I don't have the time for a detailed reply, but I'd like to draw your attention to the dessication effects that agriculture has had on the "fertile crescent," which is now mostly desert. Constant tilling did two things: crops used up a lot of the moisture that had been locked into the soil—the grains that were being cultivated there used more moisture than was naturally replenished by natural forces. The lack of crop rotation sapped much of the nutrients in the soil. Since fields were not left fallow in the early days of agriculture, the earth never had the opportunity to regenerate and as time wore on, the fields became less and less fertile. Eventually, the large populations that had depended on the fertility of that land suffered starvation. In the worst cases, the land converted to desert and the cities were abandoned.
It probably didn't help that an accepted tactic in classical period warfare was to salt the enemy's land to destroy their ability to grow food.