Crop Rotation

Now is the time to plan your vegetable garden. When you do, consider the benefits of crop rotation. Crop rotation is a method of restoring soil nutrients, which have been depleted by previous crops.

Crops vary as to the types and amounts of nutrients they extract from the soil and are thus classified as heavy feeders, light feeders or soil builders. A few of the heavy feeders are corn, cabbage, cauliflower, squash and greens. Some light feeders are potatoes and other root crops. The soil builders include beans, peas, alfalfa and clover. If you plant a heavy feeder in an area where you grew a legume like beans or peas last season, there will be plenty of nitrogen and other nutrients for the new crop to thrive. The next season, there should still be enough nutrients left over for a light feeder to do well. In the third year you should start the sequence over again by planting a member of the legume family. This is the basic concept of crop rotation.

Crop rotation is also one of the most effective ways to control insect pests and diseases in the vegetable garden without using chemicals. The theory is simple. Disease organisms such as club root fungi attack a crop (cabbage, for instance) during the growing season. During the winter, the organisms may overwinter as spores in the soil or in plant debris left in the garden. The following year, they will reinfest a new planting of cabbage or another member of the cabbage family like cauliflower or broccoli. The disease will become increasingly severe as the spores build greater and greater populations. By planting vegetables that belong to the other plant families for several years, the club root dies out because it lacks the desired host plant.

In controlling insect pests, a similar theory works. Each type of insect tends to feed on a few favorite plants. They feed all season, lay eggs and die. In the next season, the eggs hatch and the larvae emerge, feeding off the same host plant. Rotating plantings, even if they are relocated just 4 or 5 feet away, can reduce damaging insect populations.

Since plants from the same family tend to be hosts for the same insects or diseases, knowledge of plant families is necessary. It is not a true rotation if you follow a plant with a member of the same plant family. Below are the main vegetable families and some of the plants they include.

VEGETABLE FAMILIES

Compositae: Chicory, Endive, Salsify, Dandelion, Jerusalem & Globe Artichokes. Cucurbits: Cucumbers, Gourds, Melons, Squash. Goosefoot: Beets, Chard, Spinach. Grasses: Rye, Sweet Corn, Popcorn. Legumes: All Peas and Beans. Lily: Garlic, Onions, Leeks, Shallots, Chives Mallow: Okra. Mustards: Cabbage, Collards, Brussels Sprouts, Kale, Cauliflower, Broccoli, Kohlrabi, Turnips, Cress, Radish, Horseradish Parsley: Carrots, Parsley, Celery, Parsnips Solanaceous /Nightshades: Eggplant, Peppers, Potatoes, Tomatoes

To design a rotation, you need to have a sketch of what was grown in the garden and where it was grown last year. The simplest way to get started is to divide the garden plot on paper into 4 or 5 main sections. Then by referring to the vegetable families, you can place the plants you want to grow in a space that was not used last growing season by any member of its family. Plan to plant a heavy feeder such as corn, tomatoes, or cabbage the season before light feeders such as root vegetables, bulb crops, or herbs. Plant a soil-building crop such as a legume the third season.

Other rules about rotating crops to consider are :

Wait 3 to 6 years to plant related crops in a rotation to effectively break disease and insect cycles.

Never rotate within a family. Peppers following tomatoes is not a rotation.

Rye cover crops secrete an enzyme into the soil, which inhibits the sprouting of small seeds. Follow such crops with large seeded vegetables such as sweet corn, squash or cucumbers.

Precede tomato, okra, carrots, or potatoes with a member of the grass family because the grasses suppress root knot nematode, a condition caused by microscopic worms in the soil.

When turf is plowed under for new garden space, avoid following it with root crops because sod may harbor soil pests such as grubs or wireworms, which would damage the root crops.

Follow sweet corn with heavy crops such as legumes or cucurbits. They help to decay residue.

Keeping crop rotation records from year to year will make your garden planning easier, your soil more productive and your plants robust and disease and insect free.

Dee Lawrence is a Penn State Cooperative Extension Master Gardener serving Adams County. Penn State in Adams County is located at 670 Old Harrisburg Road, Gettysburg, PA 17325, phone 334-6271.