Sulphur in the soil
The sulphur content of soils varies widely, in humid climates, S concentration is typically around 0.02-2 %, moorland soils may contain 1 % and in marshland, S concentration can be as high as around 3.5 %.
Sulphur may occur in the soil inorganically or organically bound. Depending on the state of the soil, the inorganically bound sulphur can occur as elementary sulphur or in different forms of oxidation (sulphide, sulphate, thiosulfite etc.). Sulphur containing organic compounds are: amino acids, proteins, polypeptides and others.
Sulphur compounds occurring in the soil are transformed in many different ways, similar to the nitrogen cycle. For example, the organic substances are subjected to microbial mineralization and hydrolysis and the sulphate ion is always the final product. However it is possible for sulphur to be temporarily fixed by microbial activity when they incorporate sulphur into fulvic acid and humic substances Under anaerobic conditions, the sulphate-ion can be reduced by bacteria to hydrogen sulphide which is unavailable for plants.
Elemental sulphur turns into sulphate-sulphur as a result of a transformation process by soil bacteria (e.g. thiotrophic bacteria). The bacteria oxidise elemental sulphur into sulphate releasing hydrogen (H+). Consequently, elemental sulphur causes soil acidification. By contrast, in the case of calcareous soils (pH value > 7), the sulphuric acid arising in the process is neutralised by lime, which leads to the formation of hardly soluble gypsum.
Magnesium and potassium sulphate are neutral salts. The nutrients contained dissolve into the soil solution without previous conversion and have no impact on soil pH values.
As a negative ion, sulphate, like the nitrate ion, does not undergo any significant adsorption to soil colloids and is therefore at risk of being washed out, especially during the winter half-year. The risk of sulphur deficiency is high on lighter soils, especially after heavy rain, on soils with a poor structure and under any conditions which result in a restricted root system (eg. Compaction, pan formation etc.). On the other hand, when evaporation exceeds rainfall, especially during the summer months, the needs-based application of sulphate in spring ensures a sufficient supply of sulphur over the entire vegetation period.
Various methods are used to measure the S-supply of soils. The Smin test measures potentially available S from very deep soil cores or the value can be estimated from a number of soil, agronomic and weather characteristics. Alternatively an increasingly important method is that of S calculation in the plant either by N/S ratio or by the malate/sulphate ratio test which show if the plant is adequately supplied with S.
Sulphur in the plant
Only the sulphate ion is taken up by the plant from the soil. However, plants are able to take up small quantities of atmospheric sulphur (hydrogen sulphide and and sulphur dioxide)
Function of sulphur in the plant:
- increased nitrogen use efficiency
- essential for synthesis of sulphur-containing amino acids and is therefore vital for protein synthesis.
- activates important enzymes necessary for the metabolism of energy and fatty acids.
- component of chloroplast protein.
- important for synthesis of sulphur-containing, secondary plant substances such as aromatic oils and compounds which affect taste and aroma and therefore qualitative value of several cash crops.
- component of vitamin B1 (particularly for cereals and legumes).
- important for production of innate plant defence substances (phytoalexines, glutathione).
- Deficiency symptoms often occur first on younger leaves, (cf. N deficiency which tends to appear on older leaves first)
- large areas of generalised chlorosis appear on the leaves.
- the whole plant appears rigid and brittle.
- typical symptoms in oilseed rape are stunted growth, spoon-like arched leaves, pale yellow or white petals and pods can appear bladder-like and bloated.
- Until the 80’s, SO2-emissions of industrial origin were generally sufficient to supply the complete S requirement of most crops
- Several measures for lowering industrial emissions for cleaner air have resulted in a significant decrease of atmospheric sulphur emissions. This in turn has resulted in a decrease in S depositions which have now decreased to pre-industrial levels
- A huge deficit in sulphur is predicted especially for Asia, but also for other regions of the world.
- the input of sulphur in rural areas far from industrialisation reaches only an average of 5-10 kg S ha-1. This does not cover the requirements, which depending on the crop and yield level, ranges between 5-80 kg S ha-1.
- In many areas of the world, shortage of sulphur has now become the single largest limiting factor restricting plant productivity
- Many crops, such as oilseeds, legumes, onions, leek, and garlic, require S for the production of aromatic flavour compounds. For these crops in particular, fertilisation with sulphur frequently results in enormous increases in yield and quality.
- in addition, the supply of sulphur through manure and other farm wastes is often insufficient. In the first year only 5-10 % of the complex sulphur is available for the plant. For crops that have an early sulphur requirement such as oilseed rape and cereals, use of such manures over several years is still not sufficient and therefore a mineral fertilization is recommended.