It's hard to place a definite value on a vegetable garden. The produce may, on a dollar basis, easily be worth more than ten times the cost of seed, fertilizer and other materials you need. But because of the pleasure you can get from gardening and getting fresh vegetables you've grown yourself, it can be worth much more than the actual cash value.

Anyway, the time has come to get started with it. English peas, lettuce, turnip greens, kale, cabbage, radishes, onions, parsley and spinach can all be planted outdoors now, if your soil is not too wet.

Squeeze a handful of soil. If it sticks together when you open your hand, if it won't crumble when you rub it, it's too wet. But in two or three days it may be all right.

Soil is the most important thing you have to work with: Those who are most successful at gardening are the ones who have learned to improve their soil year after year and to replace the elements the plants have removed in the process of development.

After removing stones and trash, the way to improve your soil is to work organic matter into it. It's marvelous how much poor soil can be improved by adding organic matter, lime and fertilizer. Organic matter includes compost, barnyard manure, peatmoss, decayed leaves, seaweed and similar materials.

Mix a one or two-inch layer of organic matter with the top six inches of soil, along with lime and fertilizer. Apply one pound of ground limestone and one pound of 5-10-5 fertilizer per 10 square feet.

The lime is important to reduce soil acidity. Most soils in this area are too acid for good growth of most vegetables. The best way to find out how much lime to apply is to have your soil tested, but it's too late for that now.

Organic matter helps unite fine clay particles into larger ones and increases the size of channels in the soil through which air and water move. It helps sandy soils hold moisture and nutrients. More fertilizer can be applied during the growing season as the need develops.

Don't plow or spade the soil unless it's necessary to mix organic matter, fertilizer and lime into it; to prepare a seed bed; to get rid of weeds; or to eliminate a crust on top of the soil so that water can penetrate more readily.

Research has shown that soil compaction is not eliminated by merely digging the soil. After two or three rains, the soil goes back to its original condition.

Lime moves very slowly downward in the soil. An application made to the soil surface and left there may not reach the root zone for years. That's why it's so important to dig it in and mix it thoroughly with the soil before spring planting.

Practically all vegetables grow best in direct sunlight, although a few will stand more shade than others. Trees that are too close to a garden are detrimental to it because of the shade they cast and the chance that their roots may penetrate far into the garden and rob the vegetables of moisture and nutrients. No amount of fertilizer, water or care will overcome lack of sunshine.

The vegetable garden will require a moisture supply equivalent to about one inch of rain a week during the growing season. If there's not that much rainfall, the deficiency should be made up by watering. If needed, it's best to give the garden a good soaking once a week. Light sprinklings at frequent intervals do little good, if any.

Q - When is the best time to divide rh-ubarb? We have lots of plants and they're overcrowed.

A - Rhubarb usually needs to be divided when it's four or five years old and the best time to do it is early spring or late fall, when the plants are dormant. The crowns may be cut into as many pieces are there are buds. A piece of root with one strong bud should produce a good plant in one season. When old crowns are divided (six to ten years old) use only the vigorous center portion for planting. If you want to keep the rhubarb growing where it is, division can be accomplished by spading away parts of the crown. The parts removed can be used for new plants.

Q - We want to grow some tomatoes in our backyard this year. Should we buy plants or start them from seed? What kinds are best?

A - If you know how to do it, and can provide the proper environment, starting the plants from seed yourself has important advantages: You can be sure you have the varieties you want, and sometimes that can be accomplished only by planting the seed yourself. There are a lot of good kinds to choose from, one of the best is Better Boy, whose virtues include disease resistance, very good flavor and a lot of fruit.

Q - I love violets and want to raise some. Any suggestions?

A - Most violets and pansies (also of the violet family) do best in fairly good soil, with light shade and plenty of water. One of the main reasons violets stop blooming is that the clump becomes too big and needs to be divided.

Q - I planted geraniums and petunias in a somewhat shady place in the backyard last year and they didn't do very much. Is there something better I can plant?

A - Coleus and Impatiens both put on a good show in light to medium shade. You may be surprised by the wide selection of plants you have to choose from with both of these. Although both can be started from seed yourself, it may be a good idea to buy plants already in bloom so leaf patterns and colors can be chosen that please you most.

They should be planted in soil that is well drained, the soil should be prepared to a depth of about eight inches, and some organic matter should be added to the soil each year. The plants need at least an inch of water per week either from rain or watering. Usually early morning watering is best since the leaves can dry before night, which reduces possibility of disease infection.

Q - Regardless of whether I plant the seeds indoors or outdoors in the garden, it seems to take forever for them to sprout. Is there a way to speed it up?

A - Factors that affect germination time include planting depth, soil temperature and moisture. Usually the seed packet provides most of this information. Some flowers take a long time to come up, as much as 20 days or more.

The following require five to 1/ days to germinate: sweet alyssum, snapdragon, aster, calendula, callopsis, candytuft, coreopsis, cosmos, sweet william, baby's breath, lobelia, marigold, pansy, petunia, annual phlox, portulaca, zinnia and nasturtium.

Shasta daisy, coleus, foxglove, gaillardia, kochia, forget-me-not, scabosia and verbena require 10 to 20 days to germinate.

Q - I've tried several times to start parsley from seed and the seeds never come up. What is the secret of getting them to sprout?

A - Plant the seeds properly and then be patient with them. Plant them indoors in Jiffy-7s or plant pots, and keep them moist but not soggy. Soaking the seed in warm water (85 degrees to 95 degrees F) for six hours before planting will hasten germination. Keep the planting on the dark side until the seeds start to germinate and then give them good light, preferably sunlight. It usually takes two or three weeks for germination.

Q - The only place I have for a vegetable garden is low and stays wet for a long time after a rain. Can I do anything to improve its possibilities?

A - A long-range solution might be installing drainage tile. A temporary solution could be to make rows for the plants so the roots can stay above water.

Q - What kind of groundcover is best under oak trees to prevent the soil from washing away? It's on a slope and there are a lot of leaves falling from late summer on.

A - English ivy will hold the soil once established. But before planting it or anything else, think of the problem of raking leaves off of it. Maybe a good deep much of woodchips or ground bark would be best for you.

Q - I've been told the way to improve your soil for a vegetable garden is to get earthworms. Do you agree?

A - For earthworms to live in the soil there must be organic matter, moisture and oxygen. With this environment, earthworms will come to the soil of their own accord. If the soil is not up to it, bringing them in will not help because they will not stay because they cannot survive.

Q - What size vegetable garden would be needed to take care of a family of four?

A - It would depend on how intensely you garden the plot and how much you know about gardening. A plot 25 by 50 feet should provide enough food during the growing season, but if you go in for canning, freezing and other processing, a half acre may be required, and that's a lot of garden.

If you have a question for Tom Stevenson, write to him at the Weekend section, The Washington Post, 1150 15th St. NW Washington, D.C. 20071.

ORCHID AUCTION - The National Capital Orchid Society will hod a fund-raising auction Saturday, noon to 5, at the Rock Creek Knights of Columbus Hall, 5417 West Cedar Lane, Bethesda. All types of seedlings, flowering sizes (some in bloom), miniatures and hybrids will be auctioned, and a booth will offer potting mixes, fertilizer, supplies and books. Call Soph or Lou Martin, 596-9669, for details.

ADOPT A TREE - Next Wednesday, thousands of seedlings - pines, hardwoods and wildlife shrubs - from Maryland's state nursey will be given away at the 4-H and Youth Park in Denton, from noon to 6. Call Stark Mclaughlin, 301/479-0990, for details.