To bring out its best, give your garden a good thinning

The corn is up, the early tomatoes have set fruit, and the winter squash vines are beginning to sprawl out. There is an obvious vigor in the garden and the promise of abundance come Thanksgiving Day.

But if the garden is to achieve its full potential, there are two important steps to take: (1) thin out the overcrowded beds; and (2) be prepared to give the maturing plants a midseason feeding. For most gardeners the first task is infinitely more difficult than the second.

''Thin and win'' is the successful vegetable gardener's slogan. We thin for the same reason we weed - to remove unwanted competition from those plants we ultimately wish to harvest. If you still find it difficult to undertake this task, consider these facts:

Thinning -

* Promotes sturdy, vigorous growth. Crowded plants are frequently spindly. The carrot that should grow to an inch across will never do it if several of its neighbors are trying to do the same thing a mere one-quarter of an inch away.

* Reduces competition for nutrients and water. With no need to struggle, the remaining plants grow rapidly and flavor is enhanced.

* Provides good air circulation around plants, increasing the availability of carbon dioxide (yet another needed plant nutrient) while making the control of pests and other problems far simpler.

* Enables large-heading flowers (zinnias, marigolds, and the like) to develop sturdy stems that will remain erect in wet and windy weather.

Standard thinning advice is to gently pull out unwanted plants when the soil is damp so as to disturb the remaining seedlings as little as possible. Do this preferably in the evening so that any disturbed plants have the cool night hours in which to recover. Where possible, my preference is to cut off the seedlings at the ground with a pair of scissors. This way you won't disturb the neighboring plants at all. Obviously, I wouldn't do this with, say, baby carrots once they were large enough to be included in salads or thrown into the stir-fry dish.

This brings up another point. Thinning is a gradual process. Ultimately, you will want your carrots to be spaced about 3 inches apart. But the first thinning might leave them spaced only a half inch apart so that the roots can reach a small but edible size before the next thinning. This also would apply to beets, onions, lettuce, etc. - anything that can be eaten at an immature stage.

Here are some approximate final spacings for plants in a wide row or bed (plants set out in rows could be planted marginally more closely because of the 18 or so inches which separate most garden rows):

Snap beans 8-10 inches; beets 3-4 inches; carrots 3 inches; corn 12-18 inches; loose-leaf lettuce 8-10 inches; onions 5-6 inches; radishes 2 inches; and spinach 6-8 inches.

Midseason feedings are recommended so that the early vigor of spring can be maintained as the growing plants approach maturity and their nutrient needs are greatest.

A well-fertilized spring soil is generally rich enough in nutrients to carry most crops through to harvest. Nitrogen, however, is the one element most readily used up and may also be leached from the soil by heavy rains. When plants begin to set fruit, or head up, additional nitrogen is generally advisable.

My preference is to apply more compost, if I have it. It goes on as a mulch around the plants up to 1 inch thick. Aged manure can be used in the same way, or alfalfa hay (rich in nitrogen) can be applied several inches thick. Another option is to scatter alfalfa pellets which are used in certain green cat-litter products (check the label). You might also feed the plants periodically with diluted fish emulsion, available at garden centers, as well as liquid compost or manure ''teas.''

If you plan to use chemical fertilizers, apply these as a side dressing around the plants or along the rows. Open up a 2- to 3-inch-deep furrow 6 inches from the plant or around the drip line of larger plants. Sprinkle in the fertilizer (10-10-10 analysis is recommended at this stage) and cover over with soil. The plant roots will reach out and take what is needed from this band of fertilizer.

Too much of anything is a bad thing, particularly when using chemical fertilizers. Follow the directions on the bag very closely. If you do not follow the directions explicitly, spindly growth, with an overabundance of leaves and too little of the edible product, will result. Peppers are particularly vulnerable to this. Manures, too, can overstimulate, but compost seldom will.