By the same token, members of the same family often suffer from the same problems. Certain pests and diseases often attack only a closely related family of crops. Keeping the same family in the same spot in the garden each year makes it easy for pests to find their favorite target and fosters the accumulation of pests and disease in the soil.
Different families use nutrients differently and, therefore, make different demands upon the soil.
Crop rotation is the easy solution to this problem. Move crops around your garden on a regular sequence so that they do not return to the same spot for three to four years. It is sometimes argued that rotation in a small garden is not worthwhile since crops can't be moved far and because some diseases persist in the ground for more than four years. But the improvement in soil fertility and structure that results from rotation makes this a sensible practice. It certainly can't hurt.