Diammonium phosphate (DAP) (chemical formula (NH4)2HPO4 ) is one of a series of water-soluble ammonium phosphate salts which can be produced when ammonia reacts with phosphoric acid. DAP is used as a fertilizer and a fire retardant. When applied as plant food, it temporarily increases the soil pH (more basic), but over a long term the treated ground becomes more acidic than before upon nitrification of the ammonium. It is incompatible with alkaline chemicals because its ammonium ion is more likely to convert to ammonia in a high-pH environment.
DAP is sometimes used as a yeast nutrient for brewing mead, and is an additive in some brands of cigarettes.