Diammonium Phosphate (DAP) — (NH₄)₂HPO₄, CAS 7783-28-0

Diammonium phosphate is the ammonium salt of phosphoric acid in which two of the three acidic protons have been neutralized. It is the most widely traded solid phosphate fertilizer in the world, supplying both nitrogen and phosphorus in a single granule at a nutrient density that is hard to match. Its defining behavior — and its one genuine agronomic hazard — follows directly from that second ammonium ion: the solution formed as a DAP granule dissolves is alkaline, not acidic, and that alkalinity generates free ammonia at the granule surface.

Substance identity

Field	Value
Chemical name	Diammonium hydrogen phosphate
Common names	DAP, ammonium phosphate dibasic
Formula	(NH₄)₂HPO₄
CAS	7783-28-0
PubChem CID	24540
Molar mass	132.06 g/mol
EC/EINECS	listed — confirm the current EC number against the ECHA substance record

Specification

Theoretical composition of the pure compound:

Parameter	Value
N	21.2 %
P	23.4 %
P₂O₅ equivalent	53.7 %
Grade equivalent	[CAS?]

Commercial tiers — total nutrient basis:

Tier	Total N + P₂O₅	Typical split
Standard / reference grade	64 %	[CAS?]
Lower tier	57 %	≈ [CAS?]
Property	Typical commercial figure
—	—
Moisture	≤ 2.0 – 2.5 %
Water-soluble P₂O₅ as % of total	typically ≥ 85 %; declare on CoA
Granule size	≥ 90 % within 2 – 4 mm
Bulk density	≈ 0.9 – 1.0 t/m³

Properties

Appearance — white crystalline solid when pure; commercial granular product ranges through grey, brown, black and yellow depending on process route and market preference.
Solubility in water — high; commonly reported near 58 g per 100 mL at 10 °C and rising steeply with temperature. Reported values differ between sources; treat any single figure as indicative.
Solution reaction — a 1 % aqueous solution is alkaline, around pH 8. The soil solution immediately around a dissolving granule is generally reported in the range pH 7.8–8.2.
Free ammonia generation. At that pH, part of the very high local ammonium concentration exists as dissolved free ammonia, NH₃, rather than as the ammonium ion. Free ammonia is phytotoxic to germinating seed and to root tips. This is a chemical consequence of the compound, not a product defect, and it is the reason DAP placement rules exist.
Thermal instability. DAP is markedly less thermally stable than MAP. On heating it loses ammonia and reverts toward monoammonium phosphate; decomposition is commonly reported around 155 °C, and ammonia loss can begin well below that. Slow ammonia loss also occurs during prolonged warm, humid storage.
Critical relative humidity — around 82.5 % at 30 °C, distinctly lower than MAP. DAP therefore needs better moisture control in storage than MAP.
Density — approximately 1.62 g/cm³.

Applications

Basal and pre-plant N + P on cereals, maize, rice, cotton, oilseeds and sugarcane. Broadcast and incorporated, or drilled away from the seed.
Where DAP is preferred over MAP. On acid soils, the transient alkaline reaction zone around the granule is neutral to mildly beneficial, and DAP’s higher nitrogen content per tonne reduces the amount of separate N carrier needed. On strongly calcareous or alkaline soils the position reverses and MAP is normally the better choice, because the alkaline reaction zone accelerates precipitation of applied phosphate as calcium phosphates.
Seed placement — caution. Direct seed contact with DAP risks germination injury from free ammonia. Risk rises with: higher application rate, narrower row spacing and lower seed-bed utilization, coarse-textured or dry soils, already-alkaline soils, and small-seeded or sensitive crops. The standard mitigation is to side-band or otherwise separate the fertilizer from the seed row. Where seed-placed phosphate is required, MAP is the appropriate product.
Bulk blending with urea, ammonium sulfate and potash to reach target N-P-K ratios.
Non-agricultural — flame retardant formulations, and as a yeast nutrient in fermentation and winemaking.

Handling and storage

Store dry, covered and ventilated, on pallets, in sealed packaging. DAP’s lower critical relative humidity makes it more sensitive to humid storage than MAP; in hot humid conditions expect both caking and a faint ammonia odor from slow decomposition, and manage stock rotation accordingly. Do not store or mix with lime, basic slag, calcium cyanamide or any alkaline material — ammonia release is immediate and phosphate availability falls. Do not store with strong acids. Keep away from heat. Ventilate enclosed storage; ammonia can accumulate in poorly ventilated warehouses holding large quantities.

Notes

Grade figures for DAP are total N + P₂O₅. “64 % DAP” is [CAS?]. It is arithmetically impossible for it to mean 64 % P₂O₅ — the pure compound tops out at 53.7 % P₂O₅.
Ammonium nitrogen in DAP nitrifies in soil and the net long-term effect on soil pH is acidifying, despite the alkaline reaction of the granule itself.
Granule color does not indicate nutrient content — see the yellow DAP page.

Frequently asked questions

Why does fertilizer-grade DAP analyse [CAS?] when the pure compound calculates closer to [CAS?]?

Pure (NH4)2HPO4 has a molar mass of about 132.06 g/mol, which works out to roughly 21% N and about 54% P2O5. Granular fertilizer DAP is made by ammoniating wet-process phosphoric acid, so it carries iron, aluminium, calcium, magnesium, fluoride and sulphate from the phosphate rock, part of the product crystallises as monoammonium phosphate, and the granule holds free moisture. Those components dilute the analysis down to the traded [CAS?] grade. Technical and soluble grades made from purified acid sit closer to the theoretical figure. A grade below the stoichiometric maximum is therefore normal chemistry, not adulteration — judge a lot by its certificate of analysis, not by the formula.

What is the practical difference between DAP and MAP, and when should I take MAP instead?

Both are ammonium phosphates, but they behave differently at the granule. A dissolving DAP granule creates a strongly alkaline micro-zone, around pH 8, and briefly releases free ammonia; a MAP granule (NH4H2PO4) creates an acidic micro-zone, roughly pH 3.5 to 4.5. On calcareous, high-pH soils that alkaline site pushes ammonia volatilisation when the product is left on the surface and speeds calcium-phosphate fixation, so MAP usually holds phosphorus available longer. MAP is also the safer option for seed-placed applications. Nutrient ratio is the second difference: DAP brings more nitrogen per tonne, MAP more phosphate. Choose DAP where the extra nitrogen is genuinely wanted and the product will be banded or incorporated.

What should DAP never be blended or tank-mixed with?

Keep it away from alkaline materials. Ground limestone, hydrated lime, basic slag, wood ash and calcium cyanamide all strip ammonia out of DAP: nitrogen is lost and a closed warehouse fills with ammonia. In bulk blending, urea is the classic risk pair — a solid mixture has a critical relative humidity lower than either component on its own, so the blend takes up moisture and cakes at humidity neither would absorb separately. Blend shortly before dispatch and keep the blend dry and sealed. In solution, never put DAP and calcium nitrate, or DAP and hard high-calcium water, into the same concentrated stock tank; dicalcium phosphate precipitates and blocks filters and emitters. Use separate tanks and dilute before the streams meet.

How should DAP be stored, and what does an ammonia smell in the warehouse indicate?

DAP is hygroscopic, though less aggressively so than urea or ammonium nitrate — its critical relative humidity is roughly 80% at 30 °C. Above that threshold granules take up water, then cement into lumps as they dry again. Store bagged or in a sealed bulk pile, off the floor, away from direct sun and rain, with humidity control in port storage and monsoon climates, and leave sealed coated bags closed until use. A clear ammonia smell means the product is losing NH3 — driven by heat, long open storage, or contact with an alkaline material. The analysis then drifts toward MAP and the declared nitrogen falls. Find the cause and resample rather than just ventilating the shed.

How do I compare a DAP offer against a MAP or TSP offer on a fair basis?

Compare cost per unit of nutrient delivered, not per tonne of product. One tonne of [CAS?] supplies 460 kg P2O5 and 180 kg N; one tonne of [CAS?] supplies 520 kg P2O5 and 110 kg N. Value the nitrogen at whatever your cheapest nitrogen source costs, subtract it, and what remains is the implied price of phosphate — now the offers sit on one axis. Then adjust for the specification details that change delivered value: whether the quoted P2O5 is total or available, moisture and water-insoluble content, granulometry and crushing strength, since fines mean handling and spreading losses, and freight and duty calculated per unit of nutrient rather than per tonne of bulk.

When is DAP the wrong choice?

Several situations. On calcareous or high-pH soils with surface application and no incorporation, DAP loses ammonia and its phosphorus is fixed quickly — banding, incorporation or MAP does better. In direct seed contact, particularly on sandy, low-CEC or dry soils and with ammonia-sensitive crops such as canola or flax, free ammonia from the granule injures germinating seed; follow local extension guidance on safe seed-placed rates. Where a soil test already shows high phosphorus, or where local rules cap phosphorus loading, additional P2O5 is a cost and a runoff risk rather than yield. DAP also supplies no sulphur, potassium, calcium or micronutrients, so on sulphur-deficient ground an NPS or ammonium sulphate route fits better.

Can fertilizer-grade DAP be used in fertigation or foliar sprays?

Usually not. Fertilizer-grade granules contain water-insoluble matter — iron and aluminium phosphates, gypsum and rock residues — that will not dissolve, settles in the tank, and plugs filters, drippers and nozzles; anti-caking coatings add to the residue. For injection, specify a soluble or technical grade with a stated maximum water-insoluble content, and account for pH: DAP raises the pH of the stock solution, so in hard water it precipitates calcium and magnesium phosphates. Foliar use is limited by scorch risk from ammonia at higher concentrations and by the nitrogen load. Whatever grade you buy, run a jar test with your own water and your own tank partners before committing a season’s programme.