[bluGill]

When you grow a crop that takes nutrients out of the soil they must be replaced. When you eat the crop in the same location (substance farming) there isn't a problem as your excrement is fertilizer.

When you live in the city that doesn't happen - nobody transports sewage back to the fields. It isn't necessarily a good idea to try: the energy to transport sewage back to the fields needs to come from somewhere; sewage needs to not have harmful chemicals/heavy metals from other processes mixed in.