Gypsum

Gypsum is calcium sulfate dihydrate, sourced from natural mineral deposits or recovered as a by-product of flue-gas desulfurisation or certain acid manufacturing processes. Agricultural gypsum products typically guarantee around 23% calcium and 18% sulfur.

Mineral formula

CaSO₄·2H₂O

Calcium content

Approximately 23% elemental calcium

Sulfur content

Approximately 18% elemental sulfur, as sulfate

Effect on soil pH

Negligible; sulfate is not a neutralising base like carbonate

Gypsum supplies calcium and sulfate-sulfur in slightly soluble form, both immediately usable nutrients, while also acting on soil physical and chemical properties through the calcium ion’s effect on clay flocculation and sodium displacement.

• Calcium and sulfur are both supplied without a liming effect
• Slightly soluble, releasing nutrients gradually over time
• Calcium ions can displace exchangeable sodium on soil cation exchange sites

Beyond supplying calcium and sulfur to crops such as groundnut, where calcium reaching the developing pod is a specific consideration, gypsum is widely used as a soil amendment to reclaim sodic soils, where displaced sodium is leached away, and to improve aggregate stability and reduce crusting on some soils.

• Corrects calcium or sulfur deficiency without altering soil pH
• Used to reclaim sodic soils by displacing exchangeable sodium
• Can improve aggregate stability and reduce surface crusting on some soils

Where gypsum is used for sodic-soil reclamation, effective use generally requires adequate leaching water to carry displaced sodium below the root zone, and rates are typically based on a soil test of exchangeable sodium rather than a fixed figure.

Because gypsum does not affect soil pH, it can be used alongside pH-management programs without interference. Its use in sodic-soil reclamation depends on adequate drainage to carry leached sodium away; without sufficient drainage, reclamation is less effective.