Partially Acidulated Phosphate Fertilizers (PAPFs) are phosphatic fertilizers produced by partially treating natural rock phosphate with mineral acids such as sulfuric or phosphoric acid. Only a portion of the phosphate is converted into water-soluble phosphorus, while the remainder stays in citrate-soluble or slowly available forms.
This dual nature provides both immediate and long-term phosphorus availability, making PAPFs a cost-effective and agronomically efficient solution for acidic and phosphorus-deficient soils.
Chemical Composition & Phosphate Phases
| Component | Typical Range |
|---|---|
| Total P₂O₅ | 16–20% |
| Water-Soluble P₂O₅ | 5–9% |
| Citrate-Soluble P₂O₅ | 8–12% |
| Calcium (CaO) | 28–35% |
| Sulfur (S) | 4–8% |
| Fluorides | Trace–moderate |
✔ Monocalcium phosphate – water soluble
✔ Dicalcium phosphate – citrate soluble
✔ Unreacted apatite – residual slow-release P
✔ Calcium sulfate (gypsum) – matrix structure
Key Physical Properties
| Property | Typical Characteristics |
|---|---|
| Physical State | Powder / granules |
| Color | Grey, brown, off-white |
| Bulk Density | 1.2–1.6 g/cm³ |
| Water Solubility | Partial |
| Citrate Solubility | High |
| Thermal Stability | High |
✔ Stable during transport
✔ Lower caking tendency than SSP
✔ Suitable for blending operations
Key Mechanical Properties
| Property | Behavior |
|---|---|
| Granule Crushing Strength | Medium |
| Abrasion Resistance | Good |
| Flowability | Good |
| Dust Formation | Low–moderate |
| Storage Stability | High |
📌 PAPFs are mechanically more robust than SSP and less brittle than raw rock phosphate
Strengthening & Metallurgical Behavior
PAPFs do not provide mechanical strengthening but reflect controlled mineral activation similar to metallurgical beneficiation processes.
✔ Partial decomposition of apatite lattice
✔ Gypsum phase improves matrix stability
✔ Enhanced lattice defects increase reactivity
✔ Thermally stable phosphate structure
Key Characteristics
✔ Dual phosphorus availability (fast + slow)
✔ Reduced phosphorus fixation in soil
✔ Lower acid requirement than SSP/DAP
✔ Cost-effective phosphorus source
✔ Long residual soil effect
Refining & Processing
Produced using beneficiated phosphate rock and controlled partial acidulation.
✔ 30–50% acidulation level
✔ Controlled exothermic reaction
✔ Gypsum formation within matrix
✔ Curing, granulation, screening
📌 Lower acid and energy consumption than fully acidulated phosphates
Available Forms
Powdered PAPF
Granular PAPF
Blended NPK formulations
Sulfur-enriched phosphate fertilizers
Customized soil-specific blends
Applications
Agriculture: Rice, wheat, maize, sugarcane, pulses, oilseeds
Soil Types: Acidic soils, P-fixing soils, rain-fed regions
Industrial: Intermediate phosphate blends, soil conditioners
Advantages
✔ Balanced phosphorus release
✔ Improved phosphorus use efficiency
✔ Lower production cost
✔ Reduced environmental impact
✔ Better agronomic response than rock phosphate
Comparison: PAPF vs SSP vs DAP
| Feature | PAPF | SSP | DAP |
|---|---|---|---|
| Total P₂O₅ | 16–20% | 16% | 46% |
| Water-Soluble P | Partial | High | Very High |
| Citrate-Soluble P | High | Moderate | Low |
| Residual Effect | High | Moderate | Low |
Limitations
Slower response than DAP/MAP
Not suitable for fertigation
Lower P concentration
Requires proper soil moisture
Less effective in alkaline soils