Compost Questions

What you need to know about compost

November 26, 2010

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.

Q. How can I speed up decomposition?
A. Gardeners can step in and accelerate the composting process by creating optimal conditions for decomposition.

  • Get the balance right. Compost = Air + Water + 2 Parts Browns + 1 Part Greens
  • Shred, chop, or chip ingredients. Smaller pieces of material have more surfaces for microorganisms to work on, so your pile decomposes faster. Pieces ranging from 1/2 to 1 1/2 inches decompose best.
  • Have a critical mass. A pile will not heat up if it's too small. Minimum size is 3 by 3 by 3 feet.
  • Mix your ingredients. For a long time, experts (including—gasp!—this magazine) recommended piling up ingredients like a layer cake, but compost should really be more like a tossed salad—mixing all the goodies together produces the best results. "You definitely want to mix," says Dr. Cogger. "You want to get the green and brown material in intimate contact." Layer cake-style composting causes compaction and reduces biological activity.
  • Aerate your pile. Compost can be turned twice a week or never; it's up to you. But the more you turn it, the faster you'll get results.
  • Monitor moisture. Dry piles decompose slowly, so adjust moisture as needed.
  • Add some microbes. Mix in a shovelful of finished compost or garden soil to get the microbial activity rolling.

Q. What's the easiest way to make compost?
A. Mix together green and brown yard wastes (remember the C/N ratio), place them in a pile, and wait 6 to 24 months for the microorganisms, earthworms, and insects to break down the material. Add new materials to the top of the pile. You can reduce the waiting period by occasionally turning the pile and monitoring its moisture level. Compost on the bottom of the pile generally "finishes" first.


Q. When is my compost finished?
A. "If the pile isn't heating anymore and you can't identify original materials, it's done," says Dr. Maynard. It's okay to use compost that's not quite finished as mulch. You can also work it into garden beds one to two weeks before planting, because the soil microbes help finish off the decomposition process. Be sure to screen out any big, woody chunks first. If you want to use compost in a potting mix, it must be fully mature to prevent problems with nutrient availability and germination. Try this sniff test, recommended by soil scientists at Cornell University, to see if your compost makes the cut for potting mixes.

  • Mix some compost and water in a jar until the compost is soggy.
  • Seal the jar and wait a week.
  • Open the jar. If the compost smells bad, it's not done yet. If it smells earthy, then it's fine to use (though you may want to experiment with planting a small amount of seed first to be sure).

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

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