Thomas N. Tomas
Crop rotation in vegetable production depends upon an understanding of the relationship of various plant families to each other, as well as their environmental relationships. Vegetables within each plant family are likely to have the same insect and disease problems. The carrot rust fly larvae will feed on the roots of dill, celery, fennel, parsley and parsnip, as well as wild carrot or Queen Anne's lace. The Colorado potato beetle feeds on the foliage of eggplant and tomato and survives in the wild on the buffalo burr and horse nettle.
With this information, you can see that rotations should be planned between plant families and not just different crops. It also indicates that you need to know what weeds may serve as alternate host plants for vegetable diseases and insects. Field crops and cover crops in the same plant families could also perpetuate insect and disease problems. This is one reason why annual rye grass is so often used as a cover crop and soil builder in vegetable production. The only member of the grass family commonly grown as a vegetable is sweet corn.
By rotating cool season with warm season vegetables, weed seed production cycles can be broken. Planting a succession of short season crops, such as lettuce followed by green beans followed by a fall cover crop, will allow at least three flushes of weed seeds to germinate. If the crops are kept clean and the cover crop tilled in before weeds can set seed, many of the most vigorous weed seeds in the soil will have been eliminated. The following year vegetable crops can be grown on relatively weed-free soil. With weed sensitive crops such as onions, parsnips and carrots it is best to plan ahead at least two years, and give special attention to weed control in the preceding crops.
Crops that form a dense canopy that shades the soil should be used in rotation with crops that have an open architecture allowing sunlight to reach the soil. Crops that are easily cultivated, such as sweet corn, can follow cucumbers, squash or melons where weed control is difficult once the vines begin to run. By studying the particular spectrum of weed species that pose the most serious problems, rotations can be devised that reduce the production of weed seed while still ensuring a variety of marketable crops.
If there are limited markets for only a few types of vegetables, it may be possible to rotate with field crops. You can do this in cooperation with an organic grain or livestock producer if you do not have the equipment or markets for these crops. You should develop a working relationship with an organic livestock producer anyway, in order to get organic manure or compost for your soil. In our area, we think in terms of how vegetables can fit into a corn, milo, soybeans, winter wheat, alfalfa rotation.
An entire field need not be planted to the same crop. Small grains, soybeans and field corn can be planted in strips wide enough to accommodate planting and harvesting equipment. If vegetables are included in the rotation they can utilize the strips most suited to their needs. If you plan to plant vegetables following soybeans, it make sense to walk those strips a few more times to eliminate any weeds going to seed.
Rotations are a tool that must be used with common sense. If the weather does not cooperate or the market is hot, you may decide to take a chance. Without an insect or disease problem, onions following onions for two years may make more sense if the soil fertility is high enough and that is the only weed-free strip of soil available. Tomatoes following tomatoes or potatoes may work for the same reasons. The multiple long term benefits of a good rotation must be kept in mind and weighted against the short-term, one-season advantages.
Organic production depends on knowledge and understanding of the interaction between different members of the biological community in order to avoid problems, rather than seeking to remedy them when they occur. Each vegetable grower has to develop rotations that fit their particular farm. It helps to read what others have done, and to visit other growers to learn the practical application of these ideas. There is no substitute, however, for trying it on your farm and observing how things actually work in your piece of the world.
For a more comprehensive discussion of rotations in vegetable crops, read The New Organic Grower by Eliot ColemanNebraska Sustainable Agriculture Society: Home Gardening