👯Secondary Nutrients: Calcium, Magnesium & Sulphur

Why Secondary Nutrients Matter: A Farmer’s Perspective

An apple grower in Himachal Pradesh finds dark, sunken spots on his fruit — Bitter Pit, caused by calcium deficiency. A dairy farmer in Haryana loses cattle to Grass Tetany because pasture grasses have too little magnesium. A mustard farmer in Rajasthan gets poor oil content because sulphur is deficient. These three secondary nutrients — Ca, Mg, and S — are called “secondary” not because they are less important biologically, but because they are needed in smaller quantities than N, P, and K.

Quick Comparison: Ca vs. Mg vs. S

Calcium (Ca)

Sources of Soil Calcium

Agricultural example: The black cotton soils of Maharashtra (formed from basalt) are naturally rich in Ca. But the red laterite soils of Kerala, under heavy monsoon rainfall, lose Ca through leaching and need regular liming.

Forms of Calcium in Soil

These forms exist in dynamic equilibrium — when solution Ca is depleted by plant uptake or leaching, exchangeable Ca replenishes it.

Functions of Calcium

Deficiency of Calcium

Soils seldom become Ca-deficient if pH is maintained near neutral. Deficiency is more common in acidic, highly leached soils and rapidly growing crops.

Agricultural example: Blossom End Rot in tomato is often triggered by inconsistent watering rather than absolute Ca deficiency — uneven irrigation disrupts Ca transport to developing fruits. Regular, consistent watering is the best prevention.

Magnesium (Mg)

Sources of Soil Magnesium

Forms of Magnesium in Soil

• Occurs predominantly as exchangeable and solution Mg
• Coarse-textured (sandy) soils have the greatest potential for Mg deficiency — lower CEC holds less Mg, prone to leaching
• Competition between NH₄⁺ and Mg²⁺ lowers Mg availability — heavy ammonium-based fertilisation suppresses Mg uptake

Functions of Magnesium

Deficiency of Magnesium

Agricultural example: A dairy farmer in Punjab heavily fertilises his berseem (clover) pasture with ammonium sulphate and MOP. The high NH₄⁺ and K suppress Mg uptake in the forage. His cattle develop Grass Tetany — muscle tremors and seizures. Solution: apply dolomite (supplies both Ca and Mg) and reduce K fertiliser.

Toxicity of Magnesium

• Excess Mg absorption → browning of roots → growth ceases → death of roots and leaves
• Counteracted by CO₂ antagonistic action

Sulphur (S)

S vs. N Deficiency — A Common Exam Trap

Sources of Sulphur

Forms of Sulphur in Soil

S Mineralization and Immobilization

Just like nitrogen, the C:S ratio of decomposing material determines which process dominates:

Factors Affecting S Oxidation and Mineralization

Functions of Sulphur

Agricultural example: In Rajasthan’s mustard belt, farmers who apply 20-40 kg S/ha (as gypsum) along with NPK see 15-20% higher oil content and significantly better seed yield compared to NPK alone.

Deficiency of Sulphur

S is immobile in plant — deficiency appears on young leaves.

Toxicity of Sulphur

Agricultural example: In the organic-rich paddy soils of coastal Odisha, Akiochi can be severe. Applying ferrous sulphate not only provides Fe but also binds toxic H₂S, protecting rice roots.

Nutrient Uptake Mechanisms

Summary Table: Secondary Nutrients at a Glance

- Tisdale, S.L., Nelson, W.L., Beaton, J.D., Havlin, J.L. 1997. Soil Fertility and Fertilizers. 5th ed. Prentice Hall of India, New Delhi.
- Singh, S.S. 1995. Soil Fertility and Nutrient Management. Kalyani Publishers, Ludhiana.
- Maliwal, G.L. and Somani, L.L. 2011. Soil Technology. Agrotech.
- IARI Toppers Soil Science Part-9 (6th Edition 2025).

Summary Cheat Sheet