Who's at Work in the Compost Pile?

Compost seems like slow-acting magic to the human eye. You start out with materials like food scraps, grass clippings, and autumn leaves that you put in a pile or bin. If the materials are kept moist and air is added by turning, after twelve weeks of warm weather, presto, you have compost. How does this magic happen? Do the "little people" come in the middle of the night? The scientific answer is: sort of. The "beings" that make compost are very small but they are not people and they are active twenty-four hours a day. Most of them are so small that you need a microscope to see them. Such tiny organisms are given the general name microbes.

Making compost is a microbial process. Most of us are familiar with the fact that we use yeast to make bread or beer. Composting involves a much more complex mixture of microbes than we use in food preparation. Fortunately, nature looks after adding the microbes. Most compost materials come with microbes attached and ready to go to work. We can add some top soil and finished compost to the mix, just to make certain that there is a well rounded group to get the job done. Scientists have not identified every microbe at work in a compost pile but the general categories are well known; they are also coming to understand that the microbial life in compost is a very valuable addition to soil.

The types of microbes involved in the actual breakdown of the compost materials are:

Fungi are the most familiar of these organisms because they can be seen in some forms without a microscope: mushrooms, mould on food, yeast growing before it is added to bread dough. We don't see the bulk of the fungi at work in the world. They exist mostly as hyphae -- tiny tubes that can grow through soil or leaves on the forest floor. Hyphae differ in width; a few are large enough to see without a microscope and are visible in maturing compost piles and leaf litter in forests. Fungi, like animals, cannot make their own food. The ones involved in composting are decay organisms -- they can only use dead plants and animals as food. Fungi are active throughout the composting process but they tend to dominate in the later stages of breakdown because they can degrade some of the more difficult compounds like lignin (found in woody products).

Bacteria are the tiny, often single-celled, workers that we are trying to keep happy when we talk about how to make compost. Like fungi, bacteria use the compost materials as food. Bacteria are most active in the early and middle stages of the compost process. They may be invisible to the naked eye, but your nose can tell you which group of bacteria are running the show. If a newly-made compost pile heats quickly and doesn't smell bad, then the aerobic or oxygen-using bacteria are dominant. This is good news. You can help them keep up the good work by turning the materials to maintain the air levels in the pile. If things stay cool and start to smell bad, there is not enough air (oxygen) in the pile and the bacteria change to a different mode of using the food. This results in bad smells like rotten egg gas (hydrogen sulfide) being released.

Actinomycetes are a special group of organisms that are now classified with the bacteria because of their cell construction. Under the microscope, given their filamentous shape, they look more like fungi. They are active throughout the compost process but seem to be more important players in the later stages. Streptomyces is a well known actinomycete. It releases chemicals called geosmins that give soil its "earthy" odour; it is also the source of many antibiotics used in human and veterinary medicine. A compost pile is a busy place!

(Source: WasteWatch, December 2003)

Back to Composting main page

Back to Resources main page

Back to Home page