There are three reasons for crop rotation:
1. Some crops utilize more of a specific nutrient than others, and by rotating crops soil fertility can be better equalized.
2. Some crops attract specific insects. By rotating the crops the cycle of insect build-up is minimized.
3. If a crop becomes diseased, rotating to a crop that is not susceptible to that disease can break the cycle of the disease pathogen.
All of these conditions are of only minor importance, however. When plant nutrition is really understood, fertility can be maintained easily. And by keeping a weed-free, clean garden and uniform, healthy, fast-maturing crops, insect and disease build-up are seldom experienced.