Citrate-Soluble Phosphatic Fertilizers are phosphorus fertilizers in which phosphorus is not fully water-soluble but is soluble in neutral ammonium citrate solution. This indicates gradual phosphorus availability through soil moisture, root exudates, and microbial activity.
They are particularly effective in acidic to neutral soils, offering slow and sustained phosphorus release, reduced fixation losses, and improved long-term soil fertility. This solubility closely mimics natural organic acids released by plant roots.
Major Types & Chemical Composition
| Fertilizer | Source / Formula | P₂O₅ Content | Notes |
|---|---|---|---|
| Basic Slag (Thomas Phosphate) | Steelmaking slag | 14–18% | 70–90% citrate-soluble P |
| Dicalcium Phosphate (DCP) | CaHPO₄·2H₂O | 38–41% | High citrate solubility |
| Partially Acidulated Rock Phosphate (PARP) | — | 16–20% | Mixed water + citrate soluble |
| Defluorinated Rock Phosphate | — | 32–36% | Thermally enhanced solubility |
Key Physical Properties
| Property | Typical Behavior |
|---|---|
| Physical State | Powder / granules |
| Color | Grey, brown, off-white |
| Density | 1.3–2.0 g/cm³ |
| Water Solubility | Low |
| Citrate Solubility | High |
| Thermal Stability | High |
Key Mechanical Properties
| Property | Performance |
|---|---|
| Hardness | Moderate |
| Abrasion Resistance | Good |
| Flowability | Good |
| Granule Strength | Medium |
| Caking Tendency | Low |
✔ Suitable for bulk broadcasting
✔ Stable during storage and transport
Strengthening & Metallurgical Behavior
Citrate-soluble phosphatic fertilizers have no structural strengthening role but are closely linked to metallurgical processes.
✔ Calcium-rich matrix improves soil structure
✔ Magnesium enhances soil aggregation
✔ Basic slag acts as a soil conditioner
✔ Phosphate phases show high thermal stability
📌 Strong linkage between steel metallurgy and fertilizer production
Key Characteristics
✔ Slow and sustained phosphorus release
✔ Reduced phosphorus fixation losses
✔ Suitable for acidic soils
✔ Low leaching losses
✔ Improves soil physical condition
✔ Long residual phosphorus effect
Refining & Processing
Produced using phosphate rock, metallurgical slag, lime or dolomite, and thermal or partial acidulation processes.
✔ Partial acidulation or thermal treatment
✔ Controlled grinding and granulation
✔ Particle size optimization
✔ Blending with N or K fertilizers
📌 Lower acid consumption than SSP/DAP
📌 Energy-based upgrading rather than full chemical conversion
Available Forms
Fine powders
Granulated fertilizers
Blended NPK formulations
Soil-conditioning phosphate blends
Fertilizer-grade calcium phosphates
Applications
Agriculture: Paddy rice, sugarcane, oilseeds, pulses, plantation crops
Soil Management: Acid soil correction, Ca & Mg supplementation, sustainable fertility programs
Industrial: Feed phosphates (DCP), cement additives, chemical intermediates
Advantages
✔ Long-term phosphorus availability
✔ Reduced fixation losses
✔ Cost-effective phosphorus source
✔ Dual nutrient benefit (P + Ca/Mg)
✔ Suitable for rain-fed agriculture
✔ Environmentally safer than fully soluble P
Comparison: Citrate-Soluble vs Water-Soluble Phosphates
| Feature | Citrate-Soluble P | Water-Soluble P |
|---|---|---|
| Availability | Slow & steady | Immediate |
| Leaching Loss | Very low | Low |
| Fixation Loss | Low | Moderate |
| Cost | Lower | Higher |
| Application | Broadcast | Fertigation / foliar |
Limitations
Slower crop response
Less effective in alkaline soils
Not suitable for fertigation
Requires adequate soil moisture
Lower P concentration than MAP/DAP