Air is one of the keys to making compost.
In fact, some studies suggest even a temporary absence of air can lead to a reduction in compost quality.
In a previous article, we took a look at the role of oxygen in compost. That was the more sciency, technical aspect (but super interesting – do take a look!)
This time around, I wanted to show you all the different ways in which you can aerate your compost.
They range from easy to super-complicated – but one of the keys here is that you don’t need to apply all of them.
Start your compost off with a layer of sticks and twigs
One of the ways I sometimes try to introduce air into the compost from the start is to put a layer of sticks at the bottom of the compost pile.
This traps air at the bottom of the compost heap or bin.
Do note that wood takes a lot longer to rot than other compost materials, so don’t expect it to be broken down when you turn your compost. What’s more, it can be a bit of a pain when you are turning the bottom of your pile, and it gets caught in your fork.
Borrowing a tip from the Red Gardens No Rule Composting system, when I turn the compost, I tend to either put semi-decomposed branches aside for the next compost heap or chuck them in an earlier stage pile.
Utilize a compost bin with air holes
You can either buy a compost bin that has ventilation built in or make your own.
For example, a friend has an amazing compost bin that his father, an engineer at a steel works, made 40 years ago.
Made of steel, it stands on four short legs and has perforated holes that allow air to flow into it.
That’s probably a bit ambitious for most people. However, pallet bins, which are quick, easy and cheap to make, also allow plenty of access for air.
At the same time, there’s a trade-off here – more access to air via the holes means less insulation, which means less warmth.
It’s also true that many people make perfectly good compost without solid-sided bins. Still, if you are looking to maximize airflow, this is one way to help.
Use a base with air holes
You can also use a base for the compost heap which traps air under the compost.
An easy way to do this is to use a pallet to create the base of the compost bin. The pallet traps air at the base of the compost heap.
If you are using wire or pallets to create the sides of your bin – or any material with holes in it – your compost heap will then be exposed to air from every angle.
Introduce bulking material
Some compost materials (usually high-carbon browns) trap air in the compost, creating what is known as Free Air Space.
These bulking agents are one of the best ways for the home composter to aerate their compost. I now keep a large bag of sawdust in my shed so I always have some on hand.
To learn more about bulking materials, including the different options you can use, read our guide here.
Poke holes with a bar
Another way to aerate your compost is to poke holes into it with a bar or stick.
I’ve tried using a metal bar for this process, and I try to create holes both vertically and horizontally through the compost. However, this gets harder the bigger your compost heap gets.
Use a compost aerator
Another option that I’ve experimented with is using a compost aerator.
These are tools specifically designed to introduce air into the compost heap. There are different kinds of aerators – some act like a screw, while others have turning blades which release after you have pushed the aerator into the compost.
I use it after I’ve turned compost once, and it does a good job of introducing enough air to keep the compost hot. However, you do have to learn to wiggle the device as you bring it up, or the blades don’t always open.
Despite its limitations, it requires a lot less effort than turning the compost again!
Use a compost bin stirrer
If you have a compost bin, a compost stirrer (also called a compost stirring rake) is absolutely brilliant. Mine came with my hot bin. It’s essentially a straight metal bar with a handle and hook at the end.
After introducing new materials, you simply hook the stirrer down into the compost and mix up the new materials with the old. It’s very effective at keeping the compost hot.
You can buy them cheaply enough from HotBin, or simply make your own.
Turning your compost
This is deliberately further down the list because it is not the easiest way to introduce air into your compost!
There are also plenty of composters who are against turning.
Joseph Jenkins in the Humanure Handbook argues that it discourages composters and leads to nutrient loss, while 02 compost estimates that oxygen levels return to 1% in 30-45 minutes. (They were referring to windrows, which are likely to have larger oxygen requirements than the average home heap.)
In my garden, I’m shifting this year from turning my pile twice to turning it once. But I do find that single turn useful. For starters, I have found that it can restart a stalled compost heap at the end of winter, even when the moisture level is too high.
Another advantage of turning a pile is that you can present compost bacteria with less decomposed materials.
That’s because the materials in your pile decompose at different rates. The material at the center of your pile is the warmest, which means thermophilic bacteria rapidly get to work on the material. On the outside of the pile, where it is the coolest, the decomposition process can be very slow.
When you turn the pile, you work to put the most rotted material on the outside, and the least rotted on the inside.
In a large or very active pile, oxygen introduced in the pile can be used up very quickly. However, when you turn the pile, you also create new paths for air to enter into the compost.
Turning also has other advantages too.
First, it allows you to check moisture levels. I sometimes find that the heat a compost pile generates leads to very dry layers, and I need to spray layers with water. The combination of turning and adjusting moisture levels can be a very powerful way to restart a compost heap, and the heat can be maintained with a compost aerator.
Secondly, it can be an effective way to reduce the temperature of your heap if it is getting too hot. I like to try and keep my pile to 65 degrees or less, as when it gets above this you start to reduce microbial diversity in the finished product.
It does sound strange that introducing oxygen, which is one of the keys to creating heat in the first place, can cool down compost. However, by redistributing the hot compost to the outer parts of the pile, and the cool to the center, you can reduce temperature (at least in the short term!)
Also see: Best Compost Forks
Limiting the size of the pile
If you have a reasonably sized pile, some air will enter from the sides and top of the compost heap. However, if it gets too large, air will not able to access it.
For many home composters, this won’t be an issue. However, if you have a stable or a farm, size can become more of an issue.
One farm near me has an enormous manure pile, which has gained size over many years. It’s so big you could drive a tractor onto it. (In fact, they sometimes do so.)
I harvest manure from it once or twice a year, as they never use herbicides on their fields. The top level of the pile is usually well-rotted compost (perhaps because chickens are always scratching the surface!), but go more than a spade or so deep and the manure is less decomposed.
So, if you have a lot of compost material, it’s worth following Cornell University’s advice and limiting the width to 10-15 feet and the height to 6-8 feet.
Introduce pipes into the compost heap
You can also build aeration into the pile by using perforated pipes. You want the pipes ready before you start constructing the pile. Options include using PVC pipes with holes drilled in it or rolled-up chicken wire.
The pipe needs to be long enough so that it reaches right from the bottom of the pile to where the top will be when the pile is completed.
Use forced aeration
Now we start to enter the world of industrial composting technologies and/or the fanatic composter! But for the sake of comprehensiveness, it’s worth mentioning forced aeration.
As the name suggests, this uses a powered fan or pump to force air through the compost.
The pro here is that the aeration does a lot of the work for you, and can ensure you have optimum levels of oxygen. At the same time, you are increasing the setup cost, energy use and complexity of what is otherwise a fairly simple operation.
If you are interested in knowing more, I recommend checking out the Urban Worm Company’s introduction to Aerated Static Pile Composting.
What else affects air levels in compost?
To maintain oxygen levels, avoid getting the compost too wet. If too much water enters the compost, it can force oxygen out.
See Moisture Levels in Compost for more information.
Compost material size
Generally, it’s considered a good idea to shred, cut, or reduce the size of the organic material we use to make compost. That’s for good reason – the smaller it is, the more access bacteria have to the material.
However, if the particles in the compost are too small, they can cluster together, reducing the ability of air to circulate through the pile.
The same goes for bulking material – when the size is reduced, it is easier for bacteria to access carbon. But if it’s too small, it will provide less free air space.
As you probably know if you’re a keen enough composter to read this far, you want to get your compost heap hot because it encourages heat-loving bacteria.
This is one of those chicken and egg things, though – as it gets hot and thermophilic bacteria get going, they use up more oxygen in the pile.
The heat they generate also causes the air to rise, which causes a loss of oxygen.
If you have a large, hot pile it’s well worth monitoring your temperature with a thermometer.
There are a lot of ways to aerate your compost here. Which one do you use?
I am glad you asked me – that seems like a nice way to round up this article!
I’ve already mentioned I turn my compost – but, going forward, this will just be a single turn. I also use bulking materials (currently sawdust) and sometimes lay sticks at the bottom of my compost.
My insulated compost bin has some aeration built in, but my pallet bins provide plenty of aeration around the sides. When I rebuild the pallet bins (they are coming to the end of their lives) I may use an extra pallet as a base in the future.
I also have an aerator, which is not perfect but does help keep the compost warm after the first turn.
I’ll also be experimenting with more methods, and I encourage you to do so too. However, I think of everything I do the most important is using bulking material and the single turn.