When planting your vegetables you should be rotating your vegetables to different spots to avoid building up nasty diseases and insect populations that can make your job of disease and insect control much more complicated. Rotate back to the same spot in about three to five years. Generally there should be a physical separation between these spots or beds so when you are preparing your soil you are not contaminating it. How important is this concept? It has been around for many many years and is one of the most simple concepts in organic and conventional gardening. Here is something I wrote for an NGO a few years ago when working in Lebanon.
Rotating Vegetable Crops to Prevent Diseases
Rotating fields to different crops each year is one of the most important and easily implemented disease control strategies for farmers. This practice avoids the buildup of many plant diseases in the soil. The longer the rotation before coming back to the crop, the less likely a disease will occur. Because diseases usually attack members of the same plant family, it is best to avoid planting crops after each other that belong to the same family. Insect damage may increase when the same crop is planted in the same area over several years as well. Here are some common vegetables and the families they belong to.
Tomato Family: Tomato, potato, eggplant, peppers
Cucumber Family: Cucumber, melons, squash, pumpkin, gourd
Lettuce Family: Lettuce, endive, salsify, Jerusalem artichoke
Onion Family: Onion, garlic, leek, shallot, chive
Carrot Family: Carrot, parsnip, parsley, celery
Cabbage Family: Cabbage, broccoli, cauliflower, Brussels sprouts, kohlrabi, turnip, radish, Chinese cabbage, kale, collards, rhutabaga
Beet Family: Beet, Swiss chard, spinach
Pea Family: Peas, snap bean, lima bean, soybean
Okra Family: Okra
Corn Family: Sweet corn, field corn, wheat, barley, oats
Some choices of crop rotations include Pea Family to Corn Family, Lettuce Family to Cucumber Family, Cucumber Family to Cabbage Family, and Cucumber Family to Corn Family. Rotating beans with a grain crop such as barley, oats, rye, wheat, or field corn or with a forage crop is very beneficial for root-rot control. One or two years in a grain crop is often long enough to prevent severe root rot when the field is not heavily infested with this disease.
Some diseases that come from the soil are not easily controlled by rotation. Such diseases can live a long time in the soil and are not affected by rotation. Examples include clubroot that attacks the Cucumber Family, Phytophthora blight, and Fusarium wilt of several crops. Other diseases attack so many vegetables that they can survive indefinitely on many different plants including weeds. These diseases include Sclerotinia, Rhizoctonia, Verticillium and root-knot nematodes. Many diseases can survive successfully because they can live on plants and plant parts left in the field after harvest. However they are unable to survive once the plants left in the field decompose. Destruction of plants and parts of plants left in the field after harvest can eliminate this problem. Plowing the field after harvest and before letting the soil rest can reduce the amount of disease that will survive.