Summary
Composting works because living microbes break down organic material. As they do this, they release warmth, which tells us how active the process is. A good compost bin helps hold the right conditions so this natural process can keep going.
What happens inside a compost bin
Inside a compost bin, billions of microscopic organisms are feeding on organic material such as food scraps, garden waste, and plant residues. These microbes need three basic things to stay active: air, moisture, and warmth.
As microbes consume organic material, they use oxygen and release energy. Part of that energy becomes heat. This is why compost heaps often feel warm in the middle. The warmth is not added from outside — it is produced by the biology inside the bin.
At first, microbes focus on the easiest materials to digest. Soft plant tissues, sugars, and proteins are broken down quickly. During this stage, activity rises and temperatures increase. As these easy foods are used up, the pace naturally slows and temperatures fall.
Throughout the process, the compost is changing. Fresh scraps gradually lose their original form. What remains becomes darker, crumbly, and more stable. The rise and fall of temperature simply reflects how active the biology is at each stage.
The role of warmth in composting
Warmth helps microbes work efficiently, but it does not cause composting by itself. Compost heats up because microbes are active, not because heat is forcing the process to happen.
When conditions are right, compost often passes through a warm phase. During this time, microbes that prefer higher temperatures become more common. They continue breaking down tougher materials that were slower to change earlier on.
There is a useful middle range where composting tends to move along smoothly. In this range, microbes are active without being stressed. If compost becomes too cool, activity slows. If it becomes too hot, some microbes struggle and overall progress can stall.
This is why steady warmth matters more than extreme heat. Compost does not need to stay very hot to finish properly. In fact, cooling over time is a normal and healthy part of the process.
Air and moisture work with temperature
Temperature never acts alone. It is tightly linked to air and moisture.
Microbes need oxygen to do their work. As compost warms up, microbes breathe faster and use oxygen more quickly. If fresh air cannot reach the centre of the pile, oxygen levels drop and the process slows.
Moisture matters just as much. Microbes live in a thin film of water around organic particles. If compost dries out, microbial activity fades and temperatures fall. If it becomes too wet, air spaces fill with water and oxygen cannot move freely.
Healthy compost usually feels damp but not soggy. When air and moisture are balanced, warmth takes care of itself.
Why structure and bin design matter
How compost materials are held together makes a big difference to how heat behaves.
Very loose piles lose heat quickly. Very dense piles can trap heat but also restrict airflow. The most effective compost structure sits between these extremes, holding warmth while still allowing air to move through.
A compost bin helps by providing shape, containment, and some insulation. This reduces heat loss and protects the compost from drying winds or heavy rain. The result is not higher temperatures for their own sake, but more stable conditions that allow microbes to keep working.
Good design supports the biology rather than trying to control it.
What composting does — and doesn’t do
Composting turns mixed organic waste into a more stable, soil-like material that is easier to handle and use. Over time, smells fade, volume reduces, and materials become unrecognisable.
Composting does not happen instantly, and it does not stay hot forever. Cooling is not a failure; it signals that the most active stage has passed. Compost also continues to change after it cools, as slower biological processes finish the job.
Warmth can help reduce some unwanted organisms when conditions allow, but it is only one part of a wider process. Time, oxygen, and moisture all play essential roles.
How this links to choosing a bin
Different compost bins manage air, moisture, and heat in different ways. Understanding the role of temperature helps explain why some bins keep compost active more reliably than others.
On the next page, you can explore what materials can be composted and how they behave inside a bin — or look at how different bin designs support the conditions composting needs to work well.
