How to Build a Compost Bin That Doesn’t Smell
Updated: Sep 2
A healthy compost bin should have a mild, earthy smell. It cannot be guaranteed to produce no smell at all, but persistent sour, rotten-egg or ammonia odours usually mean that the system needs adjusting.
The solution is rarely a special additive. Most problems can be corrected by balancing carbon-rich and nitrogen-rich materials, moisture, air and particle size.
What a compost bin needs
Composting is carried out by microorganisms that use carbon for energy and nitrogen for growth. They also need water and oxygen.
A commonly cited starting point is a carbon-to-nitrogen ratio of about 25–30 parts carbon to one part nitrogen by weight. Home composters do not need to calculate this precisely. A practical approach is to add approximately two or three loose handfuls of brown material for each handful of wet, green material.
Brown, carbon-rich materials include:
Dry leaves
Shredded, non-glossy cardboard
Torn paper
Straw
Small amounts of untreated sawdust
Fine woody prunings
Green, nitrogen-rich materials include:
Fruit and vegetable scraps
Coffee grounds
Fresh garden clippings
Fresh weeds that have not set seed
Small amounts of grass clippings
Too much wet, nitrogen-rich material can use oxygen quickly and create ammonia or rotten odours. Too much dry, woody material usually causes a different problem: the pile decomposes very slowly.
Choose a suitable bin and location
Choose a bin that is large enough for the amount of material your household produces. It should have:
A secure lid
Openings for air
Access for mixing or turning
Protection from excessive rain
A base that prevents scraps spilling out or being easily reached by pests
Place the bin on a level, well-drained site where any escaping liquid cannot enter a drain, waterway or neighbouring property. A soil base can allow soil organisms to enter, but a rodent-resistant mesh base may be helpful where rats or mice are a problem.
Keep the bin reasonably close to the kitchen and garden. A bin that is difficult to reach is less likely to be managed regularly.
Build the first layer
Begin with a loose layer of coarse brown material, such as small twigs, dry stems or shredded cardboard. This helps maintain air spaces near the base.
Add green and brown materials in mixed layers. Do not create a thick, compact layer of food scraps or lawn clippings. Cover each addition of kitchen scraps with brown material.
Keep a lidded container of shredded cardboard, dry leaves or torn paper beside the bin. This makes it easier to add carbon every time food scraps go in.
Manage moisture
Compost should feel damp, similar to a wrung-out sponge. It should not drip when squeezed.
If the pile is too wet:
Add dry leaves, shredded cardboard or another absorbent brown material.
Mix the wet section into the rest of the pile.
Protect the bin from heavy rain.
Check whether drainage holes are blocked.
If the pile is too dry:
Add water gradually while mixing.
Add moist green material.
Cover the pile to reduce drying.
Excess water fills the spaces that would otherwise hold air. This encourages anaerobic decomposition and offensive odours. Very dry material, however, will decompose slowly.
Maintain oxygen and structure
Turn or mix the compost when it becomes compacted, wet or smelly. Move material from the edges towards the centre and break apart dense clumps.
Particle size affects both decomposition and airflow. Smaller pieces expose more surface area to microorganisms, but material that is chopped too finely may pack together.
Aim for a mixture of sizes:
Chop large food scraps and tough stems into smaller pieces.
Leave some coarse brown material to maintain air spaces.
Break up wet clumps of grass or leaves before adding them.
A compost aerator can be useful in a tall enclosed bin. A garden fork may be more practical in an open bay or tumbler.
What should stay out of a basic home compost bin?
Unless you use a system specifically designed and managed for these materials, avoid adding:
Meat, fish and bones
Dairy products
Fats and cooking oils
Large amounts of cooked food
Dog or cat faeces
Diseased plant material
Weeds carrying mature seeds or persistent underground parts
Treated or painted timber
Glossy or plastic-coated paper
Compostable packaging unless your home system can meet its stated processing requirements
Animal products, fats and cooked foods can create odour and pest problems. Pet waste may contain pathogens and should not be placed in compost intended for food gardens without an appropriately controlled treatment process.
Prevent pests
Pests usually indicate that food is exposed or unsuitable material has been added.
To reduce pest access:
Bury fresh scraps in the active compost.
Cover scraps with a generous layer of brown material.
Keep the lid closed.
Repair gaps and damaged mesh.
Avoid meat, dairy, oils and large amounts of cooked food.
Harvest finished compost rather than allowing the bin to remain full and unmanaged.
Do not use poison baits inside the compost. They may create risks for pets and wildlife.
Manage leachate safely
Liquid draining from an overly wet compost bin is leachate, not finished compost tea. It may contain concentrated nutrients and microorganisms.
Reduce leachate by correcting excess moisture and protecting the bin from rain. Do not allow it to enter stormwater drains or waterways. Wear gloves when cleaning spills and wash your hands afterwards.
Compost troubleshooting guide
It smells like rotten eggs
Likely cause: The pile is too wet, compacted or low in oxygen.
Action: Mix it thoroughly, add coarse dry brown material and protect it from rain.
It smells strongly of ammonia
Likely cause: There is too much nitrogen-rich material.
Action: Add dry carbon-rich material and mix it through the pile.
It is not breaking down
Likely cause: The pile may be too dry, too cool, too woody or too small.
Action: Check moisture, add some green material, reduce oversized pieces and mix the contents.
There are flies around the bin
Likely cause: Food scraps are exposed.
Action: Bury the scraps and cover them with dry leaves, cardboard or finished compost.
Rodents are entering the bin
Likely cause: There is an access point, exposed food or an attractive input.
Action: Remove unsuitable food, secure the lid and install rodent-resistant mesh where appropriate.
A simple weekly routine
Once a week:
Check the smell.
Squeeze a handful to assess moisture.
Look for exposed food or pest access.
Mix any wet or compacted areas.
Add brown material if the contents are dense or wet.
Clean up spills and check drainage.
Small, regular corrections are easier than rebuilding a neglected pile.
Evidence and limitations
Composting conditions vary with climate, bin design, ingredients and maintenance. A ratio of about 25–30:1 is a useful scientific guide, but household systems can be managed successfully using the simpler green-and-brown method.
This guidance draws on Evidence A sources from an Australian environmental regulator and Evidence B university compost science. Persistent offensive odour should be treated as a sign of imbalance or unsuitable material, not as a normal feature of composting.
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References
Cornell Waste Management Institute. (n.d.). Compost chemistry. Cornell University. Evidence B.
Cornell Waste Management Institute. (n.d.). Getting the right mix. Cornell University. Evidence B.
NSW Environment Protection Authority. (2012). Food and garden organics: Best practice collection manual. Evidence A.
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