Merits of lettuce for home garden

The National Garden Bureau, educational arm of the North American seed industry, has decreed 1983 as the ''Year of the Lettuce.'' There is no single overriding reason why lettuce should be so named, but few vegetables are more deserving of the honor.

Lettuce:

* Is one of the most popular vegetables and the most popular salad vegetable in much of the world. On average, North Americans eat 27 pounds of lettuce a year (or 12.27 kilos if you are Canadian). Europeans love it, too, and it is eaten with relish in Australia, South Africa, and Argentina, to name a few Southern Hemisphere countries. After World War II the Japanese took on the American custom of eating lettuce even more readily than they took to baseball.

* Is relatively simple to grow. That has to be a major plus for the home gardener.

* Demands little space and less sun than most other food crops, which means the tiniest of gardens generally has room for a lettuce or two. It is readily grown in containers or even in hanging baskets. Look for a bed with six sunny hours, if possible, but less will do provided the area is relatively bright (no heavy shade) the rest of the day.

* Can withstand light frost, which makes lettuce one of the first crops to go into the ground in spring and one of the last to come out in fall. In warm climates lettuce can be grown throughout the winter.

* Matures relatively quickly so that sowing a few seeds every other week enables the home gardener to put fresh lettuce on the table virtually all through the growing season, even at the height of summer if a little shading with latticework or netting is provided.

* Is available in many varieties. Theophrastus named three varieties in his ''History of Plants'' in 350 B.C. In ancient Rome, 12 varieties were common on the tables of the upper classes. George Washington's kitchen garden at Mount Vernon included 16 varieties, and by 1885 a report put out by the New York Agricultural Experiment Station listed 87.

All these varieties are loosely categorized into four groups - head, semi-heading, loose leaf, and cos or Romaine. Head lettuce are the iceberg types that are most commonly found at supermarkets. Less common in stores but most popular with home gardeners are butterheads, a lettuce that forms a loose head at the center. They are not as crisp but are better flavored than the iceberg types.

Loose-leaf lettuce forms a rosette of loose leaves and is the easiest of all lettuce to grow. It can be harvested one leaf at a time from the outside, or the whole plant can be cut at maturity. This type of lettuce is now available in a ruby red color, which adds considerably to the attractiveness of the vegetable garden.

Cos, or Romaine, lettuce (it was a favorite in ancient Rome) grows in an upright, cylindrical shape. Its outer leaves are dark green and the inner leaves are blanched. It is the principal ingredient in a Caesar salad.

Whatever your preference in lettuce, enrich the soil with compost and manure or a balanced 10-10-10 fertilizer before planting. I throw a trowelful of compost into every planting hole and a tablespoonful of a balanced organic fertilizer. In the absence of compost throw in a handful of alfalfa pellets used to feed gerbels and similar children's pets. Alfalfa is high in nitrogen, which leafy plants thrive on.

One of the more inexpensive ways to get these pellets is to buy cat litter made from alfalfa (Littergreen is an example). Use the new stuff. It is not wise to recycle anything from kitty's box on a food crop, although your roses would be grateful.

Sow lettuce in well-worked soil, one-quarter inch deep during cool seasons and as deep as one-half inch when the hot weather of late spring arrives. I sow seed in flats in a cold frame, transplanting the young plants when three or four true leaves have formed.

Soil temperature dramatically affects lettuce-seed germination. In low 40 degrees F. it will take around two weeks; between 65 and 75 it takes two to three days; at 95 degrees there will be no germination at all.