Ammonium phosphate (MAP): properties and applications

Ammonium phosphate (monoammonium phosphate, MAP) is a high-concentration complex nitrogen-phosphorus mineral fertilizer and one of the most in-demand and versatile phosphorus-containing fertilizers in global crop production. It is a granular or crystalline substance, white, grey or slightly coloured, readily soluble in water. Ammonium phosphate contains two key nutrients simultaneously — nitrogen in ammonium form and phosphorus in water-soluble form — making it indispensable for basic, pre-sowing and row application across a wide range of agricultural crops.

Chemical Nature and Agronomic Value

The basis of ammonium phosphate is monoammonium phosphate (NH₄H₂PO₄) with a small admixture of diammonium phosphate ((NH₄)₂HPO₄). It is the monoammonium phosphate basis that gives ammonium phosphate a physiologically slightly acidic reaction, which favourably distinguishes it from many other fertilizers. Nitrogen is in ammonium form, which is not immediately washed away by precipitation and provides prolonged nitrogen nutrition to plants, while phosphorus is in readily available water-soluble form, essential for the formation of a strong root system in the early stages of development.

Ammonium phosphate contains the highest amount of phosphorus among all common solid fertilizers. The most common agricultural grade has a composition of 12:52 (12% nitrogen and 52% P₂O₅), meaning that a minimum volume of ballast is required per unit of active ingredient. This reduces storage, logistics and application costs, and allows high-concentration fertilizer blends to be created. The ammonium form of nitrogen promotes better phosphorus uptake by plants, and the temporary slight acidification of the soil solution around the granule is especially effective on neutral and slightly alkaline soils.

Classification, Grades and Standards

Requirements for ammonium phosphate in Russia and CIS countries are regulated by GOST 18918-85 "Ammonium phosphate. Specifications". The standard applies to ammonium phosphate produced by neutralising phosphoric acid with ammonia and establishes requirements for the product intended for agriculture and retail sale as a concentrated granular nitrogen-phosphorus fertilizer.

Depending on the type of raw material, ammonium phosphate is produced in two grades:

• Grade A — from extraction phosphoric acid, including superphosphoric acid, obtained from apatite concentrate or other phosphate raw materials with a total phosphate mass fraction of at least 30%, from thermal phosphoric acid or from mixtures of these acids.

• Grade B — from extraction phosphoric acid of Karatau phosphorites or other types of raw materials with a total phosphate mass fraction of not more than 28%.

Ammonium phosphate of both grades is produced without additives and with the addition of one of the trace elements — copper, zinc or boron. For the highest class of grade A, the mass fraction of total nitrogen is 12±1%, conditional phosphates (calculated as P₂O₅) — 52%; for the first class — 50±1%. For grade B of the highest class — 10±1% nitrogen and 46% phosphates; for the first class — 10±1% and 44±1%, respectively. The mass fraction of water must not exceed 1.0%.

Granulometric composition for the highest class: mass fraction of granules less than 1 mm — not more than 3%, from 1 to 4 mm — not less than 90%, more than 6 mm — absent. Static granule strength — not less than 6 MPa (for the highest class). The granular form ensures good flowability, uniform spreading and reduced dust losses.

Modern producers also offer specialised grades with increased phosphorus content (e.g., 12:61) for use in fertigation systems and foliar feeding, as well as high-purity water-soluble crystalline grades for greenhouse operations.

Production Technology

Ammonium phosphate production is relatively simple and cost-effective. The most common industrial method is based on the reaction of ammonia (NH₃) and phosphoric acid (H₃PO₄) to produce a monoammonium phosphate slurry, which is then solidified and granulated. The reaction proceeds according to the equation: NH₃ + H₃PO₄ → NH₄H₂PO₄. The process is carried out at a controlled pH (usually in the slightly acidic range) to ensure maximum yield of monoammonium phosphate and minimise the formation of diammonium phosphate.

Another common method is carrying out the reaction in a tubular flow reactor, where the released heat is used to evaporate water and solidify the slurry. Industry also uses various modifications of these methods, including pre-neutralisation and subsequent ammoniation in a fluidised bed reactor.

An important advantage of ammonium phosphate production is that low-quality phosphoric acid with a high impurity content can be used. In the production of other types of phosphorus fertilizers (e.g., complex NPK with a high content of water-soluble phosphorus), the purity of the acid must be higher. The P₂O₅ content in ammonium phosphate is 48–61% depending on the quality of the acid used. Modern research is aimed at improving process efficiency and reducing the carbon footprint, in particular by adding amino acids before ammoniation, which reduces the content of water-insoluble impurities and improves the agronomic efficiency of the product.

Applications

Basic (pre-sowing) application. Ammonium phosphate is a universal fertilizer for all agricultural crops on all soil types. The greatest efficiency is achieved with autumn application for winter cereals and as a basic fertilizer for root crops. The ammonium form of nitrogen provides prolonged nutrition, while phosphorus stimulates root system development.

Pre-sowing (row) application. Due to its high phosphorus concentration and good solubility, ammonium phosphate is ideal for application directly into the seed row during sowing. This provides young plants with readily available phosphorus during the critical period of root system formation. Granular ammonium phosphate is applied in a band or subsoil to place it near growing roots.

Preparation of fertilizer blends and complex fertilizers. Ammonium phosphate is widely used as a component for preparing blended fertilizers and complex NPK fertilizers. Its high concentration and good physical properties (non-caking, dust-free) make it a preferred component for blending.

Fertigation systems and foliar feeding. Water-soluble grades of high-purity ammonium phosphate (e.g., 12:61) are used to prepare solutions for drip irrigation and foliar feeding. Such grades dissolve well in water to form a clear solution, do not clog nozzles and ensure precise nutrient dosing.

Other applications. Ammonium phosphate is also used as a source of phosphorus and nitrogen in the production of feed phosphates, in the microbiological industry and as a component of fire-extinguishing powders.

Safety and Storage

Ammonium phosphate belongs to substances with low toxicity (hazard class 4) and is non-flammable and explosion-proof. When working with the fertilizer, personal protective equipment (respirator, safety goggles, protective clothing) is recommended to prevent irritation of mucous membranes by dust.

Storage is carried out in closed, well-ventilated warehouse premises protecting the product from atmospheric precipitation and ground water. Ammonium phosphate practically does not cake and retains flowability when stored properly. The product is supplied in bulk, in 50 kg bags, and in big bags of 500–1000 kg. The guaranteed shelf life is 6 months from the date of manufacture; the agronomic shelf life is unlimited.

Market and Trends

The global market for ammonium phosphates (including monoammonium phosphate and diammonium phosphate) demonstrates steady growth, driven by global population growth, increasing food demand and the need to improve the efficiency of arable land use. According to industry estimates, the ammonium phosphate market is valued at tens of billions of dollars, with a compound annual growth rate of 3–6% depending on the segment and region. The Asia-Pacific region is the largest producer and consumer, with India and China actively subsidising phosphate fertilizer purchases to support farmers.

Key industry trends include the transition to variable-rate fertilization based on precision agriculture, the development of water-soluble grades for fertigation and greenhouse operations, and growing attention to phosphorus use efficiency and loss reduction. Producers with a reliable raw material base and access to logistics occupy stable positions in this strategically important market.

Conclusion

Ammonium phosphate remains one of the key phosphorus-containing fertilizers in global crop production due to its maximum phosphorus content among solid fertilizers, good solubility and versatility of application. Compliance with agronomic requirements — row application, use of fertilizer blends and water-soluble grades — allows its potential to be realized and ensures high efficiency of phosphorus nutrition. Organising supplies of ammonium phosphate, including grade and granulometry selection, logistics and financial support, requires an individual approach.