CNforholdet

CNforholdet, or C/N-forholdet, is the carbon-to-nitrogen ratio of organic material, expressed as the mass of carbon divided by the mass of nitrogen. It is used in soil science, agronomy and composting to estimate how quickly organic matter will decompose and how much nitrogen will be available to plants.

Calculation and typical values are based on chemical analysis. Carbon content in many organic materials is a few tens of percent, while nitrogen content varies widely. As a rule of thumb, plant residues can range from low C/N values around 12–30:1 for legume-rich materials to higher values of 40–60:1 for many cereals, straw or dried plant matter, and even higher for wood and sawdust.

In soils, the CNforholdet influences microbial activity and nitrogen dynamics. A high CNforhold (generally above about 25–30:1) tends to slow decomposition and cause nitrogen immobilization, making some soil N temporarily unavailable to plants. A low CNforhold (below about 20:1) tends to favor faster mineralization, releasing nitrogen more readily but sometimes risking leaching or volatilization if moisture and temperature are favorable for rapid turnover.

In composting, CNforholdet is an important management parameter. A target range around 25–30:1 is common to balance rapid decomposition with steady nitrogen availability. Materials with very high CNforholdet may require addition of nitrogen-rich components, while very low CNforholdet materials can cause odor and ammonia losses if not managed properly.

Limitations include that CNforholdet is only one predictor of nutrient dynamics. Other factors such as moisture, temperature, lignin content, and microbial communities also play critical roles in decomposition and nutrient release.