Sulphur – The Forgotten Secondary Nutrient for Crop Health

In agriculture, farmers think NPK – Nitrogen, Phosphorus, and Potassium – the major nutrients required for plant growth. But secondary nutrients like Sulphur (S) is equally important for sustainable yields and high-quality produce. Sulphur is sometimes called the “fourth major nutrient” because of its essential role in plant metabolism, yield, and resistance.

Why Plants Need Sulphur

This is a key component in:

• Protein formation – Plants use S. to produce essential amino acids like cysteine and methionine.
• Oil content – Oilseed crops require higher S. for good quality oil.
• Chlorophyll production – Improves photosynthesis efficiency.
• Enzyme activation – Many plant enzymes work only in the presence of S.
• Disease resistance – Strengthens cell walls and helps plants resist fungal and bacterial attacks.
• Flavour and aroma – Onions, garlic, and pepper get their characteristic flavour from S. compounds.

Sources of Sulphur

A. Natural Sources

• Organic matter – Compost, farmyard manure, crop residues.
• Soil minerals – Gypsum, elemental sulphur naturally occurring in some soils.
• Atmospheric deposition – Earlier, S. came from industrial emissions (acid rain), but this is reduced now.
• Green manures – Legumes and deep-rooted crops recycle S. from subsoil.

B. Chemical Sources

• Sulphate of Potash (SOP) – Contains potassium and S. (K₂SO₄).
• Single Super Phosphate (SSP) – Contains both phosphorus and S.
• Ammonium Sulphate – Nitrogen + S source.
• Magnesium Sulphate (Epsom salt) – Magnesium + S.
• Elemental Sulphur – Long-lasting S. source, slow-release.
• Gypsum (Calcium Sulphate) – Improves soil and supplies sulphur and calcium.

Sulphur Requirement for Major Horticulture Crops

(Approximate total requirement per acre per year – actual dose depends on soil test)

Deficiency Symptoms of Sulphur

S. deficiency is becoming common due to reduced use of S.-containing fertilizers.
Symptoms:

• Uniform yellowing of younger leaves (unlike nitrogen deficiency, which starts from older leaves).
• Stunted plant growth.
• Thin, spindly stems.
• Reduced flowering and fruit set.
• Poor oil content in oilseed crops.
• In pepper and banana – pale leaves, low vigour, poor bunch development.

Sulphur Loss from Soil

This can be lost by:

• Leaching – In sandy or high rainfall areas, sulphate form washes away.
• Erosion – Topsoil loss removes organic matter rich in S.
• Crop removal – High-yield crops remove large quantities of S.
• Low organic matter – Soils with less than 1% organic carbon are often S.-deficient.

Excess S. and Its Effect

While rare, excess S. can:

• Increase soil acidity (especially with ammonium sulphate).
• Cause toxicity in very high amounts – leaf burn, reduced root growth.
• Imbalance with other nutrients like molybdenum.

Role of S. in Plant Health and Resistance

• It strengthens plant cell walls, making them less prone to pest and disease attack.
• Improves flavour and aroma – important for spices like pepper and condiments like onion and garlic.
• Essential in synthesising vitamins like biotin and thiamine.
• Reduces fungal diseases like powdery mildew when used as a fungicide.

How to Add Sulphur

A. Soil Application

• Use S-containing fertilizers during basal or split application.
• Mix gypsum or elemental S with compost for long-term supply.
• Incorporate Single Super Phosphate or Ammonium Sulphate in regular fertilizer schedule.

B. Foliar Spray

• Use Magnesium Sulphate (Epsom salt) – 1–2% solution for quick correction.
• Foliar application is effective for standing crops showing deficiency symptoms.
• Avoid spraying elemental S. in hot weather to prevent leaf burn.

Practical Farmer Tips

• Always test your soil before deciding S. doses.
• Maintain organic matter levels with FYM, compost, and green manures.
• In high rainfall areas, apply sulphur in split doses to reduce leaching.
• Combine sulphate nutrition with balanced NPK for maximum yield and quality.

Remember: Without adequate sulphur, your crop may look green but fail to produce quality yield. A small investment in S. can return big profits in high-value crops.