In microbial communities like soil, the C:N ratio is a key indicator as it describes a balance between energetic foods (represented by carbon) and material to build protein with (represented by nitrogen). An optimal C:N ratio of around 24:1 provides for higher microbial activity.
The C:N ratio of soil can be modified by the addition of materials such as compost, manure, and mulch. A feedstock with a near-optimal C:N ratio will be consumed quickly. Any excess C will cause the N originally in the soil to be consumed, competing with the plant for nutrients (immobilization) &ndash; at least temporarily until the microbes die. Any excess N, on the other hand, will usually just be left behind (mineralization), but too much excess may result in losses to leaching. The recommended C:N ratio for soil materials is therefore 30:1. A soil test may be done to find the C:N ratio of soil itself.
The C:N ratio of microbes themselves is generally around 10:1. A lower ratio is correlated with higher soil productivity.