The fastest way to turn kitchen and garden waste into compost is aerobic composting.
Aerobic literally means ‘with air’ – so as you can imagine, air is pretty important for aerobic composting!
In this post, we’re going to dig a little into the science of why it’s important, what happens when it’s not present, as well as some of the ways you can ensure there is sufficient oxygen in your compost.
Why is oxygen important?
(Aerobic) bacteria need oxygen
In aerobic composting, bacteria such as mesophiles and thermophiles efficiently break down organic material.
Aerobic bacteria are also called obligates, which means they need oxygen to survive. They use oxygen to help break down sugar and fats, creating energy in the process.
At the same time, they produce enzymes that help them counteract the negative effects of oxygen.
Funghi and physical decomposers also play an important role in helping to break down tougher materials.
Fungus and Actinomycetes (another type of bacteria) help break down materials that thermophilic bacteria struggle with. Physical decomposers grind and tear compost into smaller pieces. Worms help control pathogens and add their castings to the compost.
Like aerobic bacteria, these organisms also need oxygen to survive.
In the early stages of composting, you want your compost to heat up fast.
That’s because as heat-loving thermophilic bacteria take over from mesophilic bacteria, the material is broken down faster. However, as these bacteria get more active they use more oxygen.
Do note that when it comes to heat more is not always better, and you don’t want your compost to get too hot. As we saw in our guide to pathogen elimination in compost, a temperature that is too high can destroy microbial diversity.
With aerobic composting, smells are kept to a minimum.
However, when a compost heap runs out of oxygen, anaerobic (without air) bacteria take over.
These organisms release a number of gases – which include hydrogen sulfide.
This gas is dangerous in closed spaces (although unlikely to be a risk to the home gardener), and it smells like rotten eggs.
Oxygen is also essential for reducing harm to the environment.
When waste is broken down with anaerobic bacteria, about a ton of emissions are created for every ton that is broken down.
Even worse, most of these emissions are methane, which has about 28 times the warming potential of carbon dioxide.
Even a temporary absence of oxygen can affect compost
A 2023 study suggested that limiting oxygen at the start of the process impacted the whole of the composting process.
That’s because anaerobic (without air) bacteria inhibited the formation of aerobic bacteria and helpful enzymes and lead to inferior compost.
How much air do you need?
Bacteria need a minimum of 5% oxygen to survive. Air is composed of 21% oxygen.
What’s more, the air needs to be evenly distributed throughout the heap. If the air trapped in large pores falls below 10% oxygen, bacteria in parts of the pile may not be able to access it. This can lead to pockets of the compost becoming anaerobic.
As the compost becomes active, bacteria use up oxygen and the level of air in the heap can fall.
For all those reasons, we really need a minimum of 10% oxygen in compost.
Practical Tips for the composter
For all our talk of percentage levels of oxygen in compost, the home composter doesn’t need to measure oxygen or air in compost.
Instead, it’s a simple matter of appreciating its importance and taking steps to ensure that there is plenty of air available at all stages. Here are a few things to consider:
Including bulking materials
Bulking materials create pockets in the compost which trap air for bacteria to access.
Learn more with our guide to bulking agents.
Ensure moisture levels are correct
Microorganisms need some moisture to survive, but if there is too much the water will fill up air pockets, forcing air out of the heap.
Learn more with our guide to moisture in compost.
Getting the compost heap size right
While you need some size to the compost heap, if it is too large air will not be able to flow into the heap. Cornell University suggests a maximum of 6-8 feet high, and a maximum of about 10-15 feet wide.
If you are using a bin, your size will be smaller, but they will also usually be enclosed. However, compost bins such as the HotBin are designed to both provide insulation and ensure that air flows through the compost.
Aerating the pile
You can take further action to introduce air into the pile by aerating the pile after it has been built.
There are many ways to do this. For the home composter, this can include using a compost aerator, just poking holes in the compost with a bar, or turning the compost.
Bear in mind that with a very active heap, turning the compost will only provide enough oxygen for a few hours. However, at the same time turning the compost does create air passages through which more air can enter.