The United States produces more than 400,000 tons of trash a day, about 3.5 pounds per person.

More than 20 percent of it -- yard and kitchen waste such as grass clippings, leaves and foodstuffs -- could be kept out of landfills and allowed to decompose on its own through composting, the natural process in which bacteria breaks down organic waste into nutrient-rich soil called humus.

"It's an easy process understood by few people," said Joe Keyser, program director of the American Horticultural Society, which is based in Northern Virginia.

Next month, the society will begin one of the largest composting demonstration programs in the country, with exhibits of more than 40 different ways to degrade organic waste.

It will educate area residents about composting through a series of free classes at the society's headquarters at River Farm, a house once owned by George Washington, four miles south of Alexandria.

"Yard waste can comprise up to 50 percent of all landfills during peak summer and fall months," said Keyser, a self-proclaimed compost king.

"We'd save so much room in our landfills if people just composted their leaves and grass clippings."

Of the 16,400 landfills in the nation, half are expected to be closed by 2000. The Lorton landfill, which serves the District, Alexandria and Fairfax and Arlington counties, is expected to be completely filled in 2003.

Statistics such as these have forced Virginia to impose a 1995 deadline for all jurisdictions to recycle 25 percent of their trash.

Many already have geared up to make the deadline, and a few have surpassed their own goals. Last year, Fairfax County recycled 16.5 percent of its trash, more than five points higher than expected. But other localities, such as Loudoun and Prince William counties, which each recycle about 5 percent annually, are working to catch up.

Arlington County and Alexandria recycle about 10 percent of their trash.

Composting by area residents could have a large impact on helping jurisdictions to reach their goals, said Keyser, who said that in his classes he'll "teach people how to reduce the waste at home before it ever makes it to a landfill."

The classes are the main course of a "National Backyard Composting Demonstration Park" being set up on the society's grounds.

In 60-minute classes, Keyser plans to dispel myths about bad odor, rodents and high cost, which have been associated with composting, and show area residents that anyone can compost, no matter where they live.

One easy composting method that many people can take advantage of, Keyser said, is to take a large container, such as a trash can, and punch ventilation holes in the side and bottom.

The inside of the can is then layered with about six inches of yard waste to every one inch of soil. "Like a lasagna," Keyser said.

By adding soil, which already contains the microorganisms that break up plant waste, the decaying process is accelerated.

A good time to begin a compost is in the summer or autumn, when grass is cut most often and leaves fall.

The pile must be exposed to sun and moisture for the bacteria to prosper and convert the plant matter into humus.

Keyser said that if a compost has a bad odor, it requires more air and needs to be turned, by pushing the outside of the mound inward and exposing the interior to more air and light.

Because yard waste is made up of about 85 percent water, the pile will shrink to less than half its original height during the composting process.

The length of time required will vary with the size of the material, the size of the pile and the time of year.

Materials good for composting include grass clippings, leaves, pine needles, cow and horse manure, sawdust, hair, weeds, sod, hay, feathers and vegetables.

Keyser advised against using weeds heavily laden with seeds, citrus rinds, diseased plants, grease, fat, bones, charcoal briquette ashes, manure from meat-eating animals (such as dogs and cats), oil, dairy products and inorganic matter such as plastic.

Keyser said it is important to provide a balance between materials high in carbon, such as leaves, straw and paper, and those high in nitrogen, such as fresh grass clippings, manure and green kitchen scraps.

Humus is a valuable soil conditioner with natural enzymes, plant nutrients and beneficial microorganisms. Keyser said it's perfect for gardens, trees and shrubs.

"Everyone thinks that planting trees is the answer to saving our environment," Keyser said. "But most people, especially those who live in cities, have no place to plant trees and trash keeps piling up. By composting you give something back to the earth."