There are hard limits to how high ammonia can go - It is the most widely synthesized chemical on earth, and natural gas is most emphatically not a necessary ingredient. There are producers right now making money producing it via the electrolysis pathway in places with cheap electricity. I think this is due to a cartel in ammonia production, because the level of profit that implies for people producing it the lazy way is hard to explain without a cartel limiting production - but it does mean that if ammonia goes much higher a lot of people are going to start turning electricity water and atmospheric nitrogen into fertilizer.