Composting transforms organic waste into a rich soil amendment through natural decomposition. When successful, the resulting scent is earthy and pleasant, resembling damp forest soil. A strong odor akin to feces or sewage indicates that the underlying biological process has gone severely awry. This foul smell signals a shift away from the desired environment and requires understanding the science behind the stench to correct it properly.
The Science of Foul Compost Odors
The unpleasant, sewage-like smell originates from a lack of oxygen within the compost mass, leading to anaerobic decomposition. Successful composting relies on aerobic bacteria, which require oxygen to break down materials and produce relatively odorless compounds. When the oxygen supply is depleted, these beneficial organisms die off, and anaerobic bacteria take over the process.
These anaerobic microbes thrive without air, breaking down nitrogen-rich materials like food scraps and fresh grass clippings inefficiently. This incomplete breakdown releases various smelly volatile organic compounds (VOCs). Odors are commonly caused by sulfur compounds, such as hydrogen sulfide, which creates a classic rotten egg or sewage smell, and mercaptans.
When the compost contains too much nitrogen, anaerobic bacteria produce excess ammonia gas, resulting in a sharp, pungent odor. Decomposition of fats and carbohydrates in the oxygen-deprived environment also forms volatile fatty acids (VFAs), which can smell sour or rancid. Nitrogen compounds called indoles, including skatole, contribute a distinct feces-like odor to the pile.
Immediate Steps to Eliminate the Stink
Since the odor results directly from an oxygen deficit, the first action is to thoroughly aerate the pile. Turn the entire compost mass using a pitchfork, ensuring material from the center and bottom is brought to the outside. This introduces a fresh supply of oxygen, immediately shifting the environment back toward aerobic conditions.
The second necessary step is to balance the material ratio. Nitrogen overload, typically from too many “green” materials like food scraps, fueled the odor-causing bacteria. To correct this, immediately mix in a large quantity of dry, carbon-rich “brown” materials. Good choices include shredded cardboard, dry leaves, or straw.
These dry materials absorb excessive moisture and increase the pile’s porosity, allowing air to circulate easily. Mixing these browns throughout the pile dilutes the concentrated nitrogen and opens up the structure, creating space for oxygen to penetrate. This rapid introduction of carbon and oxygen halts the production of offensive volatile compounds.
Maintaining the Ideal Composting Environment
Preventing foul odors long term requires maintaining the correct balance of materials and moisture. The key to sustained aerobic decomposition is managing the Carbon-to-Nitrogen (C:N) ratio. Microorganisms require about 30 parts carbon for every 1 part nitrogen by weight to decompose waste efficiently.
To maintain this balance, always layer nitrogen-rich “greens” with a significantly larger volume of carbon-rich “browns.” Adding fresh food scraps or grass clippings should be followed by covering them with several times their volume of dry leaves or shredded paper. This layering technique prevents pockets of dense, wet, nitrogen-heavy material from forming.
Proper moisture management is also a factor in preventing anaerobic conditions. The compost should feel like a wrung-out sponge—damp but not saturated. If the material is too wet, water fills the air pockets, displacing oxygen and leading to the return of anaerobic bacteria. Chopping all materials into smaller pieces before adding them accelerates the breakdown process and reduces the time the material spends in a state where it could become overly dense.