Composting at home is fairly straightforward, but can go wrong quickly, and your nose knows when the compost container isn’t working properly. A functioning compost pile should smell faintly like warm earth. There are several causes of foul-smelling compost, and several practical solutions.

Mark King is a compost expert with the division of solid waste management at the Maine Department of Environmental Protection. He said all stinky compost problems come down to “breakdowns in pile management,” largely due to neglect.

“Many people read how easy it is to compost and tend to think you can just toss stuff into a pile and in 6 to 12 months you will have compost,” King said. “This popular misconception has led to more than a few unhappy home owners who end up with a stinky offensive mess. However, provided with the proper technical support and educational outreach, they are able to turn things around, and happiness returns.”

The Magic of Microbes

Though compost may seem like magic, it relies on little critters — either microbes or worms. As King explains, these helpers require food, air and water.

“Microbes rely upon a balance of carbon and nitrogen. Carbon provides the energy source, and nitrogen is used as ‘building blocks’ to produce more microbes,” King said. Specifically, this recipe translates to 30 or 40 parts paper or leaves (carbon) per one part food scraps or grass clippings (nitrogen). The mix must be kept moist and well aerated.

If the mix is right, the microbes produce compost, carbon dioxide and water vapor. Without enough oxygen or the wrong mix, it could start producing stinky volatile organic acids.

Time and Temperature: The Keys to Compost

King recommends taking the temperature of a compost pile. The goal is temperatures above 120 degrees Fahrenheit.

Richard Stehouwer is an associate professor of environmental soil science at Penn State University. He explains why cooler compost isn’t doing its job.

“Anaerobic decomposition is much slower and generates far less heat than aerobic decomposition,” he said. “So heat dissipates from the pile as quickly as it is being generated and the pile temperature does not increase. In these systems anything that prevents oxygen from reaching zones in the pile where decomposition is ongoing will result in anaerobic conditions and production of malodorous gasses.”

 “Optimizing the recipe will create an odor-free, aerobic environment,” King said. “When any of these items are out of balance, odors ensue.  The good news is that with proper attention, most problems are easily fixed and odors go away.”

Stehouwer offers the following troubleshooting suggestions for cold, stinky compost bins:

• Mix the carbon and the food scraps more thoroughly, so the decomposing food can diffuse oxygen faster than it consumes oxygen.

• Mix more dry leaves or paper into a pile that may be too wet. “The rate of oxygen diffusion through water is 10,000 times slower than through air,” he said.

• “Add material that will help create porosity,” he said, such as wood chips, straw and dry leaves. Large air gaps in a compost pile improve air flow and help the microbes do their job.

• Add more carbon. Excess nitrogen, or food scraps, can generate stinky ammonia.

Preventing Compost Odors in the Kitchen

The other major source of compost-related odors is the food scrap pile in the kitchen awaiting the trip to the compost bin. To avoid odors in the kitchen, King recommends storing less than a week’s worth of food in an airtight container, then feeding it to the compost microbes along with the recommended 30 to 40 times as much paper, leaves or other carbon.

The same general rules apply to worm compost, which also can start to smell with the wrong proportions of food, carbon and water. King warns against letting food scraps get too rotten. “Even worms have their standards.”

Stehouwer warns that worm compost bins can get stinky if users add more food or larger particles than worms can handle. He recommends chopping food into smaller pieces, and always adding plenty of paper or cardboard whenever food is added. Some vermicomposters use a small old blender or food processor to grind up the worm food.

Give the worms, or the microbes in a hot compost bin, the right mix of food, air and water, and they will generate valuable, earthy-smelling compost for your garden.