In recent years, the use of micronutrient fertilizers through soil application or foliar sprays has significantly increased due to widespread deficiencies of these nutrients in soils. Micronutrients are critical for addressing deficiencies, enhancing yields, and improving soil fertility. Modern high-yield cropping systems have amplified the demand for micronutrients, making their management vital for sustainable agriculture.
Micronutrients and Their Roles
Micronutrients are essential for plants but required in smaller quantities compared to macronutrients. Their role in achieving higher yields and maintaining soil health is crucial, especially in intensive farming systems.
1. Zinc (Zn)
Role: Essential for enzyme systems and the synthesis of indoleacetic acid.
Deficiency Conditions: Common in soils with high pH, low organic matter, and excessive phosphorus.
Symptoms of Deficiency:
- Shortened stems and rosette-like terminal leaves.
- Reduced fruit bud formation.
- Mottled young leaves (interveinal chlorosis).
Recommendations:
- Foliar spray of 0.5% zinc sulfate in early stages.
- Soil application of zinc sulfate is beneficial.
2. Iron (Fe)
Role: Integral to chlorophyll formation and biochemical processes such as respiration and photosynthesis.
Deficiency Conditions: High manganese/lime levels, poor aeration, or high pH soils.
Symptoms of Deficiency:
- Interveinal chlorosis of young leaves, with green veins.
- Severe deficiency leads to twig dieback and even plant death.
Recommendations:
- Foliar spray of 1% ferrous sulfate with 0.1% citric acid.
- Chelated iron compounds (Fe-EDTA) for effective results.
3. Manganese (Mn)
Role: Activates enzymes and assists in chlorophyll formation.
Deficiency Conditions: Induced by high levels of iron or low manganese content in soil.
Symptoms of Deficiency:
- Interveinal chlorosis with gradual pale-green coloration.
- Less distinct vein-interveinal color contrast compared to iron deficiency.
Recommendations:
- Foliar spray of 0.5% manganese sulfate.
- Soil application of 15-30 kg manganese sulfate per hectare.
4. Copper (Cu)
Role: Activates plant enzymes and influences pigment development.
Deficiency Conditions: Common in organic and sandy soils.
Symptoms of Deficiency:
- Stunted growth and terminal shoot dieback.
- Poor pigmentation and wilting leaf tips.
Recommendations:
- Foliar spray of 0.2% copper sulfate.
5. Boron (B)
Role: Involved in cell differentiation, carbohydrate metabolism, and pollen viability.
Deficiency Conditions: Widespread in soils lacking boron.
Symptoms of Deficiency:
- Terminal growth death with ‘witch’s broom’ effect.
- Chlorotic and curled leaves, necrotic fruit spots, and reduced flowering.
Recommendations:
- Foliar spray of 0.2% boric acid.
- Avoid liming in boron-rich soils.
6. Molybdenum (Mo)
Role: Vital for nitrogen utilization and symbiotic nitrogen fixation in legumes.
Deficiency Conditions: Common in acidic soils.
Symptoms of Deficiency:
- Stunted growth resembling nitrogen deficiency.
- Marginal leaf scorching, leaf curling, and ‘whiptail’ in cauliflower.
Recommendations:
- Soil application of 0.5-1 kg/ha sodium or ammonium molybdate.
- Foliar spray at 0.05% concentration.
Diagnostic Methods and Recommendations
- DTPA Method: Used to extract micronutrients like Fe, Zn, Mn, and Cu.
- Critical Soil Limits in Tamil Nadu:
- DTPA Fe (non-calcareous): 3.7 ppm; calcareous: 6.3 ppm.
- DTPA Mn: 2.0 ppm.
- DTPA Zn: 1.2 ppm.
- DTPA Cu: 1.2 ppm.
- Hot water-soluble boron: 0.44 ppm.
General Recommendations for Foliar Sprays
| Element | Fertilizer | Concentration |
|---|---|---|
| Zinc | Zinc sulfate | 0.5% |
| Iron | Ferrous sulfate | 1.0% |
| Manganese | Manganese sulfate | 0.2% |
| Copper | Copper sulfate | 0.2% |
| Boron | Boric acid/Borax | 0.2% |
| Molybdenum | Sodium molybdate | 0.05% |
Commonly Used Micronutrient Fertilizers
| Fertilizer | Nutrient Content (%) |
|---|---|
| Zinc sulfate (ZnSO4·7H2O) | 21 |
| Ferrous sulfate (FeSO4·7H2O) | 19 |
| Manganese sulfate (MnSO4·3H2O) | 30.5 |
| Copper sulfate (CuSO4·5H2O) | 25 |
| Borax | 11 |
| Boric acid | 17 |
| Sodium molybdate | 39 |
Crop Response and Fertilizer Efficiency
Micronutrient deficiencies can drastically impact crop yield and quality. Balanced fertilization, combining NPK with micronutrients based on soil tests, ensures better yields and sustainability. Proper management enhances not only productivity but also farmers’ economic returns.
Conclusion
Micronutrient application is essential for high crop productivity and sustainability. While not always necessary, micronutrients should be applied judiciously when deficiencies are evident. Their effective use requires proper soil testing and an understanding of crop-specific requirements to achieve long-term agricultural success.
Dr. P. Malathi, Dr. K. M. Sellamuthu, Department of Soil Science & Agricultural Chemistry, Tamil Nadu Agricultural University, Coimbatore – 641 003.
