A practical guide to MAP, why its acidic reaction suits high-pH soils, how soluble and granular grades differ, and how it is supplied, from a US-based distributor.
Monoammonium phosphate (MAP, NH4H2PO4, CAS 7722-76-1) is the most widely used dry phosphate fertilizer, supplying nitrogen and phosphorus together. Water-soluble 12-61-0 grade is made for fertigation and foliar use; granular grades go into dry blends. Its acidic reaction keeps phosphorus available on calcareous soils where alkaline sources tie up. Stratus Chemical supplies both across the United States.
| Property | Value |
|---|---|
| Chemical name | Ammonium dihydrogen phosphate |
| Synonyms | MAP, monoammonium phosphate, monobasic ammonium phosphate, ammonium phosphate monobasic |
| CAS number | 7722-76-1 |
| Molecular formula | NH4H2PO4 |
| Molecular weight | 115.03 g/mol |
| EC / EINECS | 231-764-5 |
| Appearance | White crystalline solid |
| Grades supplied | Water-soluble 12-61-0, granular |
| Packaging | Large bags, bulk |
| Documentation | COA & SDS provided on request |
The figures below are representative of water-soluble 12-61-0 grade MAP. They represent average properties rather than a guaranteed specification: individual shipments vary, and the certificate of analysis for your material governs.
| Parameter | Typical value |
|---|---|
| Total nitrogen (N) | 12% |
| Ammonium nitrogen (NH4) | 12% |
| Total phosphate (P2O5) | 61% |
| Purity | 99.8% |
| Moisture | Below 0.2% |
| Insoluble matter | Below 0.1% at 20 °C |
| Water solubility | 360 g/L |
| Density | 1.02 kg/L |
| pH | 4.2 – 4.8 |
| Appearance | White, crystalline |
Note that all of the nitrogen is present as ammonium. There is no nitrate, urea, or free ammonia, which matters for tank compatibility and for crops sensitive to nitrate form.
Both are monoammonium phosphate, and confusing them is a common and expensive mistake. The difference is purity and what the product is built to do.
The 12-61-0 grade is highly purified crystalline material, fully water-soluble, made for delivery through an irrigation system or a spray tank. That extra refining is what the higher analysis reflects: less impurity means more nutrient per ton.
The 11-52-0 grade is the granular fertilizer product, made by a less refined route directly from phosphate rock. The impurities that remain account for the lower analysis. It is intended for dry blending and direct soil application, and dissolving it in a fertigation system will put residue into the lines.
Specify by intended use rather than by price per unit of P2O5, because on that measure the granular product always looks better and will still block your emitters.
This is the property that decides MAP against DAP, and it operates in a very small volume of soil.
When a fertilizer granule dissolves, it creates a concentrated zone of solution immediately around itself, and the pH of that zone governs what happens to the phosphorus next. MAP dissolves acidic, around pH 4 to 4.8. DAP dissolves alkaline, near pH 8.
Phosphorus is the least mobile of the major nutrients and is readily fixed into insoluble forms. On calcareous and high-pH soils it precipitates as calcium phosphates, and an alkaline zone around the granule speeds that reaction up. The acidic zone MAP creates works against it, holding phosphorus in a plant-available form longer and giving roots more time to take it up.
There is a second, more immediate consequence. The alkaline zone around a DAP granule can release free ammonia, which injures germinating seedlings if the fertilizer sits too close to the seed. MAP is markedly safer in seed-placed and starter applications for that reason.
The principal use for the soluble grade. It dissolves completely and quickly, supplies phosphorus in the orthophosphate form roots take up directly, and its acidity helps hold other nutrients in solution and discourages scale in the irrigation lines. It blends readily with most other soluble fertilizers, though calcium sources should be kept in a separate stock tank to avoid precipitating calcium phosphate.
Used where a crop needs phosphorus quickly and soil delivery would be too slow or too heavily fixed, particularly at flowering and fruit set in high-value crops.
Granular MAP is a standard component of dry NPK blends and a common starter fertilizer, supplying phosphorus where the young root system can reach it during early growth when uptake is limited.
MAP is used in fire retardant formulations, including wildland fire retardants and treatments for wood and textiles, in flux and metal treatment, and as a nutrient in fermentation.
| Phosphate source | How it behaves | Trade-off |
|---|---|---|
| MAP, water-soluble 12-61-0 | Fully soluble, acidic reaction, all nitrogen as ammonium | The right product for fertigation and foliar use, but priced well above granular grades |
| MAP, granular 11-52-0 | Acidic reaction, good handling and blending properties | Economical for dry blends and soil application, but not suitable for dissolution |
| DAP 18-46-0 | Higher nitrogen, alkaline reaction on dissolving | More nitrogen per ton, but ties up phosphorus on high-pH soils and can injure seedlings near the seed |
| Monopotassium phosphate (MKP) | Supplies phosphorus and potassium, fully soluble, nitrogen-free | Ideal where nitrogen is not wanted alongside the phosphorus, but substantially more expensive |
How to choose: the delivery method decides the grade, and soil pH decides MAP against DAP. If the product goes through an irrigation system or a spray tank, only a true soluble grade will do. For dry application, DAP delivers more nitrogen per ton and is often cheaper per unit of nutrient, so on acidic to neutral soils away from the seed row it competes well. On calcareous or high-pH ground, and in any seed-placed application, MAP's acidic reaction earns its price. Where the nitrogen is not wanted at all, MKP is the alternative to cost out.
Store MAP in a cool, dry, well-ventilated place in closed original packaging, off damp floors. It is hygroscopic enough that exposed material cakes, and caked soluble grade dissolves slowly and unevenly, which defeats the purpose of buying it.
Solutions are acidic and mildly corrosive to mild steel and galvanized surfaces over time. Use plastic or stainless equipment for stock tanks, injectors, and lines.
Keep MAP separate from strong alkalis, which liberate ammonia on contact. In fertigation, do not combine it with calcium or magnesium sources in the same concentrated stock tank, since calcium phosphate will precipitate and both the phosphorus and the calcium are then lost from solution. Follow the Safety Data Sheet for the specific grade.
Monoammonium phosphate is used as a concentrated phosphate fertilizer supplying both nitrogen and phosphorus. Water-soluble grades go into fertigation, foliar feeding, hydroponics, and liquid fertilizer blends; granular grades are used in dry NPK blends and as a starter fertilizer. It is also used industrially in fire retardants and as a raw material for other phosphate products. Stratus Chemical supplies water-soluble and granular grades across the United States.
The CAS number for monoammonium phosphate is 7722-76-1. Its molecular formula is NH4H2PO4 and its molecular weight is 115.03 g/mol. Its EC number is 231-764-5. It is also known as MAP, ammonium dihydrogen phosphate, monobasic ammonium phosphate, and ammonium phosphate monobasic.
Both are monoammonium phosphate, and the difference is purity and intended use. The 12-61-0 grade is a highly purified, fully water-soluble crystalline product made for fertigation, foliar application, and hydroponics, where undissolved residue would block emitters. The 11-52-0 grade is the granular fertilizer product made by a less refined route; it carries more impurities from the phosphate rock, which is why the analysis is lower, and it is intended for dry blending and direct soil application rather than dissolution.
Because of what happens in the soil immediately around the granule. MAP dissolves to give a slightly acidic reaction, with a solution pH around 4 to 4.8, while DAP gives an alkaline reaction near pH 8. On calcareous and high-pH soils, phosphorus is rapidly tied up as insoluble calcium phosphates, and an alkaline zone around the granule accelerates that. The acidic zone MAP creates keeps phosphorus available to the root longer. The alkaline zone DAP creates can also injure seedlings if placed too close to the seed.
A representative water-soluble 12-61-0 grade runs total nitrogen at 12%, all in ammonium form, total phosphate as P2O5 at 61%, purity at 99.8%, moisture below 0.2%, insoluble matter below 0.1% at 20 °C, water solubility of 360 g/L, density of 1.02 kg/L, and pH of 4.2 to 4.8, supplied as a white crystalline solid. Specifications represent average properties rather than guarantees, so confirm against the certificate of analysis for the material you are quoted.
Because anything that does not dissolve ends up in the irrigation system. Drip emitters and micro-sprinklers have very small orifices, and undissolved residue accumulates until flow is restricted or the emitter blocks entirely. Blockages are discovered as uneven crop growth well after the fact and are expensive to clear. An insoluble matter limit below 0.1% is the parameter that separates a true fertigation grade from a granular product that has merely been dissolved.
Stratus Chemical supplies monoammonium phosphate in large bags and bulk, in water-soluble and granular grades, with a certificate of analysis provided for each shipment. Tell us the grade, analysis, and volume and we will confirm pricing and availability within one business day.
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