For much of the twentieth century, sulphur was a nutrient that most growers in industrialised regions never had to think about. It arrived free, from the sky.
The deposition story
Coal combustion and uncontrolled industrial emissions put sulphur dioxide into the atmosphere, which returned to farmland as sulphate in rainfall. In heavily industrialised areas, atmospheric deposition alone met a meaningful share of crop sulphur demand. Sulphur was also delivered incidentally: single superphosphate carries substantial sulphate, and older ammonium- and potassium-based products contained sulphur as a matter of course.
Three trends removed that free supply. Air-quality regulation drove sulphur dioxide emissions sharply down, which was unambiguously good for public health and incidentally removed a nutrient input. Fertiliser formulation shifted toward high-analysis products — urea, DAP, MAP, MOP — which carry little or no sulphur. And yields rose, increasing crop sulphur removal per hectare.
Sulphur deficiency consequently emerged as a widespread agronomic issue in regions that had never managed it, and it continues to emerge as those trends propagate.
What sulphur does in the plant
Sulphur is a constituent of the amino acids cysteine and methionine, and therefore of essentially all proteins. It is present in glutathione, central to the plant’s management of oxidative stress; in coenzyme A; in several vitamins; in the iron-sulphur clusters of electron transport chains; and in the glucosinolates and alliins that give brassicas and alliums their characteristic chemistry and much of their commercial value.
Because protein synthesis requires both, sulphur and nitrogen are functionally coupled. A sulphur-deficient crop cannot convert absorbed nitrogen into protein efficiently. Nitrate accumulates in tissue, nitrogen use efficiency falls, and additional nitrogen produces neither yield nor quality. Sulphur deficiency thus wastes nitrogen — a point with both economic and environmental force.
Recognising deficiency
Sulphur is poorly mobile within the plant, so deficiency shows first on young leaves: uniform pale green to yellow chlorosis of the newest growth, with older leaves staying greener. This is the diagnostic contrast with nitrogen deficiency, which appears on older leaves first because nitrogen is remobilised to the growing point. Severe cases show stunting, delayed maturity and, in oilseeds, poor pod fill and reduced oil content; in cereals, reduced grain protein and impaired baking quality; in forages, a widened nitrogen-to-sulphur ratio that reduces feed value.
Tissue testing that reports the N:S ratio is more informative than either element alone.
Sulphate versus elemental sulphur
Plants take up sulphur as the sulphate ion. This has one practical implication that is regularly overlooked.
Sulphate sources — ammonium sulphate, magnesium sulphate, potassium sulphate, gypsum — deliver plant-available sulphur immediately. Sulphate is mobile in soil water and moves down the profile, so it can leach, particularly from light soils over a wet winter. Timing to demand rather than autumn application is generally the appropriate response.
Elemental sulphur is not plant-available at all as applied. It must first be oxidised to sulphate by soil bacteria, principally Thiobacillus species. That oxidation depends on particle size, soil temperature, moisture and biological activity, and takes weeks to months. Elemental sulphur is an excellent way to build reserves and carries far more sulphur per tonne, but it is unreliable for correcting an in-season deficiency.
The practical conclusion
Sulphur is now a nutrient to be planned rather than assumed. For crops with high demand — oilseed rape and other brassicas, onions and garlic, legumes, forage grasses, and high-protein cereals — a sulphate source applied close to the period of rapid uptake is the dependable approach. Ammonium sulphate is one route because it delivers sulphate alongside nitrogen in a single application, in a weight ratio close to the ratio in which many crops actually need them.