Gardener's Dictionary: C:N Ratio
C:N Ratio (n.): C:N ratio (aka carbon-to-nitrogen ratio) is the ratio of the mass of carbon to the mass of nitrogen in a substance such as soil or compost. Compost should have an initial C:N ratio of 20-30:1.

C:N Ratio (n.): C:N ratio (aka carbon-to-nitrogen ratio) is the ratio of the mass of carbon to the mass of nitrogen in a substance such as soil or compost. Compost should have an initial C:N ratio of 20-30:1.