It’s one of those coincidences that aren’t really accidental when you dig a little deeper. As we’ll see, the good-guy and bad-guy properties both stem from the fact that nitrogen gas is made up of molecules that strongly resist being torn apart. First, the fertilizer role.
Every gardener knows that nitrogen is one of the three main elements that fertilizers provide, along with phosphorus and potassium. Nitrogen is extremely abundant; it makes up about 78 percent of the air we breathe. Its molecules consist of pairs of nitrogen atoms bound together into two-atom molecules, which chemists symbolize as N2.
Those two nitrogen atoms are tied together so tightly that plants can’t split them apart to get their nitrogen fixes. They need the help of lightning, which undeniably has enough power to do the job as it cracks through the air. Also, there are certain so-called nitrogen-fixing bacteria and algae that can split nitrogen molecules, but they haven’t told us exactly how they do it.
We humans must resort to our powerful chemical technology in order to convert those nitrogen molecules into more plant-usable forms, such as ammonium compounds or nitrate compounds. The fertilizer ammonium nitrate contains nitrogen atoms in both of these forms, which makes it a doubly potent fertilizer.
Now what if the two separated nitrogen atoms in ammonium nitrate were suddenly given the chance to pair up again into strong molecules of nitrogen gas? They would grab that opportunity eagerly.
After all, if nitrogen atoms love one another so much that when paired up they strongly resist being split apart, wouldn’t they want to break out of the ammonium nitrate to reestablish their tight pairings and become nitrogen gas again? They would do that with such eagerness that they would literally explode out of the ammonium nitrate to rejoin each other and fly away into the air in blissful gaseous freedom.
I have just described an explosion: anytime a solid turns into a gas with great suddenness. The wave of released gases, which are expanding rapidly because of the heat that is also being released, is the pressure that does all the damage.
In the case of ammonium nitrate, which contains oxygen and hydrogen atoms as well as nitrogen, it’s not just the nitrogen atoms that combine suddenly into tight gas molecules. Oxygen and water molecules are almost as tightly held together as nitrogen molecules are, so the oxygen atoms pair up into oxygen gas (O2), while the hydrogen and oxygen atoms join up to form water vapor (H2O). If given the chance, then, solid ammonium nitrate will suddenly break up and turn into an enormous volume of gases: nitrogen, oxygen and water vapor.
All it takes for ammonium nitrate to decompose violently in this way is heat: enough to reach a temperature of at least 570 degrees Fahrenheit (300 degrees Celsius). Even at temperatures as low as 340 degrees Fahrenheit (170 degrees Celsius), ammonium nitrate can explode, turning somewhat less violently into nitrous oxide gas and water vapor.
Keep your powder dry, certainly. But also keep your fertilizer cool.