It may seem incredible that a substance can be both nourishing and lethal. But that's the case in the explosion at the fertilizer plant in the town of West, Texas, which witnesses likened to a nuclear blast.

The substance that killed up to 15 people, injured 180 and wrecked the buildings in a five-block radius is the same stuff that makes the beans and barley grow.

But not all fertilizers are equally dangerous. And the West plant may have been harboring the worst of them all.

Barring any criminal involvement, in which case all bets are off, the two most likely culprits in the blast are anhydrous ammonia and ammonium nitrate. Both are inorganic fertilizers that were being stored in "substantial amounts" at the West Chemical and Fertilizer Company building, the New York Times reports.

Anhydrous ammonia is a gas that can cause severe burns when it comes in contact with human skin, Richard Ferguson, professor of soil science at the University of Nebraska-Lincoln told TechNewsDaily. But it won't burst into flames.

"You need to use safety gloves and goggles around [anhydrous ammonia], but it’s not explosive," said Ferguson. "It's not very flammable at all … If an [anhydrous ammonia] tank ruptures, the tank is going to decompress explosively, but that’s not chemical, that’s physical." Think of a balloon bursting rather than erupting in a fireball.

Ammonium nitrate is another story. This fertilizer is a gravel-like solid, making it far easier to handle and transport. But when mixed with a fuel and ignited, it becomes a powerful explosive.

Ammonium nitrate was the primary explosive used in the Oklahoma City bombing on April 19, 1995. Bomber Timothy McVeigh and his accomplice Terry Nichols purchased forty 50-pound bags of the fertilizer from a supply company in Kansas, according to their testimony. The ammonium nitrate, mixed with a fuel called nitromethane and an explosive used by commercial mining companies called Tovex, was enough to level the Alfred P. Murrah Federal building, damage 324 buildings within a sixteen-block radius and kill 168 people, according to the Oklahoma City Police Department report.

At the Texas fertilizer plant, something else served as a fuel: firefighters responded to a fire on the premises at around 7:30. The explosion occurred at 7:50

"In many parts of the country, ammonium nitrate has largely disappeared from the market because of the concern for its safety," Ferguson told TechNewsDaily.

In a now-infamous report filed before the accident, Texas regulators claimed that the plant’s "worst-case scenario" would be a 10-minute release of gas that would injure no one. That sounds like anhydrous ammonia. If it were to escape into the atmosphere, it would dissipate so rapidly that it wouldn’t be able to cause anything more than skin irritation and a bad smell, said Ferguson.

Ferguson was hesitant to speculate, but did say that ammonium nitrate appears to be the more likely candidate for the force behind the West, Texas, explosion., even though the plant primarily stored anhydrous ammonia.

"I’m not aware of any state where ammonium nitrate is predominant source [of inorganic fertilizer] where 30, 40 years ago it might have been," Ferguson told TechNewsDaily. “And that’s related primarily to security concerns.”

So why is ammonium nitrate still in use? It's cheap and effective.

You could be forgiven if you associate fertilizer with manure — organic waste is the oldest and most natural way to boost soil nutrient content. And it’s very hard to make poop explode.

But manure alone doesn’t cut it. "'There just aren’t enough organic sources of fertilizer [i.e. manure] to raise the crops that we raise today," said Ferguson. "The foods that we eat are all primarily reliant on inorganic fertilizers ... Our society today would not be where it is without inorganic fertilizer."

One of the more popular synthetic options is urea, an ammonium-based compound that is safer to transport, handle and disperse than anhydrous ammonia and ammonium nitrate. But it is a bit more complicated to manufacture, and also works less efficiently than ammonium nitrate, because when it hits the soil, the resulting reaction lets a lot of nitrogen escape into the atmosphere.

This "volatile loss," as it’s called, is amplified in soils with a low acidity. And as a general rule, soil in the western part of the U.S. tends to be more alkaline (the opposite of acidic), meaning urea is a less effective fertilizer in parts of Texas.

Another option is ammonium sulfate. This is the substance primarily used at the American Plant Food Corporation, a producer and marketer of fertilizer based in Houston. Ammonium sulfate is well-suited to alkaline soils, and is less flammable, but it offers fewer nutrients for its weight than ammonium nitrate.

The West, Texas, disaster will, if anything, only accelerate the declining usage of ammonium nitrate as a fertilizer, Ferguson said. But because of its low price, effectiveness and relative ease of transport, ammonium nitrate will probably still be in use — and still pose a danger — for some time.

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