Organic fertilizer requires that the N, P, and K actually came from somewhere. Often, that's from something else that used non-organic fertilizer. So if a country goes all organic not only are yields down on the fields using organic fertilizer, the supply of organic fertilizer itself could dry up.
Nitrogen can come from the air by using nitrogen fixing crops in a rotation, so it can never run out (N2 is the most abundant gas on the atmosphere). The problem is that growing nitrogen fixers take a lot of land time that could otherwise be growing crops. Also not as nitrogen rich as fossil fertilizers (gas N2 is quite stable, difficult to fixers to extract in high quantities).
