Sulfurous – but essential

Sulfur, as nitrogen, is a vital ingredient of life.

Often linked to each other in biological processes, both elements form an inseprarable team. In agriculture, however, sulfur was confined to a second role. Today, sulfur is reconsidered at its rightful place: an essential nutrient and counterpart for optimum nitrogen efficiency.

Why sulfur?

Sulfur is a fundamental ingredient of life on earth. It is present through different forms:

  • As elementary sulfur (S), sulfite (SO32-), sulfate (SO42-) and pyrita (FeS) in the soil.
  • As hydrogenated sulfur (H2S) and sulfur dioxide (SO2) in the atmosphere.
  • As sulfate in the oceans (SO42-)

Elementary sulfur it not immediately available to plants and needs to be mineralized first. Sulfur is present in all crops and plays an important role in plant metabolism.

Sulfur stands for quality

Most compounds containing sulfur also contain ntrogen, thus highlightning the close link between these two elements. Sulfur is part of an enzyme requiered for nitrogen uptake and lack of sulfur can severely hamper nitrogen metabolism. Together with nitrogen, sulfur 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 and organoleptic properties of crop.

Sulfur is also essentialy involved in photosynthesis, overall energy metabolism and carbohydrate production.

Some cruciferoud (e.g rapeseed) use significant proportions of sulfur. Garlic and onions need sulfur to produce allins. Both secondary plant substances have a determining impact on odor and taste, but also improve plant resistance and self-defence. In leguminous, sulfur is needed for fixation of nitrogen from the air.

Sulfur deficiency – often neglected

Until the 90s, sulfur availability was not a matter of concencern, since sulfur dioxide emissions from industrial origins guaranteed a sufficient and automatic supply. Environmental regulation in general and low sulfur fuel in particular has strongly reduced suh emissions. At the same time, higher yield and quality expectations have increased sulfur withdrawal from the field. If sulfur deficiencies were fairly rare 20 years ago, they are commonly encountered today. Sulfur deficiencies are more likely to occure under the following conditions:

  • Light and sandy soils with little soil organic matter (> low sulfur content)
  • High recipitations during winter (>sulfur leaching)
  • Dry spring (>low mobility of sulfates)
  • Low temperture (>low mineralization rate)
  • Low input of organic matter and mineral sulfur (>low input)
  • Distance from industrial sites (>low depositions)

Characteristic aspect of a field with advanced localized sulfur deficiency.

Rapeseed with advanced sulfur deficiency (left). Growth is diminished, flowering is sparse and white.

Symptoms appear late

Grassland with advanced localized sulfur deficiency (in the back).

Sulfur deficiency is hard to distinguish from nitrogen deficiency, to which it may be linked. Symptoms include a yellowing of the younger leafs, as a result of low chlorophyll production. Growth is generally diminished. In cereals, the tiller number is reduced. In rapeseed, flowering turns to white and leafs are disorted. In most cases, sympotms appear too late for efficient compensation. Hidden deficiency is by far more frequent than acute deficiency.