GARDENERS frequently are perplexed when blossoms drop from their tomatoes, peppers and cucumbers without setting fruit. There may be several causes for this.

One cause is too much nitrogen fertilizer, which can result in a lot of green leaves but no fruit. Test your soil before planting and follow soil test recommendations to prevent the application of too much nitrogen.

Too much shade or not enough light is another cause of poor fruit set. This usually results in spindly or leggy plants. Most fruiting vegetables do best in full sun all day, or at least most of the day.

Poor pollination can cause poor fruit set. Vine crops like cucumbers and squash require bees for pollination. Wild bees are usually present. But when plants are sprayed with Sevin (insecticide) during the day, many of the bees may be killed.

To ensure pollination of sweet corn, plant at least three rows at each planting. Three rows 25 feet long are better than one row 75 feet long.

Tomatoes, peppers, beans and peas are self-pollinated and require no special row arrangement.

Extreme temperatures can reduce fruit set. If temperatures during flowering are too low (below 55 degrees) or too high (above 90 degrees) the pollen grains of warm season crops are usually no good. This is particularly true of tomatoes.

Constantly wet soil conditions can cause fruits to fall off the plants. This is especially true of peppers on poorly drained soils.

Growing your vegetables is only half the game. You've got to pick them right, too. Take lima beans, for example. If they stay on the bush too long, they are likely to be less tasty and flavorful. They most will have a starchy taste.

Picking too early is wasteful, so look for a happy medium. A good way to tell when the bean is ready to pick is the change in color. Green seeded types of limas turn white as they become over-mature. Speckled seeded types develop specks or blotches as the green color fades.

Picking snap beans at the right time is important, too. Pick them when pods are fully developed but when seeds are no more than 50 percent of their normal size. If you let them stay on the bush too long, you'll have beans with too much fiber.

The real test of gardening know-how is picking sweet corn at just the right time. Experience is the only thing that will make you an expert.

It's a shame to waste yield by picking the ears when they are grossly immature. On the other hand, corn left on the stalk too long isn't good for much but feeding the livestock. If it is only slightly beyond maturity, there is some reduction in quality but the volume is much greater.

If you're inexperienced, there is only one solution: trial and error. But don't pull back the shuck on the tips of too many ears. Exposed kernels attract insects, and they can destroy the ear in short order.

Pulling tomatoes early isn't so critical. You can pretty well see what you're getting when you take them from the vine. If you're canning the tomatoes, pick them fully, red ripe but not mushy.

Q. We have a lot of pumpkins, what can I do to keep them in good condition for Thanksgiving and Christmas?

A. When the rind is firm enough so that it is not easily punctured with the thumbnail, the pumpkin is close to maturity. It should be cured (also winter squash) at a temperature of 80 to 85 degrees F. for about two weeks. Then store in a dry place at 50 to 60 degrees. With good curing and storage they should keep four to five months with little loss of quality.

Q. Are organically grown vegetables more nutritious than those grown with commercial fertilizer?

A. No. Research has shown that vegetables grown under the same environmental conditions will have the same nutrient content regardless of the type of fertilizer used.

Q. Can a person become poisoned by handling a poisonous mushroom?

A. No. Part or all of the mushroom must be eaten and digested for the toxins to be effective. These toxins are not absorbed through the skin, according to specialists.

Q. How should celery from the garden be stored? Does anyone ever cook it?

A. Store celery in the refrigerator. Wash the stalks to remove any dirt and cut away any bruised spots. Store in a covered container. Slightly wilted celery can be restored by putting it in a pan of ice water for a short time. Most of the celery we consume is raw, but cooked celery is also delicious. Try it sauteed with onions or tomatoes or fresh mushrooms.

Q. We have a lot of turnips. How should they be stored for the winter?

A. Turnips, parsnips, potatoes, onions, rutabagas and salsify are best stored overwinter in outdoor pits or in moist sand in a storage room at 33 to 40 degrees F.