CORVALLIS, Ore. – As you recycle autumn leaves into compost this fall, consider the science that transforms waste into "black gold" for your soil.
"There are more microorganisms in a teaspoon of topsoil than there are people on planet Earth," said Nick Andrews, small farms specialist and compost expert for the Oregon State University Extension Service. "Compost is similar. It's teeming with billions of microorganisms for each ounce of compost."
Those billions of microorganisms aren't sitting still. Their metabolism works hard to convert organic material into fuel – activity that heats up compost. Compost must reach 130 to 135 degrees to kill weed seeds and pathogens, Andrews said. Turn the pile after its first 3-5 weeks with a garden fork to add air and break up clumps of material. If the pile is big enough – one-half to one cubic yard – and well-built with a good carbon-to-nitrogen ratio, moisture content and porosity, it should heat up within a week and stay hot long enough for you to turn the pile and "process" the raw material to kill pathogens and weed seeds.
If compost just isn't happening, Andrews offered these troubleshooting tips.
- Problem: It isn't heating up because the pile is too small. For a continuous fuel source, microorganisms need at least one-half cubic yard to one cubic yard of fresh organic material, Andrews said. During harvest time in August and September, that’s realistic for most gardeners. Solution: Make sure you have a steady source of fresh material. If you don't, you could cool-compost the rest of the year, or build a worm bin, using earthworms to decompose food waste and organic matter. "Adjust your expectations," Andrews said. "If the pile isn't heating up, allow it to decompose over a longer time period, and wait long enough for the raw material to look fully decomposed, like 'black gold.' It's the 'Don't worry, be happy' approach."
- Problem: It stinks like rotten eggs. Healthy compost should emit a rich, earthy odor. But a stinky compost pile might not have enough air and could be too wet. Compost piles thrive on a good balance of air and moisture and should contain 60 to 65 percent moisture, Andrews said. Solution: Add dry material like straw, dry leaves or shredded paper. Turn the pile with a fork as you incorporate these materials. To keep out rainwater, cover the pile with plastic tarp or enclose your bin with a roof made out of scrap material.
- Problem: It attracts raccoons, mice, rats or other critters. Material that invites varmints includes meat, poultry, fish, fat, oil, dairy products, bread, grains and bones. Solution: If this is a problem for you, avoid composting food that attracts unwanted critters. The more actively you manage and turn your pile during early decomposition, the less likely you will have problems. You can also build your composting pile to exclude mammalian pests — for example, line it with hardware cloth. Your goal is to prevent animals from nesting or feeding from your compost pile. Grass clippings, leaves, plant stalks, vines, weeds without seeds, healthy fruit and vegetable scraps, livestock manure and straw don't attract pests. Wood chips, nut shells, twigs, acorns and egg shells are also compostable, but note that these materials are slower to decompose.
To learn more about composting, see the OSU Extension guides Gardening with Composts, Mulches and Row Covers and Composting with Worms.