A crude car bomb discovered in New York's Times Square this weekend was found to contain gasoline, propane and fertilizer, according to police. Fortunately, the would-be bomber used a type of fertilizer that is not explosive. Experts say that it is actually quite difficult to make a bomb using fertilizer, because of the nature of the chemistry of the explosion.

Ammonium nitrate is the fertilizer compound that can be used in explosives, said John Goodpaster, who researches explosives at Indiana at University-Purdue University Indianapolis. This compound is not found in its pure form in the common fertilizers that are commercially available. And, even in its pure form, ammonium nitrate by itself is not explosive.

"The ammonium nitrate is like the engine behind the explosion, but the engine needs fuel," Goodpaster told Life's Little Mysteries. In fact, bombs need two components beside the fertilizer: a detonator and a fuel. The fertilizer must be mixed with a fuel in an exact ratio, and the detonator must be able to generate sufficient energy, he said.

How bombs explode

The first thing that happens during a fertilizer bomb blast is the explosion of the detonator. It contains a small amount of an explosive compound in it, and when it discharges, it creates what experts call a detonation wave. This detonation wave radiates outward from the detonator at a speed of about 2 to 3 miles per second through the mixture of ammonium nitrate and fuel, Goodpaster said.

The energy of the detonation wave causes the ammonium nitrate in the fertilizer to vaporize – the solid fertilizer becomes a gas in an instant. The ammonium and nitrate molecules break down, and a large amount of oxygen gas is suddenly formed.

The gas released from the decomposing fertilizer is what drives the explosion. The rapid release of oxygen, along with the energy from the detonation wave, ignites the fuel. When the liquid fuel ignites, it rapidly combusts, and even more gas is released.

"All that gas is generated in a very short amount of time," Goodpaster said. "That's what causes the pressure waves of the explosion."

The pressure waves travel at the speed of sound, about 1,100 feet (343 meters) per second, and can damage nearby structures or even kill bystanders if the waves are strong enough, Goodpaster said. Heat is also released during the combustion, and it may be enough to set a car on fire, but most of the damage from such explosions is due to the pressure waves.

Perfect mix needed

The fertilizer and the fuel have to be combined in a just the right proportions, Goodpaster said, or else nothing will happen.

"If they're not mixed the right way, the detonator could go off, but there will be no explosion. It would just burn," he said.

Household fertilizer contains chemicals other than ammonium nitrate, said Steven Van Kauwenbergh, principal scientist with the International Fertilizer Development Center, a non-profit industry group. When other chemicals such as ammonium sulfate or urea are mixed in, the ammonium nitrate is no longer able to explode, he said.

Accidents involving fertilizer explosion are very rare, Van Kauwenbergh said. Perhaps the worst such accident was an explosion that occurred in Germany in 1921. Workers at a fertilizer plant were using dynamite to try to break up a large mass of solidified fertilizer that had formed and was clogging up the functioning of the plant, he said. In the ensuing explosion, at least 500 people were killed.