Why does fertilizer explode?

A tremendous explosion occurred at a fertilizer plant in West, Texas, on April 17, 2013.
A tremendous explosion occurred at a fertilizer plant in West, Texas, on April 17, 2013.
© Michael Ainsworth/Dallas Morning News/Corbis

The 2013 fertilizer explosion that killed and injured residents of West, Texas, was notable not only for its terrible outcome but also because explosions from fertilizer manufacturing (or retail, as was the case in West) are a relatively rare occurrence.

For a planet that consumes about 200 million tons of fertilizer a year, accidents are not ubiquitous [source: Plumer]. However, when ammonium nitrate (one potential cause of the West, Texas, disaster and other fertilizer accidents) does blow up, the effects have been historically devastating. On April 16, 1947, a boat filled with 2,300 tons of ammonium nitrate and docked near Texas City, Texas, blew up as the result of a small fire on board. Nearly 600 people were killed [source: Beach]. That 1947 incident has been called one of the worst industrial accidents in U.S. history.

How do fertilizers -- which we often think of as just natural elements good for the earth -- explode so violently?

Let's begin with a little background on fertilizer itself. Plants need a few basic nutrients to grow, and most of them are found in the air and water: things like oxygen, carbon and hydrogen. Of course, they also need some other elements that may or may not be rich in the soil they grow in [source: IFA]:

  • If a plant just needs a little bit of a nutrient, it's called a micronutrient (think boron and chlorine).
  • If a plant needs heaps of the ingredient, it's called a macronutrient (think nitrogen, phosphorus, potassium and sulfur).

That's where fertilizer comes in: It packs the dirt with the secondary elements needed to make a plant flourish. Of course, many of us home gardeners simply would classify good old manure as fertilizer, and we wouldn't be wrong. According to the International Fertilizer Industry Association, any natural or manufactured material that contains at least 5 percent nitrogen, potassium or phosphorus is a fertilizer.

Of course, the fertilizing manufacturing industry produces synthetic -- or as they prefer to call them, mineral -- fertilizers. To manufacture mineral fertilizers, there are a few steps. First you must collect the materials, which are, in fact, found in nature. Then you have to treat them to bolster the concentration or refine the products. After that, you must convert them into a form that can be used by plants, and then you may want to combine those nutrients with others.

Let's get our hands dirty and determine what actually happens to make fertilizer blow up.