Why Your Compost Pile Smells Bad and How to Fix It

Match the Smell to the Cause

A compost pile smells bad for one of three reasons: too little air, too much water, or too much nitrogen-rich material. The type of smell tells you which. Ammonia means excess nitrogen, and the fix is to mix in carbon such as dry leaves. A rotten-egg smell means the pile has run out of oxygen, and the fix is to turn it and add dry, bulky material. Oregon State University Extension puts the standard plainly: healthy compost smells earthy, not sulfurous.

Smell	Likely cause	Fix
Ammonia	Too much nitrogen	Mix in leaves, straw or wood shavings
Rotten eggs or sulfur	Low oxygen, excess moisture	Turn the pile, add dry bulky material
Musty	Too moist	Add more coarse, dry material
Wet, with rancid butter, vinegar and rotten egg together	Not enough air, too much nitrogen, or too wet	Turn, add straw, sawdust or wood chips, provide drainage
Rotting food at the surface	Exposed scraps	Cover with 4 to 8 inches of browns

The causes overlap. A soggy pile is also short of air, and Cornell’s troubleshooting table lists all three faults against a single wet, sour-smelling pile. Turning and adding dry brown material addresses most of them at once, so start there if you cannot place the smell.

Ammonia Smell: Too Much Nitrogen

A sharp ammonia smell means the mix is too rich in nitrogen. Grass clippings and food scraps are the nitrogen sources Cornell’s troubleshooting table names. Cornell’s guidance is to mix in a carbon source such as leaves or wood shavings.

University of Maryland Extension adds a second sign to check. If the pile feels really hot and has the aroma of ammonia, it may need a little more carbon. Maryland gives the ideal blend as 30 parts carbon to 1 part nitrogen, which is a ratio by chemistry and not something to measure with a bucket.

For a working rule by volume, use the one from the US Environmental Protection Agency: add at least two to three times the volume of browns, such as dry leaves, to the volume of greens, such as food scraps. Keeping a bag of dry leaves or shredded paper next to the bin makes that easy to do each time scraps go in. Our guide to the compost browns to greens ratio lists which materials count as which.

Rotten-Egg Smell: Not Enough Air

A sulfur or rotten-egg smell signals a pile working without oxygen. Oregon State names the cause as low oxygen and excess moisture and gives the fix in one line: turn the pile and add dry, bulky materials such as straw, dry leaves or shredded paper. Cornell notes that turning an outdoor pile may be enough to correct the problem, and that the odor can get worse for a short time when the pile is first opened up. Keep going and mix thoroughly.

Pile height plays a part. Maryland warns against building a pile over 5 feet high, because the weight and volume compact the material and limit air movement.

Musty or Sour Smell: Too Much Water

A musty smell points to a mix that is too moist, and Cornell’s fix is to add more bulking agent, meaning coarse dry material. The wetter version of the problem is the pile Cornell describes as smelling like a mixture of rancid butter, vinegar and rotten eggs. The solutions it lists are to turn the pile, mix in straw, sawdust or wood chips, and provide drainage.

The moisture target is the same in every guide: damp like a wrung-out sponge. Oregon State puts a number on it, about 60 to 65 percent moisture. EPA uses the same sponge test, and Maryland adds “but not soggy.”

To check, take a handful from the middle of the pile. If it is dripping, it is wetter than a wrung-out sponge and needs dry material and a turn. EPA gives the same pair of fixes for a pile with a bad odor: add more browns or dry material and turn it.

Garbage Smell: Exposed Food Scraps

When the smell is plain rotting food, look at the surface. Scraps sitting on top or at the edges rot in the open and draw animals. EPA’s rule is to always keep food scraps covered by four to eight inches of dry leaves or other browns. Cornell gives the same instruction for a pile that is attracting animals: cover all food with brown materials such as leaves, wood chips or finished compost. Oregon State suggests burying fresh scraps within the pile.

Some materials are a problem however well they are covered. EPA says they can attract animals and that backyard piles generally do not get hot enough to break them down fully. Keep these out of a home pile:

• Meat, fish and bones
• Dairy products
• Fats, oils and greasy foods

Oregon State adds bread and grains to the list where wildlife is a problem. If animals keep coming, Cornell recommends enclosing the pile in 1/4-inch hardware cloth.

When the Smell Keeps Coming Back

If the pile has been turned, dried out and rebalanced and still goes sour, check its size and shape. Maryland gives the ideal bin size as a minimum of 3 feet by 3 feet by 3 feet, or 27 cubic feet, and Oregon State suggests aiming for 0.5 to 1 cubic yard of fresh material during peak garden season. Stay under Maryland’s 5-foot height limit so the bottom does not pack down.

Cornell lists providing drainage among its fixes for a wet pile, so check that liquid can get out at the base of the bin.

According to EPA, a well-maintained pile is finished and ready to use in about three to five months. EPA describes finished compost as dark, loose and crumbly, with a smell like fresh soil. Our guide to how much compost to add to a garden bed covers how much to spread. Fall is a convenient time to use it, as described in the fall garden bed prep routine.