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.

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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.

Ammonium Nitrate and Its Starring Role in Fertilizer Accidents

Along with ammonium nitrate, the West, Texas, fertilizer plant was thought to store large quantities of anhydrous ammonia, which is also capable of generating massive explosions under specific conditions.
Along with ammonium nitrate, the West, Texas, fertilizer plant was thought to store large quantities of anhydrous ammonia, which is also capable of generating massive explosions under specific conditions.
Hemera/Thinkstock

One of the main components in manufactured fertilizer is ammonium nitrate. Like we said, nitrogen is one of those macronutrients that plants love, so a lot of fertilizers are nitrogen-based.

Of course, we can't just bottle up some nitrogen and pour it on the ole carrot patch. Atmospheric nitrogen has a really strong chemical bond that plants can't easily break, so fertilizer companies create a nitrogen-based substance that's much easier for plants to take apart. Ammonium nitrate is one such compound. And it's used for good reason: The ammonium part sticks around longer without evaporating, so it's great for hot summer fields, and the nitrate is easily used by plants. Even more compelling in the agricultural industry, it's inexpensive to manufacture. You combine ammonia and nitric acid, and you're done.

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But what makes ammonium nitrate capable of such lethal explosions? Surprisingly, not much. Truly, ammonium nitrate is a relatively stable compound, according to the U.S. Environmental Protection Agency. In other words, when it's just sitting quietly somewhere, ammonium nitrate isn't that big of a problem because it needs a relatively high activation energy (the energy needed to cause a chemical reaction) to explode [source: Encyclopaedia Britannica]. However, if an accident where some sort of detonation -- a spark, or some kind of energy -- occurs, you better believe that ammonium nitrate is deadly. The compound essentially makes its own fuel (from the ammonium) and oxidizer (the nitrate), so its reaction is violent and long-lasting [source: Di Justo].

So it makes sense that a fire seems to be the cause -- and not the outcome -- of what happened in West, Texas. If indeed ammonium nitrate proves to be the culprit, it would most likely have gotten very, very hot (probably in an enclosed space) to react the way it did. The explosion could very well have caused the seismic activity (equal to a 2.1 magnitude earthquake) reported at the scene [source: Di Justo].

Author's Note: Why does fertilizer explode?

It's worth noting that although ammonium nitrate and other fertilizer explosions are terrible, they are rare. But we'd be remiss not to mention that some folks have also noted that fertilizer can make a deadly, intentional weapon. Timothy McVeigh, for one, used a fertilizer bomb in the 1995 Oklahoma City bombing.

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Sources

  • Beach, Patrick. "66 years ago, Texas City fertilizer blast killed 600, injured thousands." Austin American-Statesman. April 18, 2013. (April 18, 2013) http://www.statesman.com/news/news/66-years-ago-texas-city-fertilizer-blast-killed-60/nXQLC/
  • Di Justo, Patrick. "The fertilizer bomb." The New Yorker. April 18, 2013. (April 18, 2013) http://www.newyorker.com/online/blogs/elements/2013/04/texas-fertilizer-plant- -nitrogen-science.html
  • Encyclopaedia Britannica. "Activation energy." Encyclopedia Britannica Online. (April 18, 2013) http://www.britannica.com/EBchecked/topic/4535/activation-energy
  • Environmental Protection Agency. "Explosion hazard from ammonium nitrate." United States Government. December 1997. (April 18, 2013) http://www.epa.gov/osweroe1/docs/chem/ammonitr.pdf
  • International Fertilizer Industry Association. "Website." International Fertilizer Industry Association. (April 18, 2013) http://www.fertilizer.org/ifa/HomePage/FERTILIZERS-THE-INDUSTRY
  • Koerner, Brendan. "Why do we use explosive fertilizer?" Slate.com. April 18, 2013. (April 18, 2013) http://www.slate.com/articles/news_and_politics/explainer/2005/01/why_do_we_use_explosive_fertilizer.html
  • Plumer, Brad. "The Texas fertilizer plant explosion is horrific. But how common is this?" The Washington Post. April 18, 2013. (April 18, 2013) http://www.washingtonpost.com/blogs/wonkblog/wp/2013/04/18/the-texas-fertilizer-plant-explosion-is-horrific-but-how-common-is-this/