Diammonium phosphate (DAP) is a high-concentration, water-soluble chemical compound and the world’s most widely used phosphorus fertilizer. This inorganic salt is integral to modern agriculture due to its dual-nutrient capacity, but its utility extends far beyond crop fields. DAP is valued for its efficiency and consistent physical properties in various industrial processes. It is a foundational component in the global chemical supply chain, supporting food production and specialized manufacturing applications.
Fundamental Chemistry and Structure
Diammonium phosphate is chemically designated by the formula \((\text{NH}_4)_2\text{HPO}_4\), classifying it as an ammonium phosphate salt. It is produced through a reaction involving ammonia gas and phosphoric acid. The finished product is a highly water-soluble, crystalline salt, typically granulated for easy handling and uniform application. DAP is graded using the Nitrogen-Phosphate-Potash (\(\text{N-P-K}\)) system. The standard commercial grade is 18-46-0, meaning it contains 18 percent nitrogen (\(\text{N}\)) and 46 percent phosphate (\(\text{P}_2\text{O}_5\)) by weight, but no potash (\(\text{K}_2\text{O}\)). This high concentration makes DAP a cost-effective choice by reducing the required volume for transport and storage.
Primary Role in Crop Nutrition
DAP delivers two macronutrients, plant-available nitrogen and phosphorus, directly into the soil profile. Because the compound is highly soluble, it dissolves quickly upon contact with soil moisture, releasing ammonium (\(\text{NH}_4^+\)) and hydrogen phosphate (\(\text{HPO}_4^{2-}\)) ions for immediate uptake by plant roots. The initial dissolution of the granule creates a temporary, highly alkaline zone with a \(\text{pH}\) around 7.5 to 8.0 in the immediate vicinity.
This localized \(\text{pH}\) spike enhances the availability and uptake of phosphorus, especially in acidic soils where phosphorus is often fixed by soil minerals. While plants initially absorb the ammonium component, soil microbes eventually convert much of it to nitrate (\(\text{NO}_3^-\)) through nitrification. This biological conversion releases hydrogen ions (\(\text{H}^+\)), which gradually causes the soil to become more acidic over the long term, neutralizing the initial alkaline effect. Farmers must carefully manage application rates to prevent the temporary ammonia release from harming sensitive germinating seedlings.
Alternative Industrial and Food Uses
Beyond agriculture, DAP is valued in several non-agricultural sectors for its unique chemical properties. It serves as a nitrogen source, or yeast nutrient, in various fermentation industries, including the production of beer, wine, and mead. Supplementing the must or mash with DAP supports robust yeast growth and activity, ensuring complete fermentation processes.
DAP is also used in fire retardants, including those applied in forestry and for treating materials like wood, paper, and fabric. When exposed to heat, DAP decomposes, releasing non-combustible ammonia gas and forming a residue of phosphoric acid and phosphorus oxides. This residue acts as a glassy coating on the material surface, which starves the fire of oxygen and promotes the formation of a carbon char instead of flammable gases. The dual action of gas dilution and surface coating makes DAP an effective fire suppression agent.
Safe Handling and Environmental Impact
Proper handling of DAP is important, as the granular material can generate dust that may irritate the eyes, skin, and respiratory tract if inhaled. The product should be stored in a cool, dry, and well-ventilated location, away from incompatible materials and moisture to maintain stability. While DAP is considered low-hazard, its widespread use necessitates careful application to mitigate environmental consequences.
The primary ecological concern associated with DAP is nutrient runoff into surface waters. When excessive nitrogen and phosphorus are applied, or when heavy rain follows application, the excess nutrients wash into rivers, lakes, and coastal areas. This nutrient overload causes eutrophication, a process where rapid algal growth (a bloom) consumes large amounts of oxygen upon decomposition. The resulting oxygen depletion creates “dead zones” that harm fish and other aquatic life.