How Does a Compost Bin Work?

The Basics

In some ways, a compost bin works a lot like a slow cooker. When organic material is heaped inside, the compost bin acts like a crock pot, allowing the ingredients to simmer slowly over low heat for a period of time.
Unlike a crock pot, however, a compost bin doesn't rely on electricity. It uses bacteria and nitrogen instead to "cook" and break down the organic ingredients in the compost. This process of breaking down occurs in four stages.

Stage 1: Mesophilic

In the first stage of the composting process, mesophiliac, or medium-temperature, bacteria that specialize in the breaking down of organic compounds begin to proliferate, raising the core temperature of the compost heap to 113 degrees F.
After a week or two, the heap will show a noticeable sag or signs of settling. This signals that the compost is entering the next stage of breaking down.

Stage 2: Thermophilic

Mesophilic bacteria gradually give way to thermophilic, or high-temperature, bacteria. During this very active stage, things really start to heat up. The more thermophilic bacteria that are present, breaking down compounds and releasing heat as a by-product, the hotter the pile gets, often reaching temperatures of 150 degrees F. At this point, most harmful bacteria, pathogens and weed seeds are killed off and the heap begins to break down rapidly.
Depending on compost bin size and the types of organic material it contains, the thermophilic stage can last from a few days to several months. As the microbes gradually deplete the food supply, their energy output declines and so does the heat in the compost pile.

Stage 3: Psychophilic

Next comes the psychophilic or cooling stage. The microorganisms that were killed or chased off during the thermophilic stage now return to the compost to finish their work There's still lots of food left in the heap and lots of work to be done breaking it down, so beetles, larvae, worms join in and help. Finally, when the compost has been degraded into a more easily digestible form, fungi move in and break it down even further
It can take many months to break down all the organic material in the heap before moving on to the final stage.

Stage 4: Curing

The fourth and final stage of composting is also called the aging or maturing stage--and many professional gardeners believe it's the most important one.
A long curing offers a safety net ensuring that phytotoxins--substances harmful to plants--are killed off, along with pathogens that rob soil of nitrogen and water.
Curing can take four months or as long as a year, depending on the size of the compost bin and the types of organic matter it contains.
The finished product, called humus, is dark, crumbly, earthy-smelling-- and a nutrient-rich treat for your lawn and garden.

Resources

• Find out more about compost bins.