Phosphatic Fertilizers
Phosphorus (P) is vital for root development, flowering, and seed formation. Based on solubility, phosphatic fertilizers are classified into three types:
- Water-Soluble Phosphatic Fertilizers
These are fast-acting and used in neutral to alkaline soils, especially for short-duration crops like wheat.
Type | P₂O₅ Content | Notes |
Single Superphosphate (SSP) | 16–20% | Most commonly used in India |
Double Superphosphate (DSP) | 32% | Also called enriched SSP |
Triple Superphosphate (TSP) | 46–48% | Best for high-P requirement crops |
MAP (Monoammonium Phosphate) | 12% N, 48% P₂O₅ | Slightly acidic, good starter fertilizer |
DAP (Diammonium Phosphate) | 18% N, 46% P₂O₅ | Neutral pH, very popular in India |
Superphosphate (SSP):
- Produced by treating rock phosphate with sulfuric acid.
- Contains: 12% sulfur, 18–21% calcium, and 16% P₂O₅ (water-soluble).
- Recommended as a basal dose at sowing or planting time.
- Citrate-Soluble Phosphatic Fertilizers
(Soluble in ammonium citrate but not in water)
Used in acid soils for long-duration crops like sugarcane, rice, and tea.
Type | P₂O₅ Content |
Dicalcium Phosphate | 33–40% |
Basic Slag (Thomas Slag) | 14–18% |
Rhenania Phosphate | 23–26% |
Dicalcium Phosphate:
- Prepared by neutralizing phosphoric acid with lime.
- Suitable for acidic, neutral, and alkaline soils.
- Citrate and Water Insoluble Phosphates
Used for plantation crops in strongly acidic soils.
Type | Nutrient Content |
Rock Phosphate | 20–30% P₂O₅ |
Raw Bone Meal | 3–4% N, 20–25% P₂O₅ |
Steamed Bone Meal | 1–2% N, 20–30% P₂O₅ |
Potassic Fertilizers
Potassium (K) regulates water uptake, enzyme activation, and improves crop resistance.
Main Potassic Fertilizers:
Fertilizer | K₂O Content | Remarks |
Muriate of Potash (MOP) | 60% | Not suitable for tobacco, sugarcane, rice, etc., due to Cl⁻ content |
Sulfate of Potash (SOP) | 48–52% K₂O + 17–18% S | Used in chloride-sensitive crops |
Nitrate of Potash (NOP) | 44% K₂O + 13% N | Suitable for vegetables, fruits, tobacco |
MOP vs SOP:
- MOP is cheaper but contains chloride, which may reduce sugar accumulation in sugarcane and affect burning quality in tobacco.
- SOP is preferred for high-value crops grown in light soils.
- Secondary Nutrient Fertilizers
These supply Calcium (Ca), Magnesium (Mg), and Sulfur (S), which are essential in moderate amounts.
Nutrient | Function | Fertilizer Source |
Calcium | Cell wall strength | Gypsum, Lime |
Magnesium | Chlorophyll synthesis | Dolomite, Kieserite |
Sulfur | Protein and enzyme synthesis | Ammonium sulfate, SSP, SOP |
50% of Indian soils are deficient in sulfur.
- Micronutrient Fertilizers
Micronutrients are required in trace amounts but are crucial for enzyme function, growth regulation, and metabolism.
Ideal Concentration Range in Plants:
Micronutrient | Concentration (ppm) |
Fe (Iron) | 0.5 – 5.0 |
Mn (Manganese) | 0.1 – 0.5 |
Zn (Zinc) | 0.02 – 0.2 |
Cu (Copper) | 0.01 – 0.05 |
B (Boron) | 0.1 – 1.0 |
Mo (Molybdenum) | 0.01 – 0.05 |
Chelated Micronutrients
Micronutrients can become unavailable in soil due to chemical fixation. To prevent this, chelated forms are used.
Chelating Agents | Purpose |
EDTA | Fe, Zn, Cu, Mn chelation |
DTPA | Stronger than EDTA |
CDTA | For heavy-metal-rich soils |
Chelates are soluble and remain unreactive in soil, thus improving nutrient uptake.
- Nitrification Inhibitors & Slow-Release Fertilizers
Nitrogen (N) is highly mobile and prone to losses through leaching, volatilization, and denitrification. To improve nitrogen use efficiency, nitrification inhibitors and slow-release fertilizers are used.
Benefits:
- Enhanced N uptake due to uniform release
- Reduced labor cost (no split application needed)
- Lower pollution (less N leaching and runoff)
- Improved fertilizer use efficiency
- Flexible application timing
Examples:
- Neem-coated urea
- Sulfur-coated urea
- Urea-formaldehyde
- Nitrification inhibitors like Nitrapyrin, Dicyandiamide (DCD)
Key Takeaways for Exams
- DAP is the most popular phosphatic fertilizer in India.
- SSP is suitable for all soil types and crops; also provides Sulfur.
- SOP is used in chloride-sensitive crops like potato and tobacco.
- MOP is not recommended for sugarcane, rice, tobacco, and tomato.
- Rock phosphate is used in acid soils for plantation crops.
- Zinc and Sulfur are the most commonly deficient micronutrients in Indian soils.
- Chelated micronutrients improve availability and uptake in soils.
- Slow-release fertilizers reduce labor and environmental impact.
