Ammonium nitrate was the first solid nitrogen (N) fertilizer produced on a large scale, but its popularity has declined in recent years. It’s been a common N source because it contains both nitrate and ammonium, and it has a relatively high nutrient content.
Large-scale production of ammonium nitrate began in the 1940s when it was used for munitions during wartime. After the end of World War II, ammonium nitrate became available as a commercial fertilizer. The production of ammonium nitrate is relatively simple: Ammonia gas is reacted with nitric acid to form a concentrated solution and considerable heat.
Prilled fertilizer forms when a drop of concentrated ammonium nitrate solution (95 percent to 99 percent) falls from a tower and solidifies. Low-density prills are more porous than high-density prills and are preferred for industrial use, while high-density prills are used as fertilizer. Manufacturers produce granular ammonium nitrate by repeatedly spraying the concentrated solution onto small granules in a rotating drum.
Since ammonium nitrate is hygroscopic and therefore readily attracts moisture from air, it’s commonly stored in air-conditioned warehouses or in sealed bags. Manufacturers typically coat the solid fertilizer with an anti-caking compound to prevent sticking and clumping.
Small quantities of carbonate minerals are sometimes added prior to solidifying, which eliminates ammonium nitrate’s explosive properties. These additives lower the N concentration and are sparingly soluble, making the modified product less suitable for application through an irrigation system (fertigation).