There isn't enough arable land to feed 8 billion people without fertilizer.

In the late 1800s scientists were modeling that after around 2 billion people population growth would have to stop because there wouldn't be any way to feed everyone. In the early 20th century a bunch of methods to mass produce fertilizer appeared. It was one of the biggest events in human history.

Today you can do the math and determine exactly how many calories a shortage of X tons of fertilizer will cause to global food production.