Compost Problems: Causes and Simple Solutions
A compost heap is a living system, so it will not always look or behave exactly the same. Most composting problems are caused by too much moisture, too little moisture, poor airflow or an unbalanced mixture of materials.
The good news is that very little is permanently ruined. Observe what is happening, make one or two simple adjustments and allow the organisms in the heap time to respond.
The compost smells unpleasant
A strong rotten or sewage-like smell usually means the heap is too wet and contains too little air. This commonly happens when large amounts of grass clippings, kitchen waste or other soft green material become compacted.
Loosen the contents and mix in dry, bulky materials such as:
Torn cardboard
Dry leaves
Straw
Shredded woody stems
Small twigs
Leave the heap uncovered briefly if it is saturated, then protect it from further heavy rain. A healthy compost heap may have a noticeable earthy smell, but it should not smell strongly rotten.
The compost is too wet and slimy
Wet compost lacks the air needed by many of the organisms responsible for healthy decomposition.
Add absorbent brown materials and mix them throughout the wet areas. Check that excess water can drain from the base and that rain is not entering faster than it can escape.
Avoid adding thick layers of fresh grass. Allow clippings to wilt first or add them in thin layers mixed with dry material.
The compost is dry and not changing
Decomposition slows dramatically when the heap becomes too dry. This is particularly common in small bins, open heaps and hot climates.
Add water gradually while mixing the materials. Aim for the moisture level of a wrung-out sponge rather than soaking the heap.
Fresh green material can also provide moisture and nitrogen. Cover the top with cardboard, a sack or a lid to reduce evaporation.
The compost is breaking down very slowly
Cold composting naturally takes time, particularly during winter. Slow decomposition does not necessarily indicate failure.
To speed it up:
Chop large materials into smaller pieces
Add a greater variety of ingredients
Mix dry and wet materials
Check the moisture level
Turn the heap to introduce air
Increase the volume if the heap is very small
Woody stems, twigs and tough leaves decompose slowly and may still be visible when the rest of the compost is ready. Return them to the next heap.
The compost heap is not getting hot
A compost heap does not need to become hot to produce useful compost. Cooler heaps decompose more slowly through the work of fungi, bacteria and soil animals.
If you want a hotter heap, it needs sufficient volume, moisture, nitrogen-rich material and air. Build the heap in a relatively short period, mix the ingredients and aim for a size of roughly one cubic metre or more.
Small quantities added gradually are unlikely to become very hot, but they can still compost successfully.
The heap contains flies
Small fruit flies are attracted to exposed kitchen scraps. Bury fresh scraps beneath the existing material or cover each addition with dry leaves, cardboard or mature compost.
Avoid leaving fruit waste uncovered at the surface. A fitted lid can help, but the heap still needs some airflow.
Larger numbers of other flies may indicate very wet, exposed or decaying food waste. Mix in brown material and avoid adding cooked food, meat, dairy products or oils.
The compost is attracting rats or mice
Rodents may be attracted by food or by the warm, sheltered habitat.
Do not add:
Meat or fish
Dairy products
Cooked meals
Bread in significant quantities
Oils and fatty food
Use a secure bin where rodents are a persistent problem, and fit strong wire mesh beneath it while maintaining contact with the soil. Disturbing or turning the heap occasionally also makes it less attractive as a nesting site.
Rodents can still visit heaps containing only garden material, especially where they are already common, so good composting practice reduces rather than completely removes the risk.
Ants have moved into the compost
Ants usually indicate that the heap is warm and dry. They do little harm to the composting process, but their presence can show that more moisture is needed.
Water the heap gradually and mix the dry material. Regular disturbance will usually encourage the colony to relocate.
There are weeds growing from the compost
Weed seeds may survive in a cool heap. Remove flowering weeds or seed heads before adding plants to compost unless you are confident that the heap will reach and maintain temperatures capable of killing the seeds.
Persistent perennial weeds can regrow from roots or creeping stems. Dry them completely, drown them separately until they have broken down, or dispose of them through an appropriate local green-waste service.
If weeds appear after compost is spread, remove them before they produce more seed.
White growth has appeared in the heap
White fungal threads or patches are usually a normal and beneficial part of decomposition, especially where woody or dry materials are breaking down.
There is generally no need to remove them. Fungi are important decomposers and help turn tough plant material into organic matter.
Very dry, dusty material should be moistened, but visible fungi alone are not a problem.
The compost contains many worms
Compost worms and earthworms often enter a cooling heap and help process partly decomposed material. Their presence is normally a good sign.
Worm numbers may fall if the heap becomes very hot, dry or waterlogged. They will move to conditions that suit them and do not need to be added to an ordinary open-bottomed compost heap.
The finished compost still contains recognisable pieces
Homemade compost does not need to be perfectly uniform. Remove or sieve out large twigs, avocado stones, fruit labels and other pieces that have not broken down.
Return suitable woody material to the next heap. It already carries decomposing organisms and can help create air spaces in the new mixture.
Compost that is dark, crumbly and earthy-smelling can usually be used even if a few original ingredients remain visible.
The compost is attracting slugs
Slugs feed on decaying organic matter and commonly live in compost heaps. Their presence does not mean the compost has failed.
Check finished compost for slugs and eggs before placing it around vulnerable seedlings. Use mature compost as a surface mulch, but avoid piling wet, unfinished material against young plants.
The heap contains plastic or other rubbish
Fruit labels, plastic-coated cardboard, synthetic tea bags and small pieces of packaging can easily enter kitchen-waste containers.
Remove visible contamination whenever you find it. Check packaging before adding material and use only plain paper, cardboard and genuinely compostable plant-based waste.
Products labelled biodegradable or compostable may require industrial conditions and may not break down in a home heap.
Most compost problems can be corrected
Composting does not require a perfect recipe. If the heap is wet or smelly, add dry brown material and air. If it is dry, add water and fresh green material. If it is slow, reduce the size of the ingredients or simply allow more time.
Judge the process by the finished result rather than whether the heap behaves perfectly along the way. With a varied mixture, reasonable moisture and patience, garden and kitchen waste will become a useful resource for the soil.
