The Carbon-to-Nitrogen Ratio Explained
Decomposition is driven by bacteria and fungi that need both carbon for energy and nitrogen for protein synthesis. The ratio of carbon to nitrogen in the pile's bulk — written as C:N — determines how quickly those organisms multiply and how effectively they break material down. At a C:N ratio of roughly 25:1 to 30:1, conditions favour rapid aerobic decomposition. Pile temperatures can reach 55–65°C at the core within days, which kills most weed seeds and pathogens.
The terms "green" and "brown" are shorthand for nitrogen-rich and carbon-rich materials, respectively. The colour description is approximate — coffee grounds are technically brown but count as greens in composting terms because their C:N ratio is around 20:1.
What Counts as Green (Nitrogen-Rich)
Greens are materials with relatively low carbon-to-nitrogen ratios, typically under 30:1. They tend to be moist, soft, and quick to break down on their own. Common examples available in Canadian households and yards include:
- Fresh grass clippings (C:N approximately 15:1 to 20:1)
- Raw fruit and vegetable scraps from the kitchen (15:1 to 35:1 depending on material)
- Coffee grounds and paper coffee filters (C:N around 20:1)
- Fresh plant trimmings from the garden
- Spent vegetable plants at end of season
- Seaweed, if available near coastal or Great Lakes regions
Avoid adding meat, dairy, cooked food with oil, or bones. These materials decompose eventually, but they attract wildlife and produce unpleasant odours during breakdown — two problems that are amplified in dense suburban areas.
What Counts as Brown (Carbon-Rich)
Browns have high C:N ratios, often 100:1 or more. They decompose slowly on their own and provide the structural bulk that keeps the pile from compacting into an airless mat. Available brown materials in most Canadian yards:
- Dried fallen leaves (C:N 40:1 to 80:1, depending on species — oak and maple are common in Ontario)
- Shredded corrugated cardboard (C:N around 350:1; shred or soak before adding)
- Unprinted newspaper in thin layers
- Straw (C:N 75:1 to 150:1)
- Woody plant stalks, shredded
- Wood chips from untreated lumber (C:N 400:1 to 500:1; use sparingly)
- Sawdust from untreated wood (C:N 200:1 to 500:1)
Canadian municipalities often collect leaves in fall for municipal composting. Keeping a portion for the backyard pile — bagged dry and stored near the bin — provides brown material throughout winter and spring when fresh greens are absent. Even two large bags of dry leaves can balance an entire summer of kitchen scraps.
Practical Layering by Volume
Lab-precise measurement is impractical in a backyard setting. A workable rule of thumb: add roughly two to three parts brown material by volume for every one part green. This does not need to be exact. The pile provides continuous feedback through temperature, smell, and moisture levels.
A pile that smells like ammonia or sulphur has too much nitrogen. Add dry carbon material and turn. A pile that sits cold and shows no decomposition after two weeks probably has too much carbon and is too dry. Add nitrogen-rich greens and water until the interior material feels like a wrung-out sponge — damp but not dripping.
Seasonal Adjustments in Canadian Climates
Spring and early summer produce large volumes of grass clippings — a high-nitrogen input that can easily overwhelm a pile if added all at once. Spread clippings in layers no thicker than 5 cm, alternating with dry browns. Thick grass layers mat down, seal out oxygen, and ferment into a slimy anaerobic mass.
Late fall in Ontario and Quebec brings a surge of fallen leaves just as kitchen inputs remain constant. This is a natural opportunity to build a large carbon reserve in bin one, with kitchen scraps layered in as available through the winter months. Even at near-freezing temperatures, decomposition continues slowly at the pile's core, provided moisture is maintained.
Materials to Avoid Entirely
Beyond the animal product issue, certain plant materials pose problems. Diseased plants should not go into a home pile unless the pile reliably reaches 55°C at the core — which requires consistent management and is difficult to sustain through most of a Canadian winter. Perennial weeds that set seed before being added (dandelion, thistle, burdock) can survive if the pile's temperature is inconsistent. Composting these materials in the active bin during summer, when heat is highest, reduces but does not eliminate the risk.
External Resources
The Health Canada website includes information on food safety practices relevant to composting. The Ontario Ministry of Agriculture, Food and Rural Affairs publishes regional composting guidance updated annually.