How to bake a batch of compost

For members of Seattle's Interbay P-Patch community gardens, fall leaves represent a free resource for what the group trumpets as "the amazing Interbay mulch."

The mulch is a mix of "green" and "brown" organic matter culled from the urban waste stream, piled on top of the soil, and covered with burlap sacks.

The objective is to feed soil-building organisms in the ground with what they need to do their work, namely nitrogen and carbon. The nitrogen, which is provided by "green" compost materials, is used in protein synthesis, and the carbon, provided by the "brown" compost materials, is a source of energy.

There is no secret formula to success, only a list of possible raw ingredients.

High on the list of "green" compost materials are yard waste and garden debris, including everything from grass clippings and pulled weeds to the chopped, end-of-season remains of corn, bean, and squash plants. Coffee grounds, seafood waste, and the manure of farm animals are also on the list.

The "brown" ingredients include fall leaves, pine needles, straw, newspapers, wood bark and shavings, and dryer lint.

"For a vegetable garden, a 50-50 split of green and brown would give you good results, and you could even go heavier on the green," says Jon Rowley, co-director of the Interbay P-Patch. "If you're [starting] a new perennial bed, you could go heavier on the brown."

Covering the mulch with burlap, held down with stones or boards, serves several purposes. It fools nocturnal organisms into working 24 hours a day. It retains moisture, while letting the soil breathe. And it keeps birds from foraging for the worms and insects that are important to soil decomposition.

The burlap almost ensures that the pile won't need watering. But, if desired, the burlap can periodically be lifted and the moisture content checked. New material can also be added.

It's best, though, to wait a month or so before adding new raw materials, since the mulch needs time to become biologically active.

Until then, Rowley says, it can look as if nothing's happening. "Then all of a sudden, you see these various critters, and once this biology gets going, you can dig a little hole and put your kitchen scraps in. If you keep feeding the mulch, it works even faster."

Mowing or chopping large leaves also helps speed decomposition. The earlier the mulch-making begins the better, since warmth is a biological ally. "If you start early enough, by December the mulch is almost done," Rowley says. "But if you start in November, you may have to wait until April."

By spring, the burlap can be removed, transforming what began as a foot-thick layer of debris into an inch or two of humus.

Most gardeners will turn it a little with a spading fork to mix it into the soil, although that's not always necessary. "You end up with this gorgeous humus - no weeds - and plants that grow like crazy," Rowley says.