The Sulphur Cycle

Overview

The sulphur cycle links atmospheric deposition, organic matter, soil minerals, and plant uptake.

Sulphur behaves similarly to nitrogen in some respects but is less mobile and less biologically intense.

Major sulphur pools

Organic sulphur

• Dominant pool in most soils
• Bound in organic matter
• Must be mineralised before uptake

Inorganic sulphur

• Present mainly as sulphate (SO₄²⁻)
• Plant-available form

Key processes

Mineralisation

Microbial conversion of organic sulphur to sulphate.

Immobilisation

Temporary microbial uptake during decomposition of high C:N materials.

Oxidation

Conversion of reduced sulphur compounds to sulphate.

Atmospheric contribution

Historically significant due to industrial emissions. Declined sharply in many regions, increasing sulphur deficiency risk.

Plant uptake form

• SO₄²⁻ (sulphate)

Sulphur loss pathways

• Leaching (moderate)
• Crop removal

Practical implications

• Sulphur deficiency is increasingly common
• Deficiency resembles nitrogen deficiency but appears on younger leaves
• Adequate sulphur improves nitrogen use efficiency
• Regular low-level supply is more effective than infrequent large doses

Agronomic note

Sulphur should be managed as a routine nutrient, not an afterthought.