The global production of nitrogen fertilizer amounted to some 117.5 million
metric tons in 2018. China was by far the country with the largest
production, with an output of 28.9 million metric tons of nutrient. It was
followed by the United States and India, with 13.6 and 13.3 million
metric tons produced, respectively.
I think so far both Russia and Ukraine have been fighting based on stocks that are already manufactured. For instance Russia has about 2,000 S-300 missile launchers and they probably have at least 10x that number of missiles. (Part of the untold story of the cold war is that the Soviets were driven crazy by the threat of the U-2, SR-71, B-52, B-1, B-2, ... and said "no way" and spent more money than they had on air defense.)
Similarly they have been working through large stocks of artillery shells, rockets, etc.
So, a +/- of $3B is a 16% shock to the fertilizer market. That is plus one year of explosives, but if you are going to war, you might need several years, all at once (pow). We are talking about future prices in the grand scheme.
That is in gross, too, we don't know specific chemical or supply bottlenecks: one kink in ammonia might not mean much except for fertilizer production; if ammonia is produced via hydrocarbon extraction, then fertilizer is not only dependent on energy prices, but competing directly with wartime energy supply.
A B-52 can carry about 30 tons, so that is 33,000 sorties of a B-52. The US has 76 of those, that would be 434 sorties per aircraft. I can picture the B-52 fleet dropping that many bombs in a year.
https://news.ycombinator.com/item?id=29129920
At the time many felt that Russia and China had halted nitrate exports for completely peaceful reasons...
https://en.wikipedia.org/wiki/Acetone_peroxide
contain nitrogen. (TATP is not used as a military explosive because it will blow up if you look at it the wrong way.) In a lot of cases that is going to come from nitric acid which in turn is made from ammonia which could otherwise be used to make fertilizer.
As you point out, Ammonium Nitrate is a powerful explosive which is commonly used in mining, road construction and other peaceful purposes. Ammonium Nitrate is also popular among terrorists because it is easy to get and highly effective in the form of a truck bomb.
Up until WWII Ammonium Nitrate was mixed with TNT
https://en.wikipedia.org/wiki/Amatol
but AN hasn't been used as a military explosive for a long time since now they mix TNT with RDX, Aluminum powder, and other things.