Above: Image © Andrew Dunn, Wikimedia Commons

That compost pile sitting in the corner of the yard probably looks about as lively as a library on Friday night, but there is plenty of action going on under those leaves. A whole ecosystem is working hard at breaking down your food and yard waste into natural fertilizer for your garden.

Did you know? Composting is not new, just our understanding of it. Luminaries such as George Washington and Thomas Jefferson were proponents of composting.

Basically, composting is a method of treating solid waste in which the organic material you put in is broken down by organisms in the presence of oxygen. Composting enriches soils with plant nutrients like nitrogen and phosphorus, helps clean up contaminated soils, reduces landfill use, and decreases the use of water, pesticides and fertilizer. In Canada, 80% of our annual solid waste ends up in landfills. Unfortunately, landfills are oxygen-deficient environments, where little decomposition occurs.

Did you know? In 2002, Canadians disposed of 31 million tonnes of solid waste in landfills. That's 2.7 kg of waste per person per day!

Compost heaps teem with life. Macroorganisms, such as beetles and earthworms, coexist with microorganisms, such as bacteria and fungi. The macroorganisms are called physical decomposers since they break large pieces of material into smaller bits. Microorganisms, responsible for most of the decomposition, are called chemical decomposers because they change the chemical form of the input.

The most important microorganisms in the compost are aerobic bacteria, which require oxygen to survive. They carry out cellular respiration on the waste, a process that uses oxygen to convert sugar molecules into carbon dioxide, water, and heat. Therefore, the temperature of the compost increases as decomposition progresses. Temperatures can get higher than 45ºC - way hotter than Death Valley in the summer!

Did you know? There are millions of aerobic bacteria in just one gram of soil!

For optimal compost and living conditions for the macro- and micro-organisms, the oxygen level, temperature, moisture content, carbon/nitrogen ratio, and size of the compost particles must be correct. If oxygen levels fall below 5%, anaerobic bacteria, which don't require oxygen, may take over. When this happens, your neighbours will complain about the rotten egg smell of hydrogen sulfide. The optimal temperature is between 45ºC and 55ºC. If the temperature stays too high for too long, bacteria can die. So how to keep oxygen levels high and reduce the temperature? You guessed it! Turn the heap with a shovel.

Moisture levels should be less than 60%. If it's too wet, anaerobic bacterial will thrive, but if it's too dry, cellular respiration will cease. Composts should be damp, but not soggy. A good carbon/nitrogen ratio lies between 25:1 and 30:1. Carbon sources are generally dry and brown, like leaves, wood chips, and straw, whereas nitrogen sources are generally green and wet, such as food and fresh cut grass. If the ratio is too high, decomposition is slow, but if it's too low, anaerobic bacteria can multiply.

Smaller particles have larger surface area to mass ratios and therefore make composting more efficient. Generally, the larger the surface area exposed to the aerobic bacteria, the more effective the composting process.

There are several styles of composters. If you have a yard, you can make or buy a holding bin, make an old-fashioned heap, or use a turning unit. The turning unit makes finished compost faster because it has better aeration, but is more expensive. If you don't have a yard, worm composters are useful. These can be put in the corner of your kitchen. All you need to get started is a box, some damp bedding like shredded newspaper or leaves, and some red worms.

Happy composting!

Learn More!

Freudenrich, Craig. How Composting Works.

University of Illinois Extension — Composting for the Homeowner

Waste Reduction Week in Canada.

"Composting" — Environment Canada.

"Wastes — Resource Conservation — Reduce, Reuse, Recycle — Composting, U.S. Environmental Protection Agency

Cochran, Soni. "Vermicomposting: Composting with Worms", University of Nebraska-Lincoln.

Swarthout, F.L. (1993) The Science of Composting. The Science Teacher 60(6): 26-29.

De Bertoldi, M., Vallini, G., and Pera, A. (1983) The Biology of Composting: A Review. Waste Management and Research 1: 157-176.

Candace Webb received her Ph.D. in molecular biology from the University of Ottawa in 2006, but now lives in Southern California. When not writing, she spends her time volunteering, hiking, painting, and hanging out at the beach

Candace Webb

I graduated from the University of Ottawa in 2006 with a PhD in Biology. I am now a postdoctoral fellow at UCLA, studying the funky circadian rhythms of plants. Besides science, I love to write, hike, paint, bike ride, and hang out at the beach.


Une diplômée de l’Université d’Ottawa, j’ai reçu mon doctorat en biologie en 2006. Je suis présentement boursière postdoctorale à l’Université de Californie à Los Angeles, où j’étudie les rythmes circadiens des plantes. En plus des sciences, j’aime écrire, passer du temps à la plage et faire de la peinture, de la randonnée et du vélo.



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