C:N ratios

When fresh organic material undergoes decomposition in soil, both the rate of decomposition and the amount of humus formed are related to the C:N ratio of the residue. When other conditions are equal, the rate of decomposition increases as the C:N ratio narrows.

Four conditions that are constant for all residue decomposition.

1. A maximum of 35% of the carbon in fresh organic material will be converted into soil humus IF there is sufficient nitrogen present.
2. A minimum of 65% of the carbon in fresh organic material will be given off to the atmosphere as carbon dioxide due to microbial respiration.
3. The humus formed from the decomposition of fresh organic material will contain approximately 50% carbon and 5% nitrogen. In other words, the C:N ratio of the humus is 10:1.
4. Most fresh plant material contains 40% carbon. The C:N ratio varies because of differences in nitrogen content, not carbon content.

Below are the average C:N ratios for some common organic materials found in the compost bin. For our purposes, the materials containing high amounts of carbon are considered “browns,” and materials containing high amounts of nitrogen are considered “greens.”

Estimated Carbon-to-Nitrogen Ratios for some common composting materials: