The primary concept of sustainable agricuture is mimicking natural ecosystems.
Basically you look at what happens in a climate zone if you just leave it alone, the plant community that arrises, than you copy it but with harvestable crops.
So say you're in the plains, your look at the different prairie species, and you grow crops that serve the same ecological functions all together.
Preferably with perennial species so you don't have to plow every year, it totally eliminates most of the work and need for fertilizer and insecticide.