A healthy compost pile relies on aerobic decomposition, a process driven by microorganisms that thrive in oxygen. When compost becomes waterlogged and emits a foul odor, it signals that this beneficial biological activity has been interrupted. The unpleasant smell, ranging from an ammonia sting to a rotten egg stench, indicates the composting environment has become oxygen-starved. These wet, slimy conditions slow decomposition and cause nutrient loss.
Identifying the Root Cause: Anaerobic Conditions
Wet and smelly compost originates when excess moisture saturates the material, displacing the air within the pile’s pockets. This lack of oxygen causes aerobic bacteria to die off rapidly. Once aerobic microbes are gone, anaerobic bacteria take over the decomposition process.
Anaerobic decomposition proceeds much more slowly and generates malodorous compounds. The sharp, pungent smell of ammonia occurs when too much nitrogen-rich material, or “greens,” breaks down without sufficient carbon and oxygen. Nitrogen is lost to the atmosphere as ammonia gas. The sulfurous, rotten egg smell is caused by hydrogen sulfide released during the putrefaction of sulfur-containing amino acids in the absence of oxygen.
Emergency Fixes: Drying and Aerating the Pile
Immediate intervention requires simultaneously addressing the lack of oxygen and excess moisture to reverse the anaerobic state. First, physically break up the compacted, wet material to reintroduce air. Use a pitchfork or aerator to thoroughly turn the entire mass, focusing on dense, slimy clumps that hold the most water.
As you turn the pile, incorporate highly absorbent, carbon-rich materials, or “browns,” to soak up the excess liquid. Excellent choices include shredded newspaper, torn cardboard, dry straw, or fine sawdust. These materials absorb water and provide the structural integrity needed to create new air pockets, preventing immediate re-compaction.
Do not simply dump the dry materials on top; layer and mix them completely throughout the wet compost for maximum absorption. For severely wet compost, spread the material onto a tarp to air-dry briefly before mixing in the browns and returning it to the bin. This combination of turning and adding dry matter is the fastest way to stop the offensive odor and restore the environment for aerobic microbes.
Maintaining the Balance: The Brown to Green Ratio
Preventing future wet and smelly conditions relies on consistently maintaining the proper ratio of carbon-rich materials (“browns”) to nitrogen-rich materials (“greens”). Microorganisms require carbon for energy and nitrogen for protein synthesis, ideally consuming them at a Carbon-to-Nitrogen (C:N) ratio of about 25:1 to 30:1. Too many greens cause the ratio to drop too low, leading to nitrogen overload, ammonia smells, and excess moisture.
Greens are fresh, moist materials like food scraps, coffee grounds, and grass clippings, which are high in nitrogen and compact easily. Browns include dry leaves, wood chips, and paper products, which are high in carbon, absorb moisture, and provide structure for airflow. Aim for roughly two to three parts brown material for every one part green material by volume.
Proper moisture management is equally important and can be gauged using the “wrung-out sponge” test. The compost should feel damp, like a sponge that has been squeezed hard, but it should not drip water when you pick up a handful. To prevent compaction, chop or shred larger materials before adding them, as smaller pieces allow air to circulate more freely. Balancing the C:N ratio and managing moisture creates a stable environment where aerobic decomposition thrives.