As their long roots draw nutrients from deep in the ground, perennial crops become their own green manures, their own herbal leys. So the land need not be taken out of production. The longer the plants stay in the ground, the stronger their relationships with bacteria that fix nitrogen, and microbes and fungi that seek out other nutrients. This means that they should, in principle, need less fertilizer.116 One estimate suggests that perennial systems hold five times as much of the water that falls on the ground as annual crops do.
