Pests taking over garden? Try mixing your crops

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The neon sign, flashing notice of a restaurant, draws the hungry motorist unerringly off the highway. The lesson is as old as commerce itself: he who advertises gets the business.

It's a lesson (in reverse) that shouldn't be lost on gardeners either. Too many gardeners flash an ''eat here'' signal that is as unmistakable to pests as the blinking neon sign is to the motorist.

The importance of crop rotation coupled with mixed plantings for the back-yard vegetable plot can't be overstated. It is one of the keys to consistently satisfying production, improving the soil and checking the development of pests and disease.

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Take two carrot patches in my garden last year. They were of similar size, yet produced startlingly different harvests. Although no more than 8 feet separated them, the one was badly affected by larvae of the carrot fly while the second was virtually free of the root-tunneling pest. Why? I think I know the answer.

The earlier sowing (they were planted a few weeks apart) went into a bed where carrots had been regularly sown in previous years; the second sowing went into a bed that had been free of carrots for three full seasons.

Carrot-fly eggs from the previous season hatch out in the soil, feed on young carrot roots, and mature into flies that then lay more eggs. There can be several generations in a season.

That, apparently, is what happened in the first carrot bed. But a fly is a fly - and surely one of the generations from the affected bed would have flown across the intervening space and populated the second carrot bed. That they didn't, at least not to any noticeable degree, is probably due to the plantings of peppers that separated the two. The peppers apparently interfered with any aroma or other chemical signal from the second carrot bed that would attract the carrot fly.

The conclusions from that experience are obvious: don't plant the same crop in the same place on consecutive years and, where possible, split large plantings into two or more blocks separated by a different vegetable species (or even with flowers) to prevent any arriving pests from having a field day.

Gardeners who follow the square-foot gardening concept (different vegetables are planted in each square foot) report few pest problems, probably because the system most nearly duplicates the mixed-growth patterns of nature.

Most insects respond to chemical signals from plants. The jumble of signals given off by a mixed planting frequently confuses them. One-crop fields, as in agriculture, or whole beds given to a single crop in the home garden, make it simple for the the insect to find an abundant food source where it can breed prolifically.

Not all of us want to go as small as the square-foot route, but adapting the same principles to moderately larger squares would be helpful.

Besides keeping a check on pests and the build-up of harmful soil organisms, crop rotation helps maintain a good balance of nutrients in the soil, even allowing for the use of compost and manure, because differing crops take varying amounts of nutrients from the soil.

Before the advent of chemical fertilizers the gardeners of France and England considered crop rotation a labor-saving technique as well. They would dig copious amounts of manure and compost into a bed (generally in the fall) only once in four years and plant this to beets (heavy feeders) and potatoes and onions, which, although light feeders in terms of the amount of nutrients they take up, nevertheless do well in a rich soil.

The second year would see this bed given over to peas and beans that yield a food crop while fixing atmospheric nitrogen in the soil for the heavy-feeding cabbage family and lettuce crops that would follow in the third year. Light-feeding root crops - carrots and parsnips among others - would be sown in the fourth year. After this harvest, the composting and manuring would begin again.

The home gardener might use this procedure as a guide, but would not follow it exclusively because it includes no tomatoes, peppers, or vining winter squash , none of which the Northern Europeans of yesteryear grew to any great degree.

If you plan to lightly compost or manure your gardening beds every year, you might follow the advice on crop rotation given by the editors of Organic Gardening and Farming magazine, in ''Getting the Most From Your Garden'' (Emmaus , Pa.: Rodale Press). They suggest soil-building crops, followed by heavy feeders, followed by light feeders. They are:

Heavy feeders - beets, broccoli, Brussels sprouts, cabbage, cantaloupe, cauliflower, celery, collard, corn, cucumber, eggplant, endive, kale, kohlrabi, lettuce, okra, parsley, pumpkin, raddish, spinach, squash (summer and winter), sunflower, tomato, and watermelon.

Light feeders - carrot, garlic, leek, mustard greens, onion, parsnip, pepper, potato, rutabaga, shallot, sweet potato, Swiss chard, and turnip.

Soil builders - alfalfa, beans (broad, lima, snap, and soy), clover, pea, and peanut.

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