Compost Questions

What you need to know about compost

By Willi Evans Galloway


Q. What is compost?
A. Gardeners love to say "compost happens," and it's true. When formerly living things (leaves, grass clippings, and so on) die or animal manures break down, they decompose into compost—a dark, moist, crumbly type of earthy-smelling organic matter.

Q. Why should I bother composting?
A. It's an easy way to reduce your carbon footprint and reduce your household waste. Plus:

  • It slowly releases nutrients to plants when they need it.
  • Compost improves clay soil, making it easier to work; helps sandy soils hold more water; and increases plants' ability to withstand drought.
  • Compost feeds the soil microorganisms that release nutrients and help plants grow strong and healthy.
  • You can use it as a soil conditioner, mulch, potting mix ingredient, and fertilizer.

Q. What are greens and browns?
A. Browns are compost ingredients that contain a high percentage of carbon and are usually dry and brownish (hence the name) in color. Greens are ingredients high in nitrogen. They are generally green, moist plant matter or an animal by-product, such as manure. Since compost microbes use carbon as an energy source and nitrogen in the proteins that build their tiny bodies, it's important to include both brown and green components in the mix. See Compost Ingredients for examples of each.

Q. Why does compost need air and water?
A. Compost is full of living things (microorganisms, earthworms, insects, and other creatures) that need air, water, and nutrients to function. A compost pile needs to be fluffy so that there are plenty of spaces (what scientists call pores) for air to move about. Occasionally turning your pile refluffs the material, moves new material into the center, and helps improve airflow into the pile, says Craig Cogger, Ph.D., extension soil scientist at Washington State University. Compost microbes also need the right amount of water. Too much moisture reduces airflow, causes temperatures to fall, and can make the pile smell bad; too little water slows decomposition and prevents the pile from heating. "Conventional wisdom says that compost should feel like a wrung-out sponge," says Abigail Maynard, Ph.D., agricultural scientist at the Connecticut Agricultural Experiment Station.


Q. What is a C/N ratio?
A. In order for a compost pile to decompose efficiently, you need to create the right ratio of carbon (C) to nitrogen (N). Piles with too much nitrogen tend to smell sour, because the excess nitrogen converts into an ammonia gas. Carbon-rich piles break down slowly because there's not enough nitrogen for the microbe population to expand. An ideal compost pile should start with a 30:1 C/N ratio. Fresh grass clippings alone have about a 20:1 C/N ratio. Building your pile with one part grass clippings or other green matter to two parts dead leaves or other brown matter will give you the right mix.

Turn kitchen scraps into super-fertile soil!  Learn more.

New for your e-reader or tablet: Compostology 1-2-3.