The C/N ratio (C:N) or carbon-to-nitrogen ratio is a ratio of the mass of carbon to the mass of nitrogen in a substance. It can, amongst other things, be used in analysing sediments and compost. Carbon-to-nitrogen ratios are an indicator for nitrogen limitation of plants and other organisms.
C/N ratios in the range 6-8:1 are usually from marine sources, whereas higher ratios are likely to come from a terrestrial source. (Dahlhem 1988).
When composting, microbial activity utilizes a C/N ratio of 30-35:1 and a higher ratio will result in slower composting rates [1]. However, this assumes that carbon is completely consumed, which is often not the case. Thus, for practical agricultural purposes, a compost should have a initial C/N ratio of 20-30:1.[2]
An example of a device that can be used to measure this ratio is the CHN analyzer. However, for more practical applications, desired C/N ratios can be achieved by blending common used substrates of known C/N content, which are readily available and easy to use.