SEED OF several vegetables used in salads, such as lettuce and mustard, can be planted outdoors about Apri 1 in the Washington area, somewhat later further north, earlier further south.

For leaf lettuce, Salad Bowl and Black Seed Simpson are recommended. Bibb produces a loose head of tender leaves with fine fla- vor but can't take hot weather which may come early in the spring.

Tender young mustard greens really taste good in salads. They have a peppery nip and a mild distinctive taste appreciated by almost all adventurous eaters.

Substitute quick-growing mustard greens for half the lettuce in salads. They will thrive almost anywhere in the United States, if planted to mature during cool weather.

Seed of beets, kale, kohlrabi, leek, parsley, parsnips, salsify and shallots also can be planted outdoors about April 1, also plants of broccoli, brussels sprouts, cauliflower, collards, head lettuce and onions.

Beet tops are commonly used as greens. They may be used in this way until the small root becomes about a half-inch in diameter. Sliced or cubed beets may be added to tossed salad, or molded in gelatin. Cold beet slices may be used as sweet or sour hors d'oeuvres. They may be added to vegetable soup, or they may be baked or fried.

Beets must grow rapidly and continuously to develop the highest quality. They will tolerate only a slight amount of soil acidity.

Kale is a favorite of southern gardeners, as a crop for greens. Vates Dwarf Blue Curled Scotch is a good variety. The young tender shoots make delicious greens and a light frost improves their quality.

Using professionally grown plants purchased from a reliable garden center of brussels sprouts, broccoli, cauliflower, collards and onions is recommended unless you are well qualified to start them indoors yourself.

For your garden to be successful, the most important thing is the soil. Do not work clay soils when they are wet and sticky. Even walking on wet clay will compact it, reducing the size of channels for water and air movement.

Clay soils can be greatly improved by mixing organic matter such as compost, and lime and fertilizer with them. Peat, compost, partially decayed leaves and barn yard manure do a good job. Hydrated lime is your best bet, it is more readily available, is faster acting, the usual recommendation is about 4 pounds per 100 square feet.

Whether you dig the garden by hand or use a cultivator, remember the deeper the cultivation, the deeper the roots will penetrate, and the better the vegetables will withstand a dry period.