Earlier quoted context omitted.
Without excrement from animals you need synthetic fertilizer, whose production causes massive production causes massive environmental damage eg emissions and algae bloom.
There's also the "classic" option, discovered some 200 years before synthetic fertilizers - crop rotation that includes some form of legumes. IMO with the current state of agriculture and food production we went into a local maximum that is very unsustainable and bad for the environment.
The only reason we need industrial fertilisers, is because its packaged so well and easy to distribute. But if we weren't raping the soil in the first place, we wouldn't have this problem.
Industrialized farming needs to be revolutionised by the same technique that computing went through: a DIY revolution.
If you don't have a garden, now is the time to start. Everyone should have a garden.