Compost Science: What’s Happening in Your Compost Bin?

Learn the compost science behind controlling the aerobic decomposition process to turn organic waste into rich humus.

| May/June 2010

  • Kitchen Scraps Headed for the Compost Bin
    You may have heard of green and brown compost. This would be an example of green.
    iStockphoto.com/Phillip Danze

  • Kitchen Scraps Headed for the Compost Bin

At its heart, composting involves creating the best conditions possible for something that already occurs in nature: rot. In this aerobic decomposition process, organic waste is broken down by microorganisms (such as bacteria and fungi) into fiber-rich, carbon-containing humus with plenty of good-for-your-garden nutrients like nitrogen, phosphorus and potassium. Larger decomposers (like mites, centipedes, and various insects and worms) are also at work physically making the materials smaller. You can watch this process happen on the forest floor, or you can help it along in a compost bin or pile.

All you need to create compost is some special trash – characterized as organic waste, such as newspaper, leaves, grass, kitchen waste (fruits/vegetables) or woody mater-ials – a place to pile it, some moisture to help it along and a regular supply of oxygen. Then you just let nature take care of the rest.

The breakdown involves three stages – moving from the easiest to most difficult compounds to break down. Think of a partially desiccated leaf with only veins and stem remaining. The softer portions have been broken down, but the harder, woodier portions still remain to be attacked by the next group of bacteria.

The bacteria involved in the process are aerobic. Carbon is their energy source, and they use oxygen to break it down into usable forms – leaving heat and carbon dioxide in their wake. In the first stage, mesophilic bacteria work to break down the easy-to-get-at compounds (like sugar and starch). They are “mesophilic” (middle-lovers) because they are happiest and survive best at medium temperatures of 70 to 100 degrees Fahrenheit. Their reactions cause the compost pile to heat up and make way for another part of breakdown and the second wave of bacteria, the heat-lovers.



Thermophilic bacteria thrive at hotter temperatures ranging from 113 to 160 degrees. At this temperature, many of the disease-causing beasties are killed, too. The thermophiles break down proteins, fats and cellulose (a major component in plants). As these compounds are consumed, the reactions start to slow down (perhaps after as short a period as three to five days), and the temperature lowers. The third, or curing, stage then begins. Fungi and actinomycetes (also called filamentous bacteria – they are what causes the humus to smell “earthy”) prefer cooler temperatures (70 to 75 degrees) and show up to break down the toughest compounds, such as the complex carbohydrate lignin. The curing stage can take a couple of months.

Keeping the composting reactions moving requires moisture, oxygen and an appropriate mixture of organic materials. The perfect pile contains 40 percent to 60 percent moisture. If the pile gets too wet, oxygen can’t get to the reaction. The pile must be created in a way that allows air into the middle, and it must be turned periodically to keep it aerated.



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