Anaerobic compost isn’t ruined, but it does have real problems compared to compost that breaks down with oxygen. It can harm plants, smell terrible, harbor pathogens, and release methane. The good news: a soured pile can almost always be rescued with some straightforward fixes.
What Happens When Compost Goes Anaerobic
When a compost pile loses access to oxygen, a completely different set of microbes takes over. Instead of the heat-generating bacteria that power a healthy pile, you get organisms that ferment organic matter slowly and produce a range of problematic byproducts. The most notable are organic acids like acetate, propionate, and formate, which accumulate in the material and make it acidic. You also get hydrogen sulfide (the classic rotten-egg smell) and ammonia, both of which off-gas in unpleasant and potentially harmful concentrations.
The other major byproduct is methane. In aerobic composting, microbes release carbon dioxide as they work. In anaerobic decomposition, the majority of the chemical energy in the starting material gets converted to methane instead. Methane is roughly 20 times more potent as a greenhouse gas than carbon dioxide, so an anaerobic pile has a significantly larger climate footprint than a well-managed aerobic one. (This is also why engineered anaerobic digesters capture that methane for energy. In a backyard pile, it just escapes into the atmosphere.)
How It Affects Plants
The organic acids that build up in anaerobic compost are directly toxic to plant roots and seeds. Propionate in particular inhibits microbial activity even within the pile itself, slowing decomposition further. When you spread this material around plants, the acidic, oxygen-poor environment can burn roots, stunt growth, and reduce seed germination. If your compost smells sour or like vinegar, those acids are the reason, and they’re a clear sign it shouldn’t go into the garden yet.
Anaerobic compost also tends to be wetter, denser, and more compacted than aerobic compost. Applied heavily, it can create waterlogged conditions in soil that further starve plant roots of oxygen, compounding the damage from the acids themselves.
Pathogens and Weed Seeds Survive
A well-managed aerobic compost pile heats up to 55°C (131°F) or higher during its active phase. That temperature destroys most human and plant pathogens, along with weed seeds. Anaerobic composting is a low-temperature process, so according to the FAO, it leaves weed seeds and pathogens intact. If your original inputs included animal manure, kitchen scraps, or weedy plant material, an anaerobic pile offers no assurance that harmful organisms like Salmonella or E. coli have been eliminated. This is one of the most important practical differences between the two methods.
Nutrient Content Is a Mixed Picture
Nitrogen loss is one of the biggest concerns in any composting method. A study comparing dairy manure composting methods found that anaerobic composting lost about 15% of its initial nitrogen over a 39-day period, mostly through ammonia emissions and leachate. That’s comparable to forced-aeration composting (about 16%) but higher than standard aerated composting, which lost only about 13% and retained the most nitrogen overall at 27.6 g/kg versus 25.9 g/kg for the anaerobic method.
Where anaerobic composting performed notably worse was leachate. Nitrogen losses through liquid runoff were nearly ten times higher in the anaerobic treatment compared to the best-performing aerated method. That means nutrients are literally draining away rather than staying in the finished product. In practical terms, anaerobic compost still contains usable nutrients, but you’re getting less value per pound of input material, and more of those nutrients are polluting the surrounding soil or groundwater instead.
How to Tell Your Pile Has Gone Anaerobic
The smell is the most reliable indicator. A healthy aerobic pile smells earthy, like forest soil. An anaerobic pile produces distinct odors depending on how far it’s gone:
- Sour or vinegar-like smell: organic acids are accumulating. The pile is anaerobic but may not be deeply waterlogged.
- Rotten egg smell: hydrogen sulfide is present, indicating true anaerobic conditions with significant oxygen deprivation.
- Strong ammonia smell: the pile is trending anaerobic, often because it has too much nitrogen-rich material relative to carbon. This is the earliest warning sign and the easiest to fix.
Other clues include a slimy texture, a dark or black color throughout (rather than the brown of finished compost), and material that looks barely decomposed despite sitting for months. Anaerobic decomposition is dramatically slower than aerobic, so a pile that never seems to break down is likely oxygen-starved.
How to Fix an Anaerobic Pile
A soured pile doesn’t need to be thrown out. The goal is simple: reintroduce oxygen and absorb excess moisture.
Start by turning the pile thoroughly. If it’s dense and compacted, break it apart as much as possible. Then mix in dry, carbon-rich “brown” materials like straw, dried leaves, or shredded cardboard. If the material is uniformly small and fine, add chunky wood chips (up to about 25% of the pile’s current volume) to create air channels throughout. The woody pieces act as structural support, preventing the pile from collapsing back into a dense, airless mass.
If your pile smells like ammonia specifically, that’s a strong signal you need more carbon. The nitrogen-to-carbon ratio is off, and adding browns will both absorb moisture and give microbes the carbon they need to shift back into aerobic metabolism. With enough browns mixed in, an ammonia-smelling pile will often reheat within days, which is exactly what you want.
Thick layers of kitchen scraps or other wet, nitrogen-heavy materials are a common cause of anaerobic pockets even in otherwise well-managed piles. Going forward, mix wet inputs with browns as you add them rather than creating distinct layers. Keep any single layer of food waste to just a few inches at most, sandwiched between generous amounts of dry material.
Once you’ve turned and amended the pile, give it a week or two and check again. If it’s heating up and the smell has shifted to earthy or neutral, the aerobic microbes are back in control. If it still smells off, turn it again and add more dry material. Most piles recover fully within one or two corrections.