Free Air Space: The Magic Key to Making Compost?

Free Air Space (abbreviated to FAS) refers to the air pockets created in compost piles.

In many guides to composting, a huge amount of focus is given the Carbon:Nitrogen ratio. While this plays a role in composting, getting the C:N right is both difficult and less important than you might imagine. In contrast, FAS is hugely important, enabling you to achieve faster, better compost with less work. 

In this article we’ll discuss what free air space is, why it’s important and how to get it in your compost pile. 

What is free air space?

Free Air Space refers to the air pockets created in your compost by gaps around the individual particles. 

What is the benefit of free air space?

Free air space allows oxygen to circulate through the compost heap. Oxygen is one of the key components needed to turn waste material into compost, as micro-organisms need oxygen to survive and multiply

By constructing your compost heap to allow air to circulate, you can reduce or even eliminate the need to turn your compost. This can save time (for the individual composter) or money (for the commercial composter).

What is the ideal amount of free air space (FAS)?

Target ranges for the amount of air space you need vary considerably. According to Efta and McCartney, the recommended air space can vary from 26 to 61% (1), while Bramwell et al recommend targeting between 35 and 60% (2).

As always, for the non-commercial gardener, we suggest you don’t overly worry about the exact amount of air-space, and instead follow the suggestions in our article on bulking materials.   

Is it possible to measure free air space?

Yes, it is possible to do a test to determine how much air space you have. However, this is a fairly involved process which involves filling a bucket with sample compost material and dropping it ten times to compact the material! What’s more, you would need uniform compost material throughout your pile to get anywhere near being accurate. If you are interested, the New Zealand government has produced a guide to measuring free air space using the bucket test. 

How do you get free air space in your compost?

There are two key elements to achieving free air space in your compost. The first is to ensure that your compost does not get too dry. If there is too much moisture, water will be absorbed into the free air spaces, which means you will not be able to make compost. (Of course, you don’t want your compost to be too dry either, because the microbes in your compost need some moisture in order to multiply). 

The second is to introduce bulking materials. Bulking materials have several purposes, but one is to create those air pockets in your compost pile. To learn more about bulking materials, and how to use them to create air pockets, see our article: Compost Bulking Agents.