Sulphur in plant nutrition

Sulphur (S) ranks among the three secondary macronutrients, alongside calcium and magnesium, whilst nitrogen, phosphorus and potassium make up the primary macronutrient group. It forms part of essential amino acids, proteins and oils, and plays a pivotal part during the early growth stages of plants. Crops absorb it solely as the sulphate ion (SO₄²⁻), which moves through the soil in soluble form. Its typical concentration in plant dry matter falls between 0.1 and 0.4 %, a level comparable to that of phosphorus and magnesium^[1].

Sources and application forms

Truly single-nutrient sulphur fertilisers are restricted to products containing this element in its native state^[2]. All other sources deliver additional major elements or trace elements concurrently. The principal ones include ammonium sulphate (24 % S), single superphosphate (12 % S), potassium sulphate (18 % S), as well as various complex nitrogen-phosphate or NPK-enriched formulations. Liquid ammonium thiosulphate combines 12 % nitrogen with 26 % sulphur, split evenly between sulphate and elemental forms^[3]. The most sulphur-rich concentrates are preparations based on native sulphur, notably sulphur-bentonite grades at 90 % S, which require prior biological transformation in the soil before becoming available to roots^[4].

Availability and agronomic management

The various mineral sulphates are agronomically equivalent, since the sulphur they contain is in an immediately plant-available form^[5]. Routine application rates range from 20 to 50 kg S/ha, depending on soil reserves and the requirements of the crop species; sandy textures and oilseed crops call for higher fertiliser inputs^[5]. For amendments based on native sulphur or pyrites, incorporation should be carried out three to four weeks ahead of drilling, by broadcasting onto moist soil followed by mechanical mixing, allowing sufficient time for microbial oxidation into available sulphate^[5]. The kinetics of this conversion depend on particle diameter, soil temperature, moisture status and the intimacy of contact between the fertiliser and the soil matrix^[5]. A deficiency detected during the growing season can be remedied swiftly by a top-dressing of ammonium sulphate^[3][5].

Deficiencies and toxicity

When a shortage occurs, it first appears on the youngest foliage, persisting despite nitrogen application^[6]. Affected plants develop a stunted architecture, with thin, elongated stems; their growth slows and cereal heading is delayed. The nitrogen-fixing symbiosis is weakened in legumes. Fruit colour remains incomplete, tending towards pale green, and ripening is interrupted. In oilseed crops, yield collapses and seed oil content declines^[6]. Sulphide ion toxicity arises in strongly oxygen-depleted environments. Plants respond adversely to atmospheric sulphur dioxide concentrations exceeding 0.6 mg/m³, manifesting as foliar necrotic lesions; typical values lie between 0.1 and 0.2 mg/m³^[6].

Regulation and market

European legislation on fertilising products aims to reduce the Union's dependence on external nutrient supplies, notably by broadening the scope to include recycled and organic materials^[7]. The fertiliser industry contributes substantially to soil sulphur nutrition, through the sulphates incorporated in superphosphate and ammonium sulphate in particular^[8][9]. In soil sulphur dynamics, dissolved and adsorbed fractions often represent only a minor share of the total reserve, sometimes below 10 %; the bulk, immobilised in organic matter, requires mineralisation to become accessible again, a process favoured by temperatures around 40 °C and moisture at 60 % of field capacity^[10].