In a well-made compost pile, the various ingredients or feedstocks break down into a rich, dark soil with a pleasantly mild, earthy aroma. However, piles will often develop unpleasant smells if they aren't properly maintained. Smelly compost has a handful of common causes, and all of them are easily addressed.
Too Many Greens
Finding a balance between nitrogen-rich "green" ingredients and carbon-rich "brown" ingredients is one of the fundamental skills of composting. The U.S. Environmental Protection Agency recommends using three parts brown ingredients such as dead leaves or sawdust for every one part green ingredients such as grass clippings or food waste (see References 1). If your pile has too high a ratio of green ingredients, it will begin to emit a piercing ammonia smell. To correct this, add a quantity of brown ingredients to the pile and turn it with a fork to incorporate them.
Not Enough Oxygen
Backyard composting harnesses beneficial bacteria and molds to decompose plant materials. Like humans, these bacteria require oxygen and moisture to live and flourish. If the pile is too dense to admit oxygen, or if it's too wet for oxygen to penetrate, other bacteria colonize the pile and begin anaerobic decomposition. This creates both a memorable rotten-egg odor and methane, a potent greenhouse gas. To fix this problem, add brown ingredients such as sawdust, peat moss or chopped straw. These absorb moisture and lighten the pile, allowing oxygen to enter (see References 1, 2).
Kitchen scraps are a useful source of nitrogen-rich materials for your compost. Diverting food waste from your landfill is also environmentally responsible (see References 3). However, decomposing food wastes can create unpleasant smells and attract animal or insect pests. For best results, use only fruit and vegetable wastes. Cooked and uncooked foods are both acceptable, but don't compost any that contain fats or oils (see References 4). To minimize odors and pests, bury the kitchen scraps at least 10 inches deep in your compost pile. Cover them with carbon-rich brown ingredients such as straw or dead leaves.
A fourth common cause of smells in compost is the use of unsuitable materials. Although industrial composting operations can process almost any food waste, home composters should avoid adding fats or animal-based products to their piles. This includes vegetable oils as well as meat and fish scraps, eggs, dairy products and leftover processed foods (see References 4). These foods all create unpleasant smells and attract pests. Homeowners who wish to divert that type of kitchen waste from their landfill can use a commercial cone-type digester to process them without generating smells and attracting pests (see References 2, page 8).
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