An interesting NSF-funded study this week examined trends in various cities and their residents’ quests for perfect lawns. That naturally led those of us involved in celebrating 2014 International Year of Crystallography to think of fertilizer, which naturally led to ammonium nitrate, a perfect crystal of the week….
Have you ever had your shoes, luggage, or hands swabbed by TSA when passing through an airport security checkpoint? The TSA officers are looking for trace residues of compounds used to manufacture explosives. One compound that their explosive trace detection (ETD) systems search for is ammonium nitrate (NH4NO3), a common fertilizing agent. All crops require nitrogen to grow, but even though 78 percent of our atmosphere is gaseous nitrogen (N2), the natural incorporation of nitrogen into the soil in sufficient quantities to supply the rapid growth of dense crop fields is poor. For this reason, farmers use fertilizing agents to supply additional nitrogen and promote crop growth.
Originally, farmers used urea to fertilize their fields. However, even though urea has a higher density of nitrogen than ammonium nitrate, it also tends to break down more quickly once exposed to the open environment of a field. For this reason, many farmers turned to ammonium nitrate as their fertilizer of choice. Ammonium nitrate is a white, crystalline solid at standard pressure and temperature. It’s also relatively stable under normal conditions. But if you expose ammonium nitrate to high temperatures, open flames, or significant mechanical shock (i.e., the detonation of an explosive), then ammonium nitrate decomposes very rapidly and very energetically—that is, the ammonium nitrate explodes.
Remember a catastrophic fertilizer plant explosion in West, Texas in 2013? Unfortunately, that plant had a large stockpile of ammonium nitrate, which was exposed to open flames as a fire spread through the facility. Likewise, the Oklahoma City federal building attack in 1995 was also fueled by detonating barrels of ammonium nitrate, among other compounds.
So, if you use ammonium-based fertilizers in your garden, be careful with their storage! Or better yet, take a cue from organic growers who use plant rotations to infuse nitrogen into their gardens in a more natural manner. One can also till plant waste back into garden beds or mix manure into the soil to supply nitrogen slowly as the organic matter decays.