When you throw organic matter onto your compost pile, it immediately begins to decompose, thanks to the trillions of bacteria that we share our environment with, along with fungi and molds, protists, and higher organisms such as worms and insects. They all play a role.
There are two different types of decomposition, aerobic (in the presence of oxygen) and anaerobic (without oxygen), and each has its own set of associated organisms. In your compost pile, you want aerobic decomposition to be favored, because anaerobic decomposition produces bad smells and amines, some of which are toxic to plants. If your compost pile starts to smell like rotten eggs or ammonia, you need to turn it to let in more air.
Bacteria make up 80-90% of the organisms in your compost heap, and because different types of bacteria take turns doing different jobs, the process from kitchen scraps to soil happens in three distinct phases. There are billions of bacteria in a single teaspoon of healthy compost, and they use many different enzymes to break organic material down into nutrients and humates (the complex of organic matter left over after decomposition).
The first phase of decomposition is carried out by the mesophilic and psychrophilic bacteria, which consume all of the readily available, easy-to-eat organic matter. As they do this, the resulting energy production causes the temperature of the compost pile to increase. Psychrophilic bacteria are most active at around55°F, although they can even be at work (very slowly) during the cold winter months, while the mesophilic bacteria favor temperatures of 70º to 100º F.
As soon as the temperature rises above 100º F, the thermophilic (heat-loving) bacteria take over, with members of the genus Bacillus dominating the crowd. The inside of the compost pile can be raised to 130º to 160º F at this point. Bacillus bacteria have the ability to form protective endospores, which can lie in wait in the soil until conditions are favorable for them to thrive. When the compost pile reaches 160ºF, it’s too hot for the Bacilli, and they return to their spore form to wait until things cool off.
At the highest compost temperatures, 140º F and above, almost all bacterial species that might be harmful to plants or humans have died off, and above 160º F, only members of the genus Thermus might be active, if they’re present. These are the same bacteria that evolved to live in hot springs and near thermal vents.
Once the compost cools down again, micro-organisms from the surrounding environment move in, including the actinomycetes (although some species of actinomycetes are also present during the thermophilic phase as well). Actinomycetes are a bacterium that forms long, thin filaments through the compost or soil, and they may look like a spider web if you see them in your compost pile. These bacteria are very important in breaking down the “tough” stuff like the cellulose of wood or bark, the things none of the other bacteria want to eat. They are also responsible for making enzymes that give soil that wonderful ‘earthy’ smell.
This final phase of decomposition goes on indefinitely, as bacteria continue to breakdown smaller and smaller components of organic matter into all of the nutrients that make our gardens thrive.