While Afghanistan has banned the importation, sale and use of ammonium nitrate fertilizer, Pakistan has not. This fertilizer, mixed with the right amount of fuel oil, can be detonated as an explosive. The Taliban in Afghanistan are willing to pay high prices for ammonium nitrate, and the smugglers have responded. The easiest way to smuggle stuff in is to simply bribe the border guards, and off you go. If necessary, local military and police commanders may also have to be bribed, as these are the guys who enforce the ban inside Afghanistan. Depending on the attitudes of local military, police and tribal commanders, the cost of bribes can vary a lot. If the security forces are taking a lot of losses from roadside bombs, the cost of bribes may be very high or, worst of all, the local security forces may not be taking bribes at all, and are out to seize all the ammonium nitrate. Same with tribal leaders, if there have been a lot of civilian casualties (which the Taliban try to avoid). But there are a lot of border crossings, and some are always bribable. So the truckloads of ammonium nitrate may have to travel a lot farther, taking a circuitous route. But if the price is right, the stuff will get to where the buyer wants it.
The Taliban can afford to pay, because of their links to the drug gangs. Both groups want foreign troops out of the country, so heroin can be produced and smuggled without interference. Some 90 percent of the roadside bombs use ammonium nitrate. The government has, for years, been urging farmers to stop, for environmental reasons, using ammonium nitrate. Farmers don't like urea based fertilizers, which are now the only kind they can legally use.
In Afghanistan, IEDs (Improvised Explosive Device, a roadside, or suicide car bomb) now cause over 70 percent of NATO casualties. It has also been discovered that there was one big difference between the IEDs in Iraq and Afghanistan; the explosives used. In Iraq, there were thousands of tons of munitions and explosives scattered around the country after the 2003 invasion was over. This was the legacy of Saddam Hussein, and the billions he spent on weapons during his three decades in power. The Iraqi terrorists grabbed a lot of these munitions, and used them for a five year bombing campaign.
With no such abundance of leftover munitions, the Taliban had to fall back on a common local explosive; ammonium nitrate. While only about 40 percent the power of the same weight of TNT, these fertilizer bombs are effective as roadside bombs. But they are bulkier and a slurry. Moreover, the fuel oil must be mixed thoroughly and in exactly the right proportion, otherwise the explosive effect is much less than expected. But the biggest problem is that if you can't get the ammonium nitrate, you have no explosives. To help make that happen, NATO and Afghan troops have been travelling the roads, setting up random checkpoints to examine truck cargo. With the foreign troops along, no bribes are possible, and the Afghan troops can often find illegal cargoes faster.