Many kinds of wild plants can be used as food by humans.Watercress, mustard, chicory and dandelion add flavor to a salad if harvested at the right stage. Acorns used to be one of the most important nut foods of Indians and roasted sunflower seeds have long been considered a delicacy.

On the other hand, many wild plants can be harmful if eaten. The problem is correct identification. Dr. Richard A. Howard, director of the Arnold Arboretum at Harvard University, has prepared some safety rules. Although adults obviously are vulnerable, he says, our primary concern is with the inquisitive child.

Avoid eating all plants that have milky or colored juices; this includes members of the milkweed, poison ivy and poppy families. Needless to say, there are exceptions, for the young shoots of milkweed are edible and even lettuce has a milky juice.

Avoid all unknown white or red fruits. Poison ivy, poison sumac, and some species of baneberry have white fruits and are poisonous. Strawberries, apples and tomatoes are red, but these are known. The majority of unrecognized red fruits are potentially toxic.

Avoid eating wild seeds for the seed of the plant usually has the greatest accumulation of chemical which may be toxic.

Avoid all fruits which are three-angled or three-lobed and thereby eliminate the potential dangers of spurge, soapberry, horsechestnut, amaryllis and lily families. Some of the world's most infamous poisonous plants belong to these families.

Avoid all bulbs that lack the smell of onions or garlic. Some of the lily and amaryllis and related families with basal bulbs may kill you if eaten in quantity.

If you must experiment in eating unknown plant materials, cook the plants parts in two changes of water. Then sample a little bit before consuming a lot. If the cooked material tastes unpleasant, don't eat it.

If poisoning is suspected, seek information as soon as possible. Look under "Poison" in the emergency pages of your phone book. QUESTIONS and ANSWERS

Q - I planted Heinz tomato plants, had a wonderful yield for about two weeks, then no more fruit, no even little ones. What could be wrong?

A - Heinz 1350 is a determinate variety that bears and ripens almost all of its fruit at one time. The determinate kinds are the ones planted for machine harvesting - once over, that's it. Indeterminate varieties such as Better Boy, Big Girl, Beefmaster and Supersonic continue to bear all season long.

Stems of determinate varieties terminate in flower clusters while those of indeterminate kinds continue to elongate and produce more flowers and fruit.

Q - After I dig my Jerusalem artichokes in the fall, how do I keep them fresh and for how long? Can I dig some up during the summer to eat?

A - The tubers should not be dug for food until after the first heavy frost kills the tops. The best place to store them is in the ground. You can store them in a cool, dark place with moderate humidity or in the refrigerator but they will keep for only about two weeks. Leave them in the ground, mulch them with 5 or 6 inches of leaves to prevent alternate freezing and thawing and when you want some to eat, go out and dig them when the ground isn't frozen.

Q - My compost pile smells awful. Is there anything I can do to stop it?

A - Usually it is due to too much moisture and poor drainage which causes the pile to stay soggy wet. When drainage is slow, decay slows down and offensive odors occur. With good aeration, the disagreeable smell will soon be gone. Turning the pile with a pitchfrok also will help by improving aeration.

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.