The Role of Nitrogen in Composting

Carbon-to-Nitrogen Ratios

• Composters use a carbon-to-nitrogen ratio to determine a good balance of compost ingredients. The ratio compares the amount of carbon to the amount of nitrogen in each material. Typically, there is always more carbon than nitrogen in most biodegradable material. Coffee grounds, for example, have about 20 parts carbon to one part nitrogen. This means coffee grounds have carbon-to-nitrogen ratio of 20. When you compost, you want a mixture that is a ratio of about 25 to 30.

High-Nitrogen Materials

• High-nitrogen materials have a low carbon-to-nitrogen ratio, typically between 0 and 10. Nitrogen heats the compost, which increases bacterial growth. These high-nitrogen materials can be used as compost activators to increase the decomposition of organic material. Examples of high-nitrogen materials include urine, which has a ratio of 1, or fish waste, which has a ratio of about 3. Many types of manure such as poultry or pig have a ratio of between 6 and 10.

Moist Greens

• Materials that have a carbon to nitrogen ratio between 10 and 30 are referred to as moist greens. Food scraps, for example, are a moist green with a ratio of about 15. Coffee grounds, with their ratio of 20, are also moist greens. Cow and horse manure are another moist green material with a ratio of about 15 to 20. These high-nitrogen materials can increase bacterial growth to speed up decomposition.

High Carbon

• Materials that have a high carbon ratio are also called drier browns. These materials are used to balance your carbon-to-nitrogen ratios and to retain oxygen flow through the compost. Dried leaves are high-carbon material with a ratio between 35 and 80. Corn cobs have a ratio of about 75. These materials slow down rapid bacteria production.

Very High-Carbon Materials

• Materials with a carbon-to-nitrogen ratio of more than 80 are called very high-carbon or dry browns. These help moderate compost that has high-nitrogen wastes. If you compost a lot of food waste, for example, it should be balanced with a very high-carbon material such as sawdust, which has a ratio of about 300. Cardboard is another dry brown that has a very high-carbon ratio at between 300 and 500. Paper ratios can range from around 100 to 800 depending on the type of paper.

Balancing Your Compost

• To ensure you have the right balance of carbon and nitrogen materials in your compost, mix one part greens, or high-nitrogen materials, to one part browns, or high-carbon materials. Typically, these materials are layered and turned frequently, but often compost ingredients tend to be added in bits over periods of days. To ensure a good mixture, consider mixing your greens and browns as you put them in. If the composting seems to be slow, add more high-nitrogen materials. Add high-carbon materials when the mixture develops a strong odor to slow down the process.