The strong, unpleasant smell often associated with certain garden fertilizers is widely recognized. Many people liken this distinctive odor to the scent of feces, which is not a coincidence. The reason for this olfactory similarity lies in the fundamental chemistry of decomposition. Both organic fertilizers and animal waste contain the same complex biological building blocks. When these materials break down, they release an identical set of volatile chemical compounds, creating a shared scent profile.
The Chemical Signatures of Odor
The molecules responsible for the intense, fecal-like odor are volatile compounds, meaning they easily transform into a gas that can travel to the nose. One of the most potent of these odorants is skatole, a chemical compound that is a direct breakdown product of the amino acid tryptophan. Tryptophan is a common component of proteins found in nearly all biological matter, and its microbial degradation generates skatole, which is a primary contributor to the characteristic smell of mammals’ feces.
Another class of powerful odor compounds is the volatile sulfur compounds (VSCs), which originate from the decomposition of sulfur-containing amino acids such as cysteine and methionine. This group includes hydrogen sulfide, recognizable by its rotten-egg scent, and various mercaptans and organic sulfides, like dimethyl disulfide. These sulfur-based molecules are detectable by the human nose at incredibly low concentrations, making them significant contributors to any foul smell. The low molecular weight and high volatility of these nitrogen- and sulfur-containing molecules ensure they are quickly released from the decomposing material. This shared chemical process occurs in both animal waste and certain types of fertilizer, leading to the shared olfactory experience.
Fertilizer Sources That Require Microbial Breakdown
The fertilizers that produce this potent aroma are exclusively organic in nature, as they are derived from once-living materials that are rich in proteins. These sources serve as concentrated packages of the chemical precursors—nitrogen and sulfur—needed to generate the foul-smelling compounds. Materials such as animal manures and biosolids (treated sewage sludge) are obvious examples, as they are essentially waste products already saturated with the necessary biological material.
Other organic fertilizers are also high-protein sources that contribute to the smell. Blood meal, a byproduct of the meat industry, is valued for its high nitrogen content. Feather meal is similarly protein-dense, consisting largely of keratin, a protein rich in the sulfur-containing amino acid cysteine. Fish emulsion, a liquid fertilizer made from fish byproducts, also contains significant amounts of protein and sulfur, directly contributing to the odor. All these organic amendments rely on decomposition to release their nutrients, setting the stage for the odor-producing reactions.
The Shared Mechanism: Microbial Action
The unifying factor that links the smell of feces and organic fertilizer is the unseen work of microorganisms. In both animal intestines and a garden’s soil, bacteria and fungi are the primary agents of decomposition, breaking down complex organic matter into simpler, plant-available nutrients. This process is known as mineralization, and it involves the microbes consuming the proteins and amino acids present in the material.
When microbes break down the proteins in fertilizer sources like blood meal or manure, they perform the same chemical reactions that occur in an animal’s gut. For instance, the degradation of tryptophan by specialized bacteria in the soil generates skatole, just as it does in the digestive tract. Similarly, the breakdown of sulfur-containing amino acids by sulfate-reducing bacteria releases volatile sulfur compounds, including hydrogen sulfide. If decomposition occurs in an environment with limited oxygen, known as anaerobic conditions, the production and release of these odorous compounds are intensified. This lack of oxygen favors the microbial pathways that create the most pungent gases.