Composting is a process that transforms raw, nitrogen-rich organic material, such as animal waste, into a stable soil amendment. The answer to whether composted manure smells depends entirely on the quality of the composting process. When done correctly, finished composted manure does not have a foul odor; instead, it develops a pleasant, earthy scent. This transformation occurs because the composting process changes the chemical structure of the volatile compounds responsible for the initial pungent smell.
The Source of Manure’s Initial Odor
Raw, uncomposted manure has a strong, unpleasant smell because it contains a high concentration of unstable organic compounds. This odor is primarily due to the rapid, uncontrolled decomposition of nitrogen-rich materials like proteins and amino acids. When these compounds break down, they release ammonia gas (\(\text{NH}_3\)), which creates the sharp, pungent, urine-like smell associated with fresh manure.
The worst odors come from areas of the pile that lack oxygen, leading to anaerobic decomposition. This process, occurring in compacted or waterlogged sections, generates highly offensive volatile organic compounds (VOCs). These include hydrogen sulfide (\(\text{H}_2\text{S}\)), known for its rotten-egg smell, and skatole (3-methylindole), a major contributor to the distinct fecal odor. The presence of these compounds signals decomposition without sufficient oxygen, resulting in noxious byproducts.
The Role of Aerobic Composting in Odor Stabilization
The controlled process of aerobic composting eliminates foul odors by promoting the activity of oxygen-dependent microorganisms. These beneficial microbes convert unstable, volatile compounds into stable, non-volatile forms. During the initial, thermophilic phase, high temperatures stabilize the nitrogen.
The microbes oxidize nitrogen from volatile ammonia into stable mineral forms, such as nitrates, which remain bound within the finished compost structure. This chemical stabilization prevents nitrogen from escaping into the air as an odor-causing gas. This process shifts decomposition from an inefficient, anaerobic breakdown to a clean, efficient aerobic one.
Healthy, mature compost is a stable, humus-like material that emits a distinctively earthy aroma. This desirable smell is caused by geosmin, a volatile organic compound produced by beneficial soil bacteria called actinomycetes. Geosmin is the same compound responsible for the pleasant scent of soil after rain, known as petrichor, confirming the material is fully stabilized.
Identifying and Resolving Problem Odors
If composted manure emits a foul odor, it signals an imbalance in the composting environment. A strong, sharp ammonia smell indicates an excess of nitrogen relative to carbon. To resolve this, add carbon-rich materials, often called “browns,” such as dried leaves, wood shavings, or straw, to balance the ratio.
A sour, putrid, or rotten-egg smell, caused by hydrogen sulfide, clearly indicates anaerobic conditions, meaning the pile lacks sufficient oxygen. This deficiency usually results from excessive moisture or material compaction. The immediate solution is to thoroughly turn the pile to introduce fresh air and restore aerobic conditions. Adding dry, porous “browns” can also help by absorbing excess moisture and creating air pockets. These adjustments quickly shift microbial activity back to the odor-free aerobic pathway.