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
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.