Some fertilizers are explosive, but most are not. The key ingredient that makes certain fertilizers dangerous is ammonium nitrate, a nitrogen-rich compound that doubles as a cheap industrial explosive used in mining and quarrying worldwide. Other common fertilizers like urea, potassium chloride, and liquid nitrogen solutions pose no explosion risk under normal conditions.
Why Ammonium Nitrate Is Different
Ammonium nitrate contains 35% nitrogen, making it one of the most efficient fertilizers available. That high nitrogen content is also what makes it dangerous. The molecule contains both a fuel component (ammonium) and an oxidizer (nitrate) in one package. Under the right conditions, it can decompose rapidly enough to detonate.
At temperatures above 260°C (500°F), ammonium nitrate undergoes a runaway decomposition reaction. If the material is confined in a sealed space or contaminated with other substances, this reaction can accelerate into a full explosion, producing toxic nitrogen dioxide gas responsible for the orange-brown fireball seen in ammonium nitrate disasters. Without confinement or contamination, the compound typically just burns or fizzles. The combination of heat, confinement, and fuel is what turns a bag of fertilizer into a bomb.
What Turns Fertilizer Into an Explosive
When ammonium nitrate is deliberately mixed with about 6% diesel fuel by weight, it creates a blasting agent called ANFO (ammonium nitrate fuel oil). This 94/6 ratio is the standard formulation used in commercial mining operations around the world. ANFO is typically mixed on-site right before use, because the combination is far more sensitive to detonation than ammonium nitrate alone.
The problem is that unintentional contamination can have a similar effect. Petroleum-based coatings, oils, sawdust, wood shavings, hay, paint, and even grain dust can act as the fuel component. Heavy metals or gritty particles from recycled oils can further increase sensitivity. This is why so many ammonium nitrate disasters involve storage facilities where the fertilizer came into contact with something it shouldn’t have.
Disasters That Show the Risk
The history of ammonium nitrate explosions is long and grim. In 1921, workers at a German factory used small dynamite charges to break up caked ammonium nitrate in a storage silo. One charge triggered the detonation of 50,000 tonnes of material, followed moments later by a second silo nearby. The blast killed over 500 people.
In 1947, two ships carrying more than 3,000 tonnes of ammonium nitrate docked at Texas City, Texas. The fertilizer on board had been formulated with petroleum jelly and petroleum wax, both highly combustible. A fire on one ship set off a chain of explosions that killed nearly 600 people and leveled much of the port.
The most recent large-scale disaster occurred in Beirut, Lebanon, in August 2020. Approximately 2,750 tonnes of ammonium nitrate had been confiscated from a ship and stored in a port warehouse for over six years. A fire broke out in the same hangar, which also contained fireworks. Thirty minutes later, the ammonium nitrate detonated, devastating a wide area of the city.
How Manufacturers Reduce the Risk
Not all ammonium nitrate fertilizer is equally dangerous. Manufacturers can significantly reduce its explosive potential by blending it with stabilizing materials. Adding calcium carbonate (limestone), dolomite, or fly ash suppresses the decomposition reaction that leads to detonation. Research has shown that a combination of up to 25% limestone and dolomite with a fly ash coating can effectively eliminate the detonation hazard.
The stabilizers work through several mechanisms. Fly ash forms a thin film over each ammonium nitrate granule, physically blocking contact with external fuels. Limestone and dolomite absorb energy during the reaction, reducing the heat available to sustain a chain reaction. Minerals like manganese and iron in the fly ash create compounds during any attempted detonation that pull energy out of the reaction, further dampening it. Even when known detonation-boosting chemicals were added to calcium ammonium nitrate in laboratory tests, the stabilized product still refused to detonate at high velocity.
This is why the fertilizer sold under the name “calcium ammonium nitrate” or CAN is considered much safer than pure ammonium nitrate. Many countries restrict or regulate the sale of high-concentration ammonium nitrate and encourage CAN as an alternative.
How to Identify Higher-Risk Fertilizer
Pure ammonium nitrate fertilizer with a concentration around 33.5% nitrogen is classified as a Class 5.1 oxidizer under UN transportation standards. You’ll see this on the packaging as an orange diamond-shaped label with a flame-over-circle symbol. This classification means the product isn’t technically labeled as an explosive, but it can intensify fires and, under the right conditions, detonate. Calcium ammonium nitrate products carry different, lower-risk hazard labels reflecting their reduced sensitivity.
If you’re buying fertilizer for a garden or lawn, you’re unlikely to encounter pure ammonium nitrate at a consumer retail store. Most consumer-grade products use urea, ammonium sulfate, or blended formulations that carry no explosion risk. Ammonium nitrate in large quantities is primarily sold through agricultural and industrial supply channels.
Storage Rules That Prevent Disasters
Federal regulations from OSHA kick in at just 1,000 pounds of stored ammonium nitrate. Buildings must have adequate ventilation or be designed to self-ventilate during a fire, so that decomposition gases don’t build up pressure and create the confinement conditions needed for detonation. No more than 2,500 tons of bagged ammonium nitrate can be stored in a building without an automatic sprinkler system.
The EPA recommends keeping ammonium nitrate at least 30 feet from any combustible or contaminating material, including grain, seeds, oils, hay, sawdust, and paint. Within 50 feet of combustible buildings or materials, fire-resistant walls are required. Storage bins cannot be made from galvanized iron, copper, lead, or zinc, because these metals can react with ammonium nitrate and increase its sensitivity. Even the partitions separating ammonium nitrate from other stored products must be sealed tightly to prevent cross-contamination.
For anyone storing smaller quantities on a farm or property, the same principles apply on a practical level: keep it dry, keep it away from fuels and organic materials, keep it ventilated, and never store it in a sealed metal container. Ammonium nitrate sitting in a well-ventilated, clean, dry space with nothing combustible nearby is a stable, useful fertilizer. The same material stored carelessly next to diesel drums in a locked shed becomes a serious hazard.