Not all compost smells mean something is wrong. In practice, three broad odour types are commonly experienced, each with different causes and implications.
Mild, cabbage‑like or earthy smells are often associated with periods of high microbial activity. These smells are usually not unpleasant to most people and commonly occur as fresh material is actively decomposed. In enclosed hot composting systems, such odours may be reduced as air passes through carbon filters fitted in the lid. They are typically short‑lived and resolve naturally.
Sharp or ammonia‑type smells are most often linked to large additions of fresh grass clippings or other nitrogen‑rich materials. As these break down, excess nitrogen can be released faster than it can be stabilised, producing a pungent but usually temporary smell.
Strong, unpleasant, or putrid odours are more indicative of oxygen limitation. These develop when compost becomes too wet, compacted, or structurally collapsed, allowing anaerobic conditions to dominate locally.
Enclosed compost bins can reduce odour risk by supporting airflow and separating excess liquid, but no bin can fully prevent odours if material balance and structure are lost.
Tip to improve:
- For mild cabbage‑like smells, leave the bin undisturbed; these often fade within a day or two.
- For ammonia smells after adding grass, mix in dry, shredded paper or other carbon‑rich material to rebalance inputs.
- For strong, unpleasant odours, gently stir the contents and add coarse bulking material and dry paper to restore air spaces and reduce excess moisture.
