Introduction
In recent years, the use of micronutrient fertilizers—through soil application and foliar spray—has increased significantly. This is due to widespread micronutrient deficiencies in soils and the decline in overall soil fertility. To address these challenges and improve crop yield, it is essential to adopt best practices in fertilizer management.
Micro Nutrients are vital elements required by plants in small quantities. However, their impact on crop health and productivity is immense. In modern high-yielding cropping systems, these nutrients are the next major source of yield improvement.
Essential Micronutrients and Their Roles
1. Zinc (Zn)
Function: Vital for enzyme systems and the synthesis of indoleacetic acid.
Deficiency Occurs In: Alkaline soils, low organic matter, or high phosphorus levels.
Symptoms:
Shortened stems
Rosette formation of terminal leaves
Mottled young leaves (interveinal chlorosis)
2. Iron (Fe)
Function: Essential for chlorophyll synthesis; activates photosynthesis and nitrogen fixation.
Deficiency Occurs In: High pH soils, waterlogged conditions, or soils rich in manganese/zinc.
Symptoms:
Interveinal chlorosis in young leaves
Dieback of twigs, in severe cases, whole plant death
3. Manganese (Mn)
Function: Enzyme activator; supports chlorophyll formation.
Deficiency Symptoms:
Pale green leaves with darker veins
Interveinal chlorosis (similar to Fe deficiency, but with less distinct vein contrast)
4. Copper (Cu)
Function: Involved in enzyme activation and pigment formation.
Deficiency Occurs In: Organic soils and sandy soils.
Symptoms:
Stunted growth, terminal shoot dieback
Poor pigmentation
Wilting of leaf tips
5. Boron (B)
Function: Helps in pollen viability, cell differentiation, and carbohydrate metabolism.
Deficiency Symptoms:
Death of terminal growth; lateral shoot development (witch’s broom)
Curled, thickened, chlorotic leaves
Soft/necrotic fruit spots
Reduced flowering and poor pollination
6. Molybdenum (Mo)
Function: Required for nitrate reduction and nitrogen fixation in legumes.
Deficiency Symptoms:
Nitrogen deficiency-like stunting
Leaf cupping, marginal scorching
Whiptail in cauliflower
Diagnosing Micronutrient Deficiencies
Testing Methods:
DTPA method: For Fe, Zn, Mn, Cu
Hot Water Extraction: For Boron
Critical Limits in Tamil Nadu Soils (ppm)
| Nutrient | Critical Limit |
|---|---|
| Fe (Non-calcareous) | 3.70 |
| Fe (Calcareous) | 6.30 |
| Mn | 2.00 |
| Zn | 1.20 |
| Cu | 1.20 |
| Boron | 0.44 |
| Sulphur | 10.0 |
Recommended Foliar Spray Concentrations
| Element | Fertilizer Used | Recommended Concentration |
|---|---|---|
| Zinc | Zinc sulphate | 0.5% |
| Iron | Ferrous sulphate | 1.0% + 0.1% citric acid |
| Manganese | Manganese sulphate | 0.2% |
| Copper | Copper sulphate | 0.2% |
| Boron | Boric acid / Borax | 0.2% |
| Molybdenum | Sodium molybdate | 0.05% |
Commonly Used Micronutrient Fertilizers
| Fertilizer Type | Nutrient Content (%) |
|---|---|
| Zinc sulphate (ZnSO₄.7H₂O) | 21 |
| Ferrous sulphate (FeSO₄.7H₂O) | 19 |
| Manganese sulphate (MnSO₄.3H₂O) | 30.5 |
| Copper sulphate (CuSO₄.5H₂O) | 25 |
| Borax | 11 |
| Boric acid | 17 |
| Granubor | 14.6 |
| Solubor | 19.0 |
| Sodium molybdate | 39 |
| Ammonium molybdate | 54 |
Micronutrient Fertilizer Application Tips
Zinc: 0.5% foliar spray or soil application.
Iron: 1% ferrous sulphate + 0.1% citric acid or chelated Fe-EDTA for quick uptake.
Manganese: 0.5% foliar spray or 15–30 kg/ha soil application.
Copper: 0.2% foliar spray.
Boron: 0.2% boric acid spray; avoid liming when applying boron.
Molybdenum: 0.05% foliar spray or 0.5–1.0 kg/ha soil application.
Crop Response and Efficiency
Micronutrient responses vary based on soil condition, crop type, and deficiency severity.
Balanced fertilization with NPK + micronutrients ensures:
Improved crop yield
Better grain quality
Higher nutrient use efficiency
Better economic returns to farmers
Conclusion
Balanced and need-based application of micronutrients is essential for sustaining crop productivity and soil health. While they are required in small amounts, their deficiency can cause significant yield loss. Use micronutrients wisely, based on soil tests, to avoid underuse or over-application.
Dr. P. Malathi & Dr. K.M. Sellamuthu, Department of Soil Science & Agricultural Chemistry, TNAU, Coimbatore – 641 003