Extremely flammable compound in fertilizer speaks to its deadly history

Fertilizer has helped feed the world, but it can also be extremely dangerous, as demonstrated by the apparently accidental explosion Wednesday night in Texas, and by the 1995 terror bombing in Oklahoma City.

Commercial fertilizer typically contains ammonium nitrate, a compound that helps absorb moisture from the atmosphere. While it remains solid at room temperature, it can become extremely volatile if ignited or even heated, causing explosions or fires. Farmers who use it to get more crops out of their fields have also long harnessed its explosive power to clear fields and even form ponds.

Long before Wednesday night’s explosion in West, Texas, a small town near Waco, ammonium nitrate caused a massive explosion in Texas City in 1947. In that case, a fire broke out aboard a French ship docked at the tiny coastal town and ignited 2,300 tons of ammonium nitrate, killing nearly 600 people.

It has also been used widely to cause intentional deaths. In the Oklahoma City bombing, nearly 100 50-pound bags of ammonium nitrate, bought from a farming supply store, were used to make the truck bomb that killed 168 people when it exploded outside a federal courthouse.

Ammonium nitrate has been used as an oxidizing agent in explosives, including the homemade bombs known as improvised explosive devices.

Because of its extreme volatility, there are complex regulations governing the handling of ammonium nitrate, one of the main ones being a prohibition against storing it near combustible materials.