All plants are inflicted with certain pests or diseases that affect them and will reduce their yield or even kill them completely. That is the reason that crop rotation is so important in the vegetable garden. If you grow the same type of plant in the same location year after year you increase the likelihood that the microorganisms that cause disease will multiply in the soil each year as the plants they survive on are grown in the same spot . The same holds true for insect pest who will overwinter and increase in numbers each year. By moving the crop to a different area of the garden you are disrupting this cycle thus making it harder for the diseases and pests to get established.
Crop rotation does not only apply to the build up of pests and diseases but also to the overall health of the soil and it’s ability to provide proper nutrition to the plant you are growing. The different type crops require more of certain nutrients than others, so dependent on what you are growing the soil will become depleted on one or more of the macro nutrients. Leaf crops like lettuce, cabbage, and spinach need more nitrogen and will quickly deplete the supply if not rotated.
The legumes such as peas actually help to fix nitrogen into the soil so they should be rotated to help restore the soil instead of worry whether they will deplete the soil. Root crops such as carrots, parsnips and onions love potassium. The deep roots of carrots and parsnips do have a positive side as well they help break up the soil. The crops that deplete most of the phosphorus are the ones that flower heavily such as cucumbers and tomatoes. Again by moving these crops around you are assuring that no one area is being depleted of too much of a macro nutrient.
To use the crop rotation system is really very simple, when you plan this years vegetable garden you will want to take out last years garden plan so you can review where each type of vegetable was grown last season. If you do not have it then try and draw a rough sketch just so you will have a general idea. Now while planning this years garden try an plant leaf crops where legumes were last year. Plant flower crops where root crops were. Not only does this disrupt the pest and disease cycle it will take advantage of some of the contributions of the previous years crop.
Be sure and mix in several inches of fresh compost each spring and fall and utilize crop rotation to minimize pests and diseases.