In June 2016 Turkey put restrictions on the sale of nitrate based fertilizers because it had found that for the last few months most of the locally made terrorist bombs used these types of fertilizers. This is possible because nitrate based fertilizers are sold as a powder that, when mixed with diesel or fuel oil, can be exploded with a detonator. While only about 40 percent of the power as the same weight of TNT, these fertilizer bombs are effective as roadside bombs or for car or truck suicide bombs. Fertilizer bombs are bulkier and a slurry, usually mixed in a plastic jug or a barrel. Moreover, the fuel oil must be mixed thoroughly and in exactly the right proportion, otherwise the explosive effect is much less than expected. It takes at least 3-4 kg (6.6-8.8 pounds) of ammonium nitrate (mixed with some fuel oil) for an average roadside bomb and at more than twice as much for a vehicle based bomb. Thus these bombs weigh at least 5-10 kg (11-22 pounds) and appear to be small plastic or metal containers of thickened fuel oil. When trucks are used there will often be several barrels of slurry all wired together. Turkey believes both ISIL (Islamic State in Iraq and the Levant) and PKK Kurdish separatist rebels are using nitrate based fertilizers for explosives because access to commercial or military explosives has become so difficult.
Earlier attempts to keep nitrate based fertilizers from being used by criminals, or even anyone (as in Northern Ireland) often hurt local farmers more than terrorists. That was because farmers needed a lot more of this stuff than terrorists and at worst bans made nitrate based fertilizers a lot more expensive for terrorists but unreasonably so. Another approach is to add things like calcium carbonate or urea granules to make the nitrate fertilizer less explosive. But in Afghanistan and Pakistan, where these additives were used a lot, if was found that local terrorists soon discovered that the additives were easily removed by using simple, if time consuming, procedures. Thus additives don't solve the problem, it just makes ammonium nitrate a little more expensive for the terrorists to use. The bomb makers have lots more to worry about than additives.
For farmers nitrate based fertilizers cost about $400 a ton. A terrorist could often buy a tenth of that from a legal user for $400 and have enough for 25 bombs. But the experience since the 1980s has been that putting restrictions on the purchase and movement of nitrate based fertilizers will reduce the number of bombs terrorists can build and set off. Turkey has noted that their efforts to halt ISIL and others from smuggling weapons (including explosives) across the Syrian border also hurt terrorist bomb use. Thus the Turks are using whatever tools they have available to limit terrorist bomb activity.