Successful composting relies on a specific balance of materials and conditions to facilitate natural decomposition. A healthy compost pile should have an earthy, forest-floor aroma, signaling that aerobic, oxygen-loving microorganisms are hard at work. A foul smell, whether the pungent scent of ammonia or the odor of rotten eggs, indicates that the decomposition process has failed. This stench is a clear sign that the microbial balance has shifted, requiring immediate intervention to restore the pile.
Understanding the Source of the Odor
The root cause of a bad-smelling compost pile is a lack of oxygen, which results in anaerobic decomposition. This happens when the air channels within the pile become blocked, often due to compaction or excessive moisture. Without sufficient oxygen, the beneficial aerobic bacteria die off, and anaerobic bacteria take over the task of breaking down the organic material.
These anaerobic microorganisms work slower and produce odorous byproducts. For example, a rotten egg smell indicates the presence of hydrogen sulfide gas, created when the pile is waterlogged and tightly packed. A strong ammonia smell signals excess nitrogen, which cannot be properly processed by the microbes and is released as a gas.
Immediate Action: Aeration and Turning
If your compost pile suddenly smells, the quickest and most effective remedy is to introduce oxygen by turning or aerating the material. This action immediately revives the beneficial aerobic microorganisms that require air to thrive. Use a pitchfork or a specialized compost crank to vigorously mix the contents of the pile, ensuring that material from the compacted center is brought to the outside.
Turning the pile breaks up dense, waterlogged clumps and re-establishes the necessary air pockets throughout the mass. The goal is to fluff the mixture, stopping the anaerobic breakdown and encouraging the aerobic bacteria to resume their faster, odorless work. When dealing with a severely smelly pile, you may need to turn it daily until the stench is completely replaced by an earthy scent.
Balancing Inputs: The Green-to-Brown Ratio
Maintaining the correct ratio of carbon-rich “brown” materials to nitrogen-rich “green” materials is the most important long-term strategy for odor prevention. Microorganisms require both carbon for energy and nitrogen for protein production, ideally in a Carbon-to-Nitrogen (C:N) ratio of approximately 25:1 to 30:1 by weight. Too much nitrogen, usually from adding too many fresh grass clippings or food scraps (Greens), causes the excess nitrogen to be released as ammonia gas, resulting in the sharp, acrid smell.
To correct an imbalance and prevent future odors, incorporate a greater volume of Brown materials. Common Brown sources include dry leaves, shredded paper, cardboard, and straw, which are high in carbon and also add structure for airflow. Green materials, such as fruit and vegetable scraps, coffee grounds, and fresh clippings, should be balanced with two to three times their volume in Browns. This layering technique ensures a consistent C:N ratio and prevents the nitrogen-rich Greens from clumping together and creating anaerobic conditions.
Managing Moisture and Particle Size
Moisture control and particle size are secondary factors that significantly influence the availability of oxygen in the pile. The compost mixture should ideally have a moisture content of 40 to 60 percent, feeling like a wrung-out sponge when squeezed. If the pile is too wet, water fills the pore spaces, displacing the oxygen and leading to anaerobic breakdown and a putrid smell.
To fix an overly wet pile, immediately mix in a large quantity of dry Brown materials, such as sawdust or shredded newspaper, to absorb the excess liquid. Conversely, if the pile is too dry, microbial activity slows down dramatically, but this is less likely to cause a foul odor.
The size of the materials also matters. Smaller pieces increase the surface area for microbes, but if chopped too finely, they can compact too easily. Aim for pieces no larger than one to two inches.