Compost smells bad? How to fix common composting problems in Bristol

If your compost smells bad, you’re not alone. Smelly compost is one of the most common reasons people give up on composting, especially in urban areas like Bristol where space is limited and food waste volumes can be high.

The good news is this:

Bad-smelling compost is always fixable.

Healthy compost should smell earthy and fresh, like soil after rain. If it smells rotten, sour, or strongly unpleasant, it’s a sign that something in the composting process needs adjusting, not that composting has failed.

This guide explains why compost smells, how to fix it, and when to seek extra support.

Why does compost smell bad?

Smells are caused by the wrong biological conditions. Composting relies on microorganisms that need oxygen and balance to work properly. When those conditions are missing, different microbes take over and produce unpleasant smells.

Common causes of smelly compost in Bristol

In urban composting systems, the most common causes are:

• Too much food waste and not enough carbon material

• Compost becoming waterlogged from rain

• Lack of airflow

• Food waste breaking down anaerobically (without oxygen)

• Adding unsuitable materials

Bristol’s climate, with regular rainfall and mild winters, can make compost more prone to becoming too wet if systems aren’t protected.

What do different compost smells mean?

Understanding the smell can help diagnose the problem.

Rotten egg or sewage smell

This usually means the compost has become anaerobic. There isn’t enough oxygen, and food waste is fermenting in the wrong way.

Sour or vinegar smell

This often indicates too much food waste and not enough carbon material to balance it.

Ammonia or a strong sharp smell.

This can happen when compost is too rich in nitrogen, often from large volumes of food waste or grass clippings without enough browns.

How to fix smelly compost.

Most compost smells can be resolved with a few practical steps.

Add more carbon-rich material.

Carbon materials absorb moisture and provide structure for airflow.

Good carbon materials include:

• Woodchip

• Shredded cardboard

• Scrunched paper

• Dry leaves

• Sawdust (in small amounts)

As a general rule, whenever you add food waste, add a similar volume of carbon material.

Improve airflow

Oxygen is essential for healthy composting.

To increase airflow:

• Gently turn or mix the compost if your system allows

• Avoid compacting food waste into dense layers

• Loosen areas that look wet or compacted

If you’re using a sealed system, ensure it’s used as designed and not overfilled.

Reduce excess moisture

Compost should feel like a wrung-out sponge, not dripping wet.

If your compost is soggy:

• Stop adding food waste temporarily

• Add extra dry material

• Check that rainwater isn’t getting into the bin

• Improve drainage if possible

In Bristol gardens, uncovered compost heaps can easily become waterlogged during wet periods.

Check what you’re adding.

Some materials cause problems if added in large quantities or to the wrong system.

Avoid adding:

• Large amounts of cooked food

• Liquids

• Oils and fats

• Compostable packaging unless your system can handle it

If you regularly compost cooked food, a managed or community composting system may be more suitable.

Preventing smells before they start.

Prevention is easier than fixing problems later.

Cover food waste every time.

Always bury food scraps beneath a layer of compost or carbon material. This reduces smells and deters pests.

Add food waste little and often.

Large dumps of food waste can overwhelm a compost system. Smaller, regular additions are easier to manage.

Choose the right composting system.

In urban settings, systems for managing food waste, such as bokashi or managed community composting, are often more reliable than open heaps.

When smells keep coming back.

If compost smells return repeatedly despite adjustments, the issue may be:

• Contamination

• System design

• Location (too wet, too shaded, poor drainage)

At this point, general advice online often isn’t enough because composting is highly context-specific.

Get practical composting support in Bristol

If you’re based in Bristol and struggling with compost smells, you don’t have to figure it out alone.

Generation Soil runs Compost Clinics that provide hands-on, practical support to:

• Diagnose what’s going wrong

• Fix composting problems on site

• Build confidence with food waste composting

Compost Clinics are ideal if:

• Your compost smells bad despite trying common fixes

• You’re composting in a shared or urban space

• You want reassurance that you’re doing things safely

👉 Explore the Generation Soil Compost Clinic

👉You can also explore our composting and soil workshops to build skills from the ground up

Composting doesn’t have to be stressful.

Smelly compost is a sign that something needs adjusting, not that composting isn’t for you.

With the right balance, airflow, and support, compost can become what it’s meant to be:

Living soil that supports food, biodiversity, and healthy urban ecosystems.

If you’d like help getting there, we’re here.