Crop rotation reduces the transmission of certain plant diseases and pests, and improves overall soil fertility. As a rule, farmers and gardeners try not to plant crops from the same plant family in the same location for at least three years. Tomatoes, potatoes and peppers, for example, are all members of the Solanaceae, or nightshade, family. There are several diseases (eg fungi, viruses and blights) and pests (eg beetles and larvae) that target the nightshade family and can survive in the soil over the winter. That’s why farmers don’t plant nightshades in the same location year after year. Different crops also require different soil nutrients to thrive. Farmers usually plant a grain crop, eg wheat, after a legume, like soy, because legumes raise nitrogen levels in the earth.
Answered by Dave Roos