Diammonium Phosphate (DAP) — Frequently Asked Questions

By one ammonium ion, and the consequences are practical. DAP carries more nitrogen at 18 percent and slightly less phosphorus at 46 percent P2O5, and it dissolves to an alkaline solution around pH 8 where MAP is acidic. That alkalinity suits acidic soils but can release free ammonia near the granule, which is why MAP is preferred for seed-placed application.

It supplies readily assimilable nitrogen, which yeast requires to build protein and complete a fermentation. A must or wort short of assimilable nitrogen ferments sluggishly or sticks, and in winemaking can produce hydrogen sulphide off-odours. DAP is the standard addition for correcting that, dosed against a measurement of yeast assimilable nitrogen rather than by rule of thumb.

On heating it decomposes and releases ammonia and phosphoric acid species. The acid promotes dehydration of cellulose to form a carbonaceous char rather than volatile flammable fragments, and that char acts as a barrier. It is used to treat timber, paper, textiles and, in solution, as a component of aerial fire-fighting retardants.

18-46-0, meaning 18 percent nitrogen, 46 percent P2O5 and no potassium. It is the most traded phosphate fertiliser in the world, largely because that combination of nitrogen and phosphorus suits base dressing across a wide range of crops and it handles and blends well as a granule.