Why Carbon and Nitrogen Both Matter
Decomposition in a compost pile is driven largely by microorganisms. Those organisms require carbon as an energy source and nitrogen to build proteins and reproduce. When one is present in much greater amounts than the other, the microbial community either runs out of energy (too little carbon) or becomes nitrogen-limited and sluggish (too much carbon).
The ratio most often cited in composting literature is between 25:1 and 30:1 by weight, carbon to nitrogen. This is not a figure to measure precisely at home — it is a framework for thinking about proportions when deciding what and how much to add.
A pile that smells like ammonia has too much nitrogen. A pile that takes many months with no visible change typically lacks nitrogen. Both problems are corrected by adjusting what you add next.
Brown Materials: Carbon Sources
Brown materials are dry, fibrous, or woody items. They decompose slowly on their own and provide structural air pockets in the pile that support aerobic bacteria.
| Material | Approximate C:N Ratio | Notes |
|---|---|---|
| Dry autumn leaves | 60:1 – 80:1 | Shredding speeds breakdown; whole leaves mat together |
| Straw | 75:1 – 100:1 | Good bulking agent; avoid straw with herbicide residue |
| Cardboard (plain) | 100:1 – 150:1 | Tear into pieces; remove tape and glossy coatings |
| Sawdust (untreated) | 200:1 – 500:1 | Very high carbon; use sparingly or mix well |
| Wood chips | 100:1 – 150:1 | Best used in a slower, cold-composting setup |
| Newspaper (black ink) | 150:1 – 175:1 | Shred first; coloured ink pages are generally not recommended |
Green Materials: Nitrogen Sources
Green materials are typically moist, recently harvested, or food-derived. They decompose faster and raise the temperature of the pile when present in sufficient quantity.
| Material | Approximate C:N Ratio | Notes |
|---|---|---|
| Fresh grass clippings | 15:1 – 25:1 | Layer thinly; clippings mat and go anaerobic in thick layers |
| Vegetable and fruit scraps | 15:1 – 35:1 | Chop large pieces; bury under brown layers to limit pests |
| Coffee grounds | 20:1 | Paper filters can go in as well |
| Fresh plant cuttings | 15:1 – 30:1 | Avoid plants that have gone to seed or show disease |
| Eggshells | — | Minimal nitrogen; adds calcium and improves aeration |
How to Layer in Practice
A simple approach for a typical household is to add a thin layer of green material whenever you have it, then cover that layer with roughly two to three times as much brown material by volume. Volume is an imprecise measure of weight and C:N ratio, but it works as a practical guide because most brown materials are lighter and fluffier than green ones.
Starting a New Pile
Place a coarse layer of sticks or wood chips at the base, roughly 10 cm deep. This elevates the pile slightly and allows air to enter from below. Add a 5–8 cm layer of green material — kitchen scraps, grass clippings, or garden trimmings — followed by 10–15 cm of brown material. Repeat the layering as material becomes available.
Adding to an Existing Pile
Most backyard bins receive material in small, irregular amounts. When adding kitchen scraps or fresh garden waste, bury the material in the center of the pile and cover it with browns. This reduces surface contact with the air and discourages flies. If browns run low, a bag of dry autumn leaves stored beside the bin through winter gives a reliable carbon reserve.
Seasonal Adjustments in Canada
Canadian winters slow decomposition significantly. In most provinces, an outdoor compost pile enters a near-dormant state between December and March. Browns accumulate during this period. When spring arrives and nitrogen-rich green material starts generating again, the pile often needs less added carbon rather than more. Monitoring the pile's smell and texture in April and May can guide what to prioritize.
In colder regions like the Prairie provinces, insulating the bin with straw bales can maintain enough microbial activity to prevent full freezing, though the pace of decomposition remains slower than in summer.
Signs the Ratio Is Off
If the pile smells strongly of ammonia, nitrogen is exceeding carbon. Add dry leaves, cardboard, or straw and mix the pile to distribute them. If the pile is very dry, cold, and not visibly breaking down after several weeks of warmer weather, it likely needs nitrogen input — fresh grass clippings or vegetable scraps mixed in will restart activity. A completed, usable compost is dark, crumbly, and smells like damp earth.
References
Royal Horticultural Society — Composting
Environment and Climate Change Canada — Composting