As the catastrophic fertilizer plant explosion in West continues to elicit debate about regulation, one factor that hasn’t been discussed as much is the soil of Central Texas.
Texas’ diverse geology and soil compositions have led to an equally diverse system of fertilizer distribution. Each type of fertilizer has different uses and risks — some work better on particular crops and soils than others.
According to Mark L. McFarland, a soil fertility specialist at Texas A&M University, there are five different agricultural fertilizers widely used in Texas. Ammonium nitrate was stored in large quantities at a depot in West that exploded last month, killing at least 14 people and injuring hundreds more. The other types used in the state include anhydrous ammonia, urea, ammonium sulfate, and a liquid solution of urea and ammonium nitrate, often called UAN.
Much of the soil in Central Texas, where many farmers got their fertilizer from the West plant, is alkaline. That attribute is related to the large amount of calcium carbonate — the chief component of limestone — in the region's geology.
Alkaline soils exacerbate the problem of fertilizer evaporation. Farmers tilling alkaline face decreased returns from using liquid fertilizer.
"Dry, straight ammonium nitrate has an extremely low volatility risk on any soil," McFarland said, referring to the fertilizer’s low evaporation rates. In addition, ammonium nitrate has high levels of nitrogen, which along with ammonia is critical for plant growth, and is ideal for growing grain.
While anhydrous ammonia, a gas, is the cheapest and easiest fertilizer to produce, it must be injected into soil using specialized equipment, or else much of the benefit is lost. The gas poses a combustion risk if subjected to a heat source. But according to the most recent reports, the substance, held at Adair Grain Co.’s facility in West, didn't contribute to the blast — and tanks holding it remained intact even after the explosion.
The other four types of fertilizer are deployed in either liquid or powder form. Urea is a good source of nitrogen, but its nutrients evaporate at a high rate. It poses a moderate combustion risk, depending greatly on how it's stored.
The largest stockpile of any type of ammonium nitrate combination in the state, as reported to the Department of State Health Services, is a depot northeastern Amarillo depot owned by Gavilon, an Omaha-based company. The facility reported holding more than 2.1 million pounds of such a liquid UAN solution on an average day in 2012.
Holding these volatile chemicals in a liquid solution almost eliminates the risk of a catastrophic explosion. But that safety comes with a trade-off: UAN volatilizes faster than pure ammonium nitrate. Improperly applied, farmers lose some of the benefit of the fertilizer.
The fertilizer that poses the smallest risk of evaporation, and is the easiest to apply, also happens to be the most dangerous: powdered ammonium nitrate.
But the attribute that makes it versatile — its powdered form — also makes it dangerous. Ammonium nitrate was responsible for the largest non-nuclear explosion to take place on American soil: the Texas City Disaster of 1947.
More than 7,700 tons of the powder exploded, killing 581 people and injuring thousands. It was also the primary chemical used in the 1994 Oklahoma City bombing.
In recent years, ammonium nitrate has become harder for farmers to get, partially as a result of the safety and security practices mandated by the federal government after the Oklahoma City bombing and 9/11.
The Department of Homeland Security requires depots that hold large amounts of high-risk chemicals to report their stockpiles to DHS. Under new rules, facilities that hold large amounts must make security improvements, including security camera coverage and perimeter fencing, which sometimes proves too expensive for smaller operators.
Charles C. Mitchell Jr., an extension agronomist and professor of soil sciences at Auburn University, says he was surprised to hear that there were facilities in Texas holding substantial amounts of ammonium nitrate.
“Ammonium nitrate has pretty much become impossible to get in Alabama, because of all the regulations,” he said. “I was surprised they were still storing it and using it in West.”
As restrictions on the use and storage of ammonium nitrate have increased nationwide, some smaller dealers have stopped selling the fertilizer. Ammonium nitrate is easier to obtain in Texas in part because larger companies, better able to handle the costs regarding safety and security improvements, play a role in the state's diverse fertilizer distribution network, Mitchell says.
But the overall decrease of availability in ammonium nitrate poses risks for Hill Country farmers, who may find it harder to get the fertilizer best-suited for their needs.
*Editor's Note: An earlier version of this story inaccurately referred to the Hill Country's soils as being acidic.