GARDENERS say that spring comes when the earth's soil is soft enough for planting back yard lettuce and spinach. It's here when the family tracks mud into the house, grumbles the housekeeper. Birdwatchers tune spring's arrival to the first clear call of the veery returning to Glover Archbold Park. Homeowners know spring is imminent when bags of mulch begin to pile up outside the Safeways. Children scramble after kites in the sharp March wind along the Mall; and on a Utah Avenue corner, a forsythia bush suddenly lights up with bright yellow as if someone had struck a match.

Weathermen talk about spring- like weather when wind patterns begin to change, and the temperatures shoot up some 20 degrees into the 50s and 60s. Newspapers carry spring stories about tornadoes rampaging in the south and west, and waters swollen with melted snow flow over river banks.

A naturalist sees the earliest spring in the first tinge of yellow- orange of weeping willows, so subtle that when he stares straight at them, the color seems to recede before his eyes. At the edges of the woods, the hoops of wineberries glow brighter red, the blackberries more lilac.

Even the scruffy grackles and starlings along Pennsylvania Avenue feel the light and warmth -- their hoarse croaks break, and become almost melodious.

The hiker pacing through mountain woods where patches of snow still cling to cool shadows suddenly comes upon the brown-purple hood of the skunk cabbage. The folded leaves of this malodorous plant encase the life-giving warmth released by oxidation of carbohydrates at temperatures sometimes 27 degrees above the temperatures of the air -- enough to melt the snow as the plant pushes toward the sun.

Spring comes to the C&O Canal's woodchuck one fine day when the temperature is just right to wake him from his winter sleep. In a matter of a few hours, his body temperature rises from about 38 degrees Fahrenheit to 97 degrees, and his breathing quickens -- from 10 to 12 breaths an hour up to 30 or 40 a minute. His metabolism soars quickly, and after a few shakes, he hastens to find food.

Pond watchers at the Audubon Naturalist Society's Woodend have their own definition of the season: In spring, the pond turns over. The sun has melted the ice at the surface and as the water becomes warmer, its density approaches that of the water at the bottom. The breeze stirs up the pond, and the warm surface water works its way down, awakening the sleeping organisms at its bottom, and churning the rich nutritious debris and cold water of the bottom up to the top. Duckweed, those tiny green plants that migrate to the pond bottom in autumn, now generate bubbles of gas that lift them again to the surface, miniature parachutes in reverse.

Spring comes also to the great masses of plants and animals of the tidewaters of the Potomac and Patuxent rivers. Shad, striped bass and perch begin to move upstream where thickets of fishing poles dip into the water on a warm sunny day. Triggered by spring, millions of baby eels from the mid-Atlantic begin their voyage to the coasts of Europe and North America. Here they work their way up the rivers, creeks and brooks to fine fresh- water pools.

Then suddenly comes the full rush of spring. Along the sidewalks north of Dupont Circle, wheelbarrows overflow with tulips, lilacs and hyacinths, and robins are seen bobbing along on newly greening lawns. Only the careful observer knows that lilac buds have been fat on shrubs all winter, and that many robins have remained in the area throughout the winter. It's really only the eyes of the spectators that have awakened to spring.

Over all this burgeoning commotion and flood of fancies stands the lonely astronomer, like a stern monitor. Quietly he marks the exact scientific minute and hour of the arrival of spring on our side of the planet.

Spring comes to us, he states, at that moment when the sun is directly above the equator, on its northern journey as the earth tips to lean its Northern Hemisphere toward the sun. At this moment, and at one particular spot on the equator, he says, the sun rises due east, and the hours of the night are exactly equal to those of the day. This is the vernal equinox.

This year, says the astronomer speaking from the U.S. Naval Observatory, spring began this past Thursday, March 20, at exactly 5:03 p.m. EST. At that precise moment, the sun was directly over the equator, on longitude 149 degrees west, about 600 miles east of Christmas Island in the Pacific Ocean, and more than 6,000 miles southwest of Washington, D.C. By Barbara Tufty; Barbara Tufty is a Washington writer.