Ammonium nitrate is produced by combining ammonia with nitric acid in a neutralization reaction. This is one of the simplest reactions in industrial chemistry: NH₃ + HNO₃ → NH₄NO₃. The result is a white, crystalline salt that serves as the world’s most common nitrogen fertilizer, delivering nitrogen in two forms that plants can absorb. Nearly all ammonium nitrate is manufactured at industrial scale, and purchasing it commercially is far more practical than producing it yourself.
The Core Chemical Reaction
The synthesis is straightforward on paper. Gaseous ammonia reacts with nitric acid to produce ammonium nitrate in solution. The reaction is highly exothermic, meaning it releases a significant amount of heat. In an industrial setting, this heat must be carefully managed with cooling systems to prevent dangerous temperature spikes. The resulting solution is essentially ammonium nitrate dissolved in water, which then needs to be concentrated and solidified into a usable form.
The nitric acid used in production typically ranges from 30 to 70 percent concentration, though high-strength acid above 90 percent also exists. The ammonia is usually supplied as anhydrous gas. Getting the proportions right matters: the reaction consumes one molecule of each reactant in a 1:1 ratio, and any leftover acid or ammonia creates an impure, potentially unstable product.
How It’s Made at Industrial Scale
Commercial production follows a well-established sequence. First, ammonia and nitric acid are fed into a neutralization reactor, producing a hot ammonium nitrate solution. That solution is then evaporated to increase its concentration, typically to 95 or 99 percent ammonium nitrate depending on the intended use.
The concentrated molten solution is then solidified through a process called prilling. The hot liquid is sprayed from the top of a tall tower (called a prilling tower) in small droplets. As the droplets fall through a stream of cool air rising from below, they solidify from the outside in. The outermost surface cools and hardens first, with solidification moving progressively toward the center of each droplet. This process, also called spray crystallization, produces the small, round pellets (prills) familiar to anyone who has handled granular fertilizer.
Once collected at the bottom of the tower, the prills are cooled further and coated with an anticaking conditioner. This coating prevents the pellets from clumping together during storage and makes them easier to blend with other fertilizers for uniform field application.
Fertilizer Grade vs. Industrial Grade
Not all ammonium nitrate prills are the same. The concentration of the molten solution before prilling determines the final product’s density and porosity, which in turn determines its use.
- High-density prills (fertilizer grade) are made from solution concentrated to about 99 to 99.5 percent ammonium nitrate. These produce solid, dense pellets designed for agricultural spreading.
- Low-density prills (industrial/explosive grade) are made from solution concentrated to roughly 95 percent. Some water remains trapped inside the prill during solidification. When that residual moisture is later dried out, it leaves behind a porous internal structure. Those pores can absorb fuel oil, which is how the commercial explosive ANFO is made for mining and construction.
For agricultural purposes, the high-density fertilizer grade is what you’ll encounter at farm supply stores. It contains about 34 percent nitrogen by weight, split between ammonium and nitrate forms, giving plants both a quick-release and a slower-release nitrogen source.
Safety and Handling Risks
Ammonium nitrate is not flammable on its own. Its NFPA hazard rating gives it a zero for both health risk and flammability. However, it earns a 3 out of 4 for reactivity and carries a special oxidizer designation. That oxidizer property is the source of its danger: ammonium nitrate aggressively feeds fire in other materials, and under specific conditions, it can detonate.
Pure ammonium nitrate begins to decompose irreversibly at around 170°C (338°F), breaking down into nitrous oxide and water vapor. In an open environment with small quantities, this decomposition actually absorbs heat and stops on its own. The situation becomes dangerous when the decomposition gases are trapped, such as in a sealed container or a large, confined pile. Trapped gases can undergo secondary reactions that release enormous energy, turning a slow decomposition into a runaway explosion.
Contamination dramatically lowers the danger threshold. When mixed with oil, charcoal, or other organic materials, ammonium nitrate becomes shock-sensitive and can explode far more easily. Contact with certain minerals like pyrite (iron sulfide, commonly found in some soils and rock) can trigger spontaneous decomposition at temperatures as low as 25 to 50°C, essentially room temperature in warm climates. This is why the Beirut port explosion in 2020 and the Texas City disaster of 1947 were so devastating: large quantities, confinement, and contamination created catastrophic conditions.
Ammonium nitrate must be stored away from acids, metal powders, flammable liquids, sulfur, chlorates, nitrites, and any finely divided organic or combustible material. Storage areas should be cool, dry, and well-ventilated. Moisture causes caking and can degrade the product, while heat and confinement create the conditions for thermal runaway.
Regulations on Purchase and Sale
Because of its potential for misuse, ammonium nitrate is regulated in the United States under the Secure Handling of Ammonium Nitrate provisions of the 2008 Consolidated Appropriations Act. This law directs the Department of Homeland Security to regulate the sale and transfer of ammonium nitrate to prevent its use in terrorism. The Cybersecurity and Infrastructure Security Agency (CISA) oversees a proposed Ammonium Nitrate Security Program that would require registration for buyers and sellers.
In practice, fertilizer-grade ammonium nitrate is still available through agricultural supply channels, but purchases may require identification, and sellers are expected to report suspicious buying patterns. Some states have additional restrictions. If you’re a farmer or landowner looking to buy ammonium nitrate fertilizer, purchasing through an established agricultural retailer is the standard path, and you should expect some documentation requirements. Large-quantity purchases for any purpose will draw regulatory scrutiny.