Diammonium phosphate (DAP): properties and applications

Diammonium phosphate (DAP) is a high-concentration complex nitrogen-phosphorus mineral fertilizer and a multifunctional technical raw material. It is a crystalline or granular substance, white, light grey or yellowish in colour, readily soluble in water. Diammonium phosphate contains two key nutrients simultaneously — nitrogen in ammonium form and phosphorus in water-soluble form — making it one of the most in-demand phosphorus-containing fertilizers in global crop production, as well as a valuable component in industry, food and pharmaceutical sectors.

Chemical Nature and Physicochemical Properties

The chemical formula of diammonium phosphate is (NH₄)₂HPO₄, with a molecular mass of 132.06 g/mol. The substance consists of colourless transparent monoclinic crystals or a white powder. Its density is about 1.619 g/cm³. Diammonium phosphate is readily soluble in water — at 10 °C, solubility reaches 58 g per 100 ml — but insoluble in alcohols, acetone and liquid ammonia. Aqueous solutions have a weakly alkaline reaction: pH 7.8–8.2 for 50 g/L at 25 °C.

A key feature of diammonium phosphate is its ability to create a temporary slightly alkaline environment around the granule when dissolved in soil. This distinguishes it from monoammonium phosphate (MAP) and requires caution when applied in close proximity to germinating seeds, since the released ammonia can have a suppressive effect on young plants. However, over time, as ammonium nitrogen nitrifies, the pH around the granule decreases and this effect disappears.

The standard agricultural grade of diammonium phosphate has a composition of 18:46:0, meaning it contains 18% total nitrogen (all in ammonium form) and 46% phosphorus calculated as P₂O₅. It is this high concentration of nutrients combined with excellent physical properties — non-hygroscopicity, absence of dust and tendency to cake — that makes DAP one of the most cost-effective phosphorus fertilizers in the world.

Classification, Grades and Standards

Requirements for technical diammonium phosphate in Russia and CIS countries are regulated by GOST 8515-75 "Diammonium phosphate. Specifications". According to the standard, the product is obtained by neutralising thermal or extraction phosphoric acid with ammonia, followed by crystallisation and centrifuging. The standard provides for two grades:

• Grade A — food grade, with a nitrogen content of about 22.4% and P₂O₅ of about 50.5%. Used in the food and pharmaceutical industries.

• Grade B — technical grade, with a nitrogen content of about 21.5% and P₂O₅ of about 48.5%. Used for impregnating wood and fabrics to impart fire resistance, as well as for other technical purposes.

For agricultural use, diammonium phosphate is produced in the form of granules predominantly 2 to 5 mm in size (at least 95%), with a content of particles less than 1 mm not exceeding 2%. The mass fraction of moisture must not exceed 1.0%. The product is non-hygroscopic, non-dusting and has good flowability, ensuring ease of storage, transportation and application accuracy.

Production Technology

Industrial production of diammonium phosphate is based on the neutralisation reaction of phosphoric acid with ammonia. The process is carried out in two stages. In the first stage, phosphoric acid reacts with ammonia to form monoammonium phosphate (NH₄H₂PO₄). In the second stage, additional ammonia is added to the monoammonium phosphate, resulting in the formation of diammonium phosphate: NH₄H₂PO₄ + NH₃ → (NH₄)₂HPO₄.

The reaction is accompanied by the release of a significant amount of heat, which is used to evaporate water and concentrate the solution. The resulting slurry is granulated, dried and cooled. Various technologies are used to obtain granules with good physical properties: drum granulators, tubular reactors and fluidised bed apparatuses.

Both thermal (purer) and extraction phosphoric acid can be used as raw materials. For fertilizer production, extraction acid is more commonly used, which reduces the cost of the product. An important advantage of diammonium phosphate is the possibility of using phosphoric acid with a relatively high impurity content, making it an economically efficient product.

Agronomic Value and Applications

Basic (pre-sowing) application. Diammonium phosphate is a universal fertilizer for all soil types and a wide range of agricultural crops. It is applied in autumn before autumn ploughing or in spring before pre-sowing cultivation. The high phosphorus content ensures powerful root system development, while ammonium nitrogen has a prolonged effect, gradually transforming in the soil.

Pre-sowing (row) application. Due to its high phosphorus concentration and good solubility, DAP is widely used 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. However, care should be taken not to allow direct contact of granules with seeds due to the temporary alkalisation of the soil solution.

Preparation of fertilizer blends and complex fertilizers. Diammonium phosphate is a preferred component for preparing blended fertilizers and complex NPK fertilizers due to its high concentration, non-hygroscopicity and excellent physical properties. It mixes well with other granular fertilizers, does not cake and ensures homogeneity of the mixture.

Fertigation systems and foliar feeding. Diammonium phosphate is fully soluble in water, allowing its use in drip irrigation systems and for preparing solutions for foliar feeding. However, due to the weakly alkaline reaction of its solutions, its use in fertigation requires pH control and compatibility with other components.

Technical and industrial applications. In addition to agriculture, diammonium phosphate finds wide application in industry. It is used as a flame retardant for impregnating wood, fabrics, paper and cardboard. In the food industry, it is used as an acidity regulator, emulsifier and stabiliser, as well as in yeast production and in winemaking to maintain fermentation. In pharmaceuticals — as an excipient and phosphorus source. In metalworking — for phosphating surfaces. Diammonium phosphate is also used in match production, in photography and as a component of fire-extinguishing powders.

Safety and Storage

Diammonium phosphate belongs to substances with low toxicity (hazard class 4). When working with the fertilizer, personal protective equipment is recommended: respirator, safety goggles and protective clothing to prevent irritation of mucous membranes by dust. The product is non-flammable and explosion-proof, but when heated above 70 °C it can decompose with the release of ammonia.

Storage is carried out in closed, well-ventilated warehouse premises protecting the product from atmospheric precipitation and ground water. Diammonium phosphate is non-hygroscopic and practically does not cake when stored properly. Joint storage with alkalis, hypochlorites, oxidisers and strong acids is not allowed. 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 mono- and diammonium phosphates demonstrates steady growth, driven by global population growth, increasing food demand and the need to improve the efficiency of arable land use. Diammonium phosphate remains one of the most widely traded phosphorus fertilizers in the world. The largest producers are China, the USA, India, Russia and the Middle East. The Asia-Pacific region dominates consumption, 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

Diammonium phosphate (DAP) remains one of the key phosphorus-containing fertilizers in global crop production due to its high nutrient concentration, good solubility and versatility of application. Compliance with agronomic requirements — proper granule placement, use of fertilizer blends and water-soluble grades — allows its potential to be realized and ensures high efficiency of phosphorus nutrition. Organising supplies of diammonium phosphate, including grade and granulometry selection, logistics and financial support, requires an individual approach.