Do you often wonder if it’s really necessary to rotate your vegetable crops, or is it ok to continually plant the same things in the same spots? Rotation is useful for two reasons: it helps prevent the buildup of pests and diseases specific to particular plants or plant families, and it maximizes the use of soil nutrients.
Although there are some equal-opportunity scourges — slugs come to mind — that will attach everything in the garden, they are in the minority. Many of the nastiest mosaics, for example, attack legumes (peas and beans) but have no interest in nightshades (tomatoes, potatoes, peppers, and eggplants). Similarly, earworms are a real problem in corn but couldn’t care less about lettuce. By moving their favorite targets around, you make it harder for these bad characters to establish themselves.
While you’re foiling the pests, you’re also managing the soil. Corn and squash require a lot of nitrogen; peas and beans make nitrogen (or, more accurately, fix the nitrogen from the air into a form that roots can use). By alternating one with the other, you can reduce the amount of supplemental fertilizer you’ll have to add.
Unfortunately, corn will always be a lot taller than beets, so where space is at a premium and crops must be closely planted, it will always be wise to put the tall crops to the north of short ones so they don’t cause shade problems. One more reason people who have large gardens in full sun have an advantage over those who don’t!