Many gardeners compost their garden waste, but quite a few don’t make the regular effort to turn their compost!
Composting is a process where microorganisms such as bacteria, fungi and protozoa are utilised to break down plant matter and other biodegradable materials into a dark, crumbly, decomposed organic matter that resists further breakdown, known as humus, or more commonly as compost.
The composting process emulates what occurs on the forest floor, where fallen leaves, branches and bark are broken down slowly to build soil, and their nutrients recycled to feed plants and trees.
Looking to nature, we can see that there’s no compost turning happening in that process, so why bother turning compost?
Well, to put things into perspective, it might be helpful to quote the United States Department of Agriculture (USDA) right here.
An often asked question is, “How long does it take to form an inch of topsoil?” This question has many different answers but most soil scientists agree that it takes at least 100 years and it varies depending on climate, vegetation, and other factors.
Put simply, it takes a very long time for nature to create soil, and that’s the reason why soil erosion is such a huge problem, but that’s another story.
When we build a compost pile, it breaks down fastest in the central core area near the bottom, where it’s most protected, often in contact with the soil, where it retains heat and moisture better. The material on the outside breaks down the slowest, especially if it’s exposed.
Why Does Compost Need Aeration?
Once the composting materials break down, lose moisture and collapse in size, the air spaces between them reduces. This is a problem because garden composting is an aerobic process, meaning it requires the presence of air, because the composting microorganisms need oxygen!
When there is not enough aeration in the compost pile, the composting process slows down, and it can take a very long time for materials to be composted.
In the worst case scenario, when there is literally no air getting into the compost pile, because it’s been sealed by a huge pile of waterlogged lawn clippings for example, then a different group of microorganisms take over, ones which live in the absence or oxygen. They employ different chemical processes to break down plant materials without oxygen, these anaerobic processes are closer to fermentation, and when this happens the compost pile will smell unpleasant. It can have a sharp, acidic, vinegar-like smell, or smell like rotten eggs, due to the production of hydrogen sulphide.
The reason for turning compost is to maintains aeration throughout the compost pile, and to mix materials that aren’t broken down from the outer part of the pile deeper into the centre so they can break down faster.
If a compost pile is just left sitting, and not turned, it will take 6-12 months or longer to completely break down, depending of the climate and weather. The cooler it is, the longer it will take. The material at the bottom will break down first (and can be removed fro there to use), and the rest will follow progressively until the material at the top is reached. The pile will reduce in volume to 1/5 or 20% of its original size.
Turning the compost speeds up the process considerably, and in compost bins, which are not as easy to turn because of their shape, there are two solutions. One is to use a tumbler compost bin, which can be rotated to mix the ingredients. The other is to use a corkscrew-shaped compost turner which grabs broken down compost and pulls it to the surface, covering the uncomposted materials, to mix the ingredients in the bin.