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The Texas fertilizer plant explosion is horrific. But how common is this?

Yet another bit of awful news to add to this week: A massive explosion at a fertilizer retail facility in central Texas on Wednesday killed as many as 15 people and left more than 160 wounded.

Investigators are still trying to determine the exact cause of the blast. But the explosion does call attention to the $10-billion dollar U.S. fertilizer industry, which underpins our agricultural system and has been expanding of late.

Fertilizer production and storage comes with some risks. The West, Texas plant stored and blended anhydrous ammonia — a pungent gas with suffocating fumes used as a fertilizer. It also contained as much as 270 tons of ammonium nitrate, which can explode if mixed with fuel and ignited. That's fairly rare, but it does happen — there have been at least 16 major explosions worldwide since 1921.

So here's a basic overview of the fertilizer industry — how big it is, how common explosions are, how often these facilities are inspected:

How big is the U.S. fertilizer industry?

In 2011, the U.S. fertilizer industry reported some $10 billion in revenues. The United States as a whole shipped about $4.5 billion worth of fertilizer overseas and imported another $13 billion worth. We still import about half the synthetic nitrogen fertilizer we use.

Most of our imported nitrogen fertilizer come from Canada, Russia and Trinidad and Tobago — all places with plenty of natural gas to make the stuff. (More on that in a sec.)

Is the U.S. fertilizer industry growing? 

Yes, and fast. Mainly because the United States is now awash in cheap natural gas. A great deal of fertilizer is synthesized from atmospheric nitrogen and natural gas — that was likely the case with the ammonia stored in the retail facility in West, Texas.

During the early 2000s, the fertilizer industry had been moving abroad to places with natural gas — like Trinidad and Tobago. But the fracking boom has given the United States its own cheap shale gas, and producers are now returning home. One Egyptian company, for example, is investing $1.4 billion in a fertilizer plant in Iowa near a gas pipeline.

Meanwhile, the global demand for fertilizer keeps growing, particularly after widespread shortages and food price spikes in 2007 and 2008. As the chart below shows, the world's appetite for nitrogen, phosphate and potash has been rising quickly since then:

So how many fertilizer plants are there in the United States?

According to a report from the Fertilizer Institute, there are 44 production plants around the country. And 30 of those are nitrogen plants:

But notice that West, Texas isn't on that map. That's because the fertilizer facility that exploded wasn't a production plant. It was a retail facility, one of approximately 6,000 around the country that sells directly to farmers in a 50- to 100-mile radius.

"There is no national list of retail facilities, but each state registers and regulates them," Kathy Mathers, VP of Public Affairs at The Fertilizer Institute.*

How common are explosions?

Based on data from the Guardian, there have been at least 16 unintended explosions of ammonium nitrate since 1921 that have led to casualties. Six of those have occurred in the United States.

The largest and deadliest took place in 1947, when a fire on board a French vessel docked in the Port of Texas City detonated some 2,300 tons of ammonium nitrate. All told, 581 people died — including most of Texas City's fire department. It still ranks as the deadliest industrial accident the country has ever seen.

Early reports suggest there was as much as 270 tons of ammonium nitrate present at the facility in West, Texas. The facility also stored and blended anhydrous ammonia, which is relatively more stable and can only ignite at very high temperatures of 1,562 degrees Fahrenheit.

Okay, so why do we use ammonium nitrate if it's so dangerous?

Brendan Koerner actually answered this exact question for Slate in 2005. Mainly, it's far too convenient to ignore: "[A]mmonium nitrate is in many ways one of the best (and certainly one of the cheapest) sources of crop-nourishing nitrogen available. For starters, ammonium nitrate is inexpensive to manufacture. ... Ammonium nitrate is also well-suited to bolstering certain types of crops. It's quite effective with fruit trees, for example, providing more efficient nitrogen delivery than ammonium sulfate."

What about safety precautions? Do regulators inspect these facilities?

Yes, although it's not at all clear that they was sufficient. The facility in West, Texas had been fined $2,300 by the Environmental Protection Agency back in 2006 for not having a risk-management plan that was up to federal standards, according to WFAA. After that citation, West Fertilizer Co. vowed to meet standards for its ammonia storage tanks — including daily in-house inspections and water-spray systems in case of accidental releases.

For its part, the operators of the West Texas facility thought an explosion was impossible. The Dallas Morning News obtained a copy of the facility's internal safety review for fire or explosive risks. "The worst possible scenario, the report said, would be a 10-minute release of ammonia gas that would kill or injure no one."

As for other oversight: The Occupational Safety and Health Administration tends to be understaffed and inspections are relatively infrequent. The Texas fertilizer industry has only seen six inspections in the past five years — and the West Texas Fertilizer Co. facility was not one of them.

Are there other downsides to synthetic fertilizer besides occasional explosions?

Sure. Here's Tom Philpott with a short rundown: "Industrial agriculture's reliance on plentiful synthetic nitrogen brings with it a whole bevy of environmental liabilities: excess nitrogen that seeps into streams and eventually into the Mississippi River, feeding a massive annual algae bloom that blots out sea lifeemissions of nitrous oxide, a greenhouse gas 300 times more potent than carbon; and the destruction of organic matter in soil."

Can we do without a fertilizer industry?

Not easily. As Fred Pearce in a Yale Environment360 essay, chemical fertilizer currently helps feed some 3 billion people worldwide. And synthetic nitrogen fertilizer has lifted the “carrying capacity” of soil around the world from 1.9 people per hectare of farmland to 4.3 people.

That said, some scientists have argued that we need to look for ways to reduce our reliance on synthetic fertilizer — for many of the reasons Philpott mentions above. Suggested strategies include breeding crops that are more efficient at absorbing nitrogen, diversifying what we grow (the main U.S. crop, corn, is extremely nitrogen-intensive) and developing farming systems that manage nitrogen better.

*Correction: Replaced the Guardian's map of fertilizer plants, which needed a tweak.