Monoammonium Phosphate (MAP) — NH₄H₂PO₄, CAS 7722-76-1

Monoammonium phosphate is the ammonium salt of phosphoric acid in which one of the three acidic protons has been neutralized. It is the most concentrated widely traded solid ammonium phosphate on a phosphate basis, and it is the reference “starter” phosphate in agriculture because the solution formed as a granule dissolves is mildly acidic rather than alkaline. The same compound is manufactured in several purity tiers, from wet-process powder used as an intermediate in compound fertilizer plants through to purified technical grade used in soluble fertilizer solutions and in dry-chemical fire extinguishers.

Substance identity

Specification

Theoretical composition of the pure compound (calculated from the formula):

Typical commercial tiers — ranges only, verify on certificate of analysis:

Properties

• Appearance — colorless to white tetragonal crystals when pure; commercial fertilizer material ranges from white through grey and brown depending on the phosphate rock and process route.

• Solubility in water — approximately 37–40 g per 100 mL at 25 °C, rising steeply with temperature.

• Solution reaction — a 1 % aqueous solution is mildly acidic, typically pH 4.0–4.5. In soil, the saturated solution immediately around a dissolving granule is reported at roughly pH 3.5–4.5.
• Thermal behavior — melts with decomposition around 190 °C, releasing ammonia; thermal decomposition products include ammonia and phosphorus oxides.
• Density — approximately 1.80 g/cm³ (crystal).
• Critical relative humidity — MAP is among the least hygroscopic of the common solid fertilizers; a CRH near 91.6 % at 30 °C is the figure usually published for it, against roughly 82.5 % for DAP and 72.5 % for urea at the same temperature.
• Chemical incompatibility — reacts with strong alkalis (lime, basic slag, calcium cyanamide) to liberate ammonia and to convert soluble phosphate toward less available calcium phosphates.

Applications

• Starter and band-placed phosphorus. Phosphorus moves only millimetres in soil, so early root contact governs uptake. MAP concentrates both N and P in a small volume, and the acidic reaction zone around the granule temporarily depresses local pH, which increases phosphate solubility on calcareous and high-pH soils where P otherwise precipitates as calcium phosphates. This is the specific agronomic reason MAP is preferred over DAP on alkaline soils.

• Seed-safety relative to DAP. Because the dissolving granule does not raise local pH, MAP does not generate the free ammonia that makes DAP hazardous in seed contact. MAP is still a soluble salt and can cause osmotic injury at high rates in close seed placement, so rate limits by crop, row spacing and soil texture still apply.

• Blending base. MAP granules blend physically with urea, ammonium sulfate and potassium chloride to build custom N-P-K ratios.
• Compound fertilizer manufacture. Powder grades are the phosphate feedstock in NPK granulation plants.
• Fertigation and foliar — purified technical grade only. Wet-process powder grades are not appropriate for drip systems.
• Non-agricultural. MAP is the principal active component of ABC dry-chemical fire extinguishing powder, and is used as a flame retardant and in fermentation media.

Handling and storage

Store in a dry, covered, ventilated area on pallets, clear of floors and walls. Keep bags closed and sealed; while MAP is comparatively resistant to atmospheric moisture, blends containing MAP take on the critical relative humidity of the mixture, which is always lower than that of either component alone — blended product must be stored more carefully than straight MAP. Segregate from lime, alkaline materials and strong acids. Avoid dust generation with powder grades; use eye protection and a dust mask when handling loose powder. Heating produces ammonia and phosphorus oxides. Keep out of drains and surface water.

Notes

• The grade figure quoted commercially for MAP is total nutrient (N + P₂O₅), not P₂O₅ alone. A “60 % MAP” carries roughly 11–12 % N and 48–49 % P₂O₅.

• P₂O₅ is a historical oxide convention, not a species present in the product. To convert: P₂O₅ × 0.436 = elemental P; P × 2.291 = P₂O₅.

• Both MAP and DAP acidify soil over time through nitrification of their ammonium nitrogen, independent of the short-lived reaction of the granule itself.

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Frequently asked questions

What is the practical difference between granular [CAS?] MAP and water-soluble [CAS?] MAP?

Both are the same compound, NH4H2PO4, but they are not the same product. Fertilizer-grade granular MAP is produced from wet-process phosphoric acid and typically carries around 11% N and 52% P2O5; the balance is moisture plus iron and aluminum phosphates and other insolubles carried over from the phosphate rock. Technical or water-soluble grade is purified and approaches the theoretical composition of the pure salt, about 12.2% N and 61.7% P2O5, with water-insolubles specified in tenths of a percent. Granular material is intended for broadcasting and bulk blending. Only the soluble grade belongs in drip lines, sprayers or foliar tanks, where insolubles from granular product settle out and block filters and emitters.

On a specification sheet, what is the difference between total, available and water-soluble P2O5?

Total P2O5 is everything present in the sample. Available P2O5 is the fraction dissolving in water plus the fraction dissolving in neutral ammonium citrate, the laboratory proxy for what a root can reach within a season. Water-soluble P2O5 is the strictest of the three and is the figure that governs fertigation behavior and how fast a granule starts working in soil. In sound granular MAP most of the total is water-soluble; a wide gap between total and water-soluble usually points to iron and aluminum phosphates from the acid, or to over-ammoniation during granulation. Always ask which figure is quoted and by which method, because a total-P2O5 number alone says nothing about usability.

How do I compare two MAP offers with different analyses so the comparison is fair?

Convert everything to cost per unit of nutrient before comparing tonnes. A tonne of [CAS?] and a tonne of [CAS?] do not carry the same phosphate, so the cheaper tonne is often the more expensive phosphate. The common method is to value the nitrogen at the cost of the cheapest straight nitrogen source available to you, subtract that, then divide the remainder by the kilograms of P2O5 in the tonne. Then normalize the rest: moisture, since you pay freight on water; water-insolubles; the water-soluble share of P2O5; particle size distribution if the product goes into a blend; and the analytical methods behind each figure. For EU-bound material, also check the cadmium limit expressed per unit of P2O5.

What should MAP not be blended or tank-mixed with?

Three groups. First, alkaline materials: lime, limestone or cement dust, basic slag, calcium cyanamide. Ammonium salts release ammonia on contact with them, so nitrogen is lost and the material cakes; keep them apart in storage as well as in blends. Second, concentrated calcium and magnesium solutions. In fertigation, MAP and calcium nitrate must never share a stock tank, because calcium phosphates precipitate; standard practice keeps phosphates and sulfates in one tank, calcium and chelated iron in the other, with both diluted before they meet. Third, physically mismatched blend partners. If the particle size distribution of MAP differs much from the urea or potash blended with it, the blend segregates during handling and the field receives an uneven application.

When is MAP the wrong choice?

Several cases, and they matter. If soil tests already show high available phosphorus, more phosphate buys no yield and raises runoff losses; this is the most frequent misuse. If sulfur, calcium or potassium is the actual limiting nutrient, MAP supplies none of them and a blend or a different product is the correct answer. If nitrogen is the main requirement, MAP is an expensive nitrogen source; its N is there mainly to carry the phosphate. Granular fertilizer-grade MAP does not belong in drip systems or foliar sprays regardless of price advantage. And on strongly acidic soils, an acidifying phosphate source applied before the pH problem is addressed will lose much of its phosphate to iron and aluminum fixation.

How should MAP be stored, and why do bags cake?

MAP is comparatively well behaved: its critical relative humidity is among the higher values for common fertilizers, roughly 90% at 30 °C, so it takes up atmospheric moisture less readily than urea or ammonium nitrate. Problems come from three directions. Blending lowers that threshold, since a mixture generally has a lower critical humidity than either component alone, so a MAP blend can absorb moisture where straight MAP would not. Temperature swings make moisture inside sealed bags migrate and recrystallize at particle contact points, which is what actually cements a bag solid. Pressure completes the job, so limit stack height and rotate stock. Store off concrete floors, away from lime and cement dust, and protected from sun and rain.

Why is MAP usually preferred over DAP for placement close to the seed?

The difference is the pH of the solution forming around a dissolving granule. MAP dissolves acidic, with a saturated solution near pH 4, so the nitrogen stays as ammonium and no free ammonia appears in the seed zone. DAP dissolves alkaline, near pH 8, and in that band part of the ammonium converts to free ammonia, which can injure germinating seed and young roots at close placement. That is why MAP is the usual choice for in-furrow or seed-placed starter phosphate, while DAP is more often broadcast, where granules are not sitting against seed. Safe placement rates still depend on crop, row spacing, seedbed moisture and soil texture, so follow local agronomic recommendations rather than any general figure.

Related substances

• A dissolving MAP granule creates an acidic zone while a DAP granule creates an alkaline one that releases ammonia – the single difference that governs seed placement and micronutrient availability around the granule.
• When the crop should not receive further nitrogen at that growth stage, MKP supplies the same water-soluble phosphate with potassium instead of ammonium.
• Heavy phosphate applications are the classic trigger of induced zinc deficiency, so the zinc source is best planned alongside the phosphate rather than after the symptoms appear.
• Dicalcium phosphate holds its phosphate in a water-insoluble calcium form, which is why it goes into a feed mixer while MAP goes through a drip line.