Diammonium phosphate at 64 % total nutrient is the world reference phosphate fertilizer grade, and in practice it is what “DAP” means without further qualification in international trade. The figure resolves as 18 % nitrogen plus 46 % P₂O₅, and 18 + 46 = 64. Against a theoretical maximum of about 75 % total nutrient for the pure compound, a 64 % commercial grade represents a high-purity granular product. It is the benchmark against which other phosphate sources are usually priced and formulated.
Substance identity
| Field | Value |
|---|---|
| Active compound | Diammonium hydrogen phosphate, (NH₄)₂HPO₄ |
| CAS | 7783-28-0 |
| PubChem CID | 24540 |
| Molar mass | 132.06 g/mol |
| Theoretical maximum (pure compound) | 21.2 % N + 53.7 % P₂O₅ ≈ 74.9 % total |
| Grade basis | 64 % = 18 % N + 46 % P₂O₅ |
Specification
Standard grade — verify against certificate of analysis:
| Parameter | Typical |
|---|---|
| Total nutrient (N + P₂O₅) | ≥ 64 % |
| Nitrogen (N), ammoniacal | 18 % |
| P₂O₅, total | 46 % |
| Water-soluble P₂O₅ as % of total | typically ≥ 90 %; declare on CoA |
| Moisture | ≤ 2.0 % |
| Particle size | ≥ 90 % within 2 – 4 mm |
| Bulk density | ≈ 0.9 – 1.0 t/m³ |
| Appearance | granular; grey, brown, black or yellow by market |
Properties
-
Highest routinely traded nutrient density in a solid straight phosphate. One tonne carries 180 kg N and 460 kg P₂O₅ — equivalent to roughly 201 kg of elemental P.
-
Alkaline granule reaction. Solution around the dissolving granule is generally reported at pH 7.8–8.2, with the associated free-ammonia effect described below.
- Freight efficiency. At 64 % total nutrient, 64 % of the shipped mass is declared nutrient. This is the reason the grade dominates long-haul international trade.
- Thermal and humidity behavior. Decomposition with ammonia release reported near 155 °C; critical relative humidity around 82.5 % at 30 °C, requiring better moisture control than MAP.
- Blend behavior. Standard 2–4 mm granulation matches granular urea and standard-grade potash for physical blending. Blends have a lower critical relative humidity than any single component.
Applications
-
Primary basal N + P source for cereals, maize, rice, cotton, oilseeds, sugarcane and pasture, drilled or broadcast and incorporated ahead of or at sowing.
-
Backbone of bulk-blend programs. In most blending operations, DAP [CAS?] supplies the whole phosphate requirement and a substantial share of the nitrogen; urea or ammonium sulfate closes the N gap and potassium chloride or sulfate supplies K.
- Soil pH context. Best suited to neutral and acid soils. On strongly calcareous or high-pH soils, the alkaline reaction zone accelerates fixation of applied phosphate as calcium phosphates and MAP is normally the better selection.
- Seed-placement caution — the key handling decision. Free ammonia generated at pH ~8 around the granule injures germinating seed and root tips. Do not place DAP in direct seed contact at agronomic rates. Risk increases with higher rate, narrow rows, low seed-bed utilization, coarse or dry soils, alkaline soils, and sensitive or small-seeded crops. Side-band, or use MAP for seed-placed phosphate.
Handling and storage
Store dry, covered and ventilated, on pallets, in sealed bags or in a moisture-controlled bulk store. Rotate stock — extended warm humid storage causes slow ammonia loss and caking. Keep strictly segregated from lime, basic slag, calcium cyanamide and other alkaline materials; contact liberates ammonia and reverts soluble phosphate. Keep away from strong acids and from heat sources. Ventilate enclosed bulk storage. Minimize transfer points to limit fines generation.
Notes
-
64 % is total N + P₂O₅, and this is provable: the pure compound cannot exceed 53.7 % P₂O₅, so no DAP anywhere carries 64 % P₂O₅.
-
[CAS?] supplies N and P in a fixed 1 : 2.6 ratio. Most crop programs need proportionally more N than that, so DAP is almost always paired with a separate nitrogen source rather than used alone.
- Long-term soil effect is acidifying through nitrification of the ammonium nitrogen, notwithstanding the alkaline granule reaction.