops. Without sufficient sulphur (and other elements) nitrogen alone may not be particularly effective.
Low Sulphur causes poor grain quality
Sulphur deficiency also slows plant growth and maturity. Affected plants show reduced leaf area and lower chlorophyll levels, directly reducing photosynthetic output, which must affect yield. In addition to reduced yield, sulphur deficient plants have grains of low sulphur status. Grain storage protein levels reduce, resulting in low grain protein and high grain nitrogen:sulphur ratios. The protein quality may also be affected which will affect breadmaking quality.
OSR will suffer from low oil levels. Maize and legumes will also suffer from a decrease in low-sulphur proteins within the seed. Nitrogen use efficiency will also be challenged as nitrogen is also a constituent of amino acids and proteins. If the plant is sulphur deficient it will be unable to synthesis protein effectively and therefore unable to utilise the applied nitrogen efficiently. The inhibition of protein synthesis is correlated with an accumulation of soluble organic nitrogen and nitrate within the plant tissues. Excess nitrate can be a precursor to reduced straw strength and lead to increased disease.
Cold wet conditions accelerate Sulphur Deficiency
After the wet winter of 2019/2020 soil sulphur reserves will be very low. Saturated soils and low soil temperatures will slow biological activity, consequently organic sulphur release will be very compromised this spring. The most widely used sulphur application routine seem to be to use ammonium sulphate (Double Top) as the first nitrogen application on most crops which supplies the RB209 recommended amount of sulphur. This is usually applied in February when plant growth is still slow, there is little crop biomass and a high risk of leaching. It is however reasonably sensible to apply some sulphur when mineralization maybe low. Considering the importance of sulphur it seems sensible to apply sulphur with all nitrogen to ensure the crop has access to sufficient sulphur and is able to fully exploit the applied nitrogen. There is gathering evidence to suggest that the RB209 recommendation for nitrogen:sulphur applications ratio of 5-6:1 is too wide and we need to be making better use of sulphur.
For legumes which receive no additional nitrogen, sulphur applications are still required to ensure adequate nitrogenase activity. Nitrogenase is the enzyme responsible for nitrogen fixation in the root nodules of legumes.
How do we recognise sulphur deficiency…?
The classic symptom in cereals and OSR is similar to that of nitrogen deficiency except that it affects the youngest growth. OSR can show pale yellow flowers as a symptom. Leaf yellowing/ chlorosis and stunted growth maybe indicators of sulphur shortage but need to be correlated against other factors such as environmental conditions and recent fertiliser applications etc.
If we are to reduce our reliance upon artificial nitrogen inputs (one of the biggest carbon sources attributable to agriculture) whilst increasing nitrogen use efficiency perhaps we need to pay more attention to a more balanced nutritional offering. This in tandem with increasing the organic matter content of our soils is surely some of the low hanging fruit we can use as a marketing success story.