"First, remember to feed the soil, not the plants," says Shepherd Ogden, a master of organic gardening. "If you take care of the soil and constantly enrich it, the plants will thrive." A nationally known market gardener, nurseryman and author, Ogden offers this cardinal principle in what he calls a "green thumb manifesto" for raising vegetables: Sure, you can give the plants an occasional shot of fish emulsion, Ogden says, but in the long term "do whatever you can to enrich the soil by concentrating nutrient, whether by composting, growing green manures and cover crops or simply mulching and then turning under the mulch." In his latest book, Step-by-Step Organic Vegetable Gardening (Harper Collins, 1992, $23), Ogden revises and updates the theories and practices expounded in a work originally written by his grandfather, Samuel. The new author runs the Cook's Garden, a mail-order seed and supply house in Londonderry, Vt., where he has become known especially for gourmet and foreign vegetables. Growing things organically generally means avoiding pesticides and synthetic fertilizers. The chemicals needed by the plants are obtained instead from unrefined "natural" sources such as manure, rotted vegetable matter and other compost. After enriching the soil, a second important step in Ogden's manifesto is to "take only what you need." He explains that, after harvesting the part of the plant that you eat, you should compost the rest, returning "as much as possible of the biological yield from the earth to the earth just as is done in nature." Another piece of advice Ogden offers is to "embrace diversity." He notes that one of the greatest farming and gardening problems arises from the fact that large groups of identical plants become prone to massive outbreaks of pests and diseases. By growing a wide range of crops and varieties of each plant, "you hedge your bets," he says. Ogden also says you should "work with the natural, seasonal cycles in your garden," a know-how usually learned through experience rather than plucked out of a book. "Plant to miss the peaks of insect reproduction," he says. "Schedule your harvest at the best time for flavor and ripening. To do so takes attention on our part to the daily details of life in the garden, to observing the subtleties of the interaction among the plants, animals, the weather, the natural cycles of earth, air, water and sun." Ogden recommends walking around in your garden early in the morning and at twilight to try to spot any problems. "Often it is just a matter of pulling a stray weed or crushing a cluster of recently laid eggs," he says. If you've grown robust plants, they are less subject to insect and disease problems and thus require less control, Ogden says. The 299-page book covers not only the organic scene but the art of gardening in general, from design to tools and equipment. There are recommendations for growing 40 vegetables, including 10 pages on tomatoes alone. A sense of the soil and its fascinating creatures is ever present in this book. In a lively section on earthworms, Ogden tells us, "The total mass of their bodies is equal to the weight of all the power tools in the average gardener's garage combined, including the rototiller, lawn mower, chain saw, snow blower, even a garden tractor, but the earthworm is much more powerful for its size than any of these _ able to move stone up to 50 times its own weight as it burrows through the soil, sometimes to a depth of 12 feet or more. "The "castings' left behind by earthworms are more than five times as rich in nitrogen, phosphorus and potassium than the surrounding soil, and their tunnels represent a significant source of drainage and aeration for the soil in which they live."