Sulfur

Sulfur is a structural component of the amino acids cysteine and methionine, making it essential for building proteins and enzymes. It contributes to chlorophyll formation and, in Allium and brassica crops, to the sulfur-containing compounds responsible for characteristic flavour and odour.

Because sulfur is required alongside nitrogen for protein synthesis, an adequate sulfur supply is often necessary for a crop to make full use of applied nitrogen.

Most soil sulfur is held in organic matter and is released as plant-available sulfate through microbial mineralisation, in a pattern that parallels nitrogen. Historically, atmospheric deposition from industrial and vehicle emissions supplied significant sulfur to farmland in many regions; as air-quality regulations have reduced these emissions, sulfur deficiency has become more widely reported.

• Mineralisation of soil organic matter releases plant-available sulfate
• Atmospheric deposition has historically supplied sulfur, though this has declined with cleaner air regulations
• Sulfate is water-soluble and can be leached from soil, particularly on light, well-drained soils

Sulfur deficiency typically appears as a pale yellowing of young leaves, distinguishing it from nitrogen deficiency, which affects older leaves first — though the two can be confused in the field and both reduce growth and protein content.

Direct sulfur toxicity to plants is uncommon under normal fertilization. Sulfur-containing fertilizers and amendments, including ammonium sulfate and elemental sulfur, acidify soil over time, which can affect the availability of other nutrients. Sulfur dioxide air pollution is a separate, atmospheric pathway of excess sulfur exposure to crops.

Nitrogen and sulfur act together in protein synthesis, and sulfur deficiency can limit a crop’s response to nitrogen fertilization. Sulfur also interacts with molybdenum: high sulfate availability can reduce molybdenum uptake, which is particularly relevant for legume crops that depend on molybdenum for nitrogen fixation.