Those of you that are down voting this comment just because this skepticism doesn't match your worldview or what you were taught from a textbook (which never tell the whole story) should stop and do a bit of research on your own. There is plenty of accessible evidence that points to nitrogenous fertilizers harming the environment and contributing to global warming without even digging into primary scientific publications.
It doesn't mean that the comment about chemical fertilizers are wrong, that's a more difficult claim to check (fertilizers increase crop yields, but could we support our populations without them if we didn't focus on overproduction). That said, it's what's driving much of the recent research into alternative fertilization methods right now. Chemical fertilizers are damaging and we need alternatives.