In the soil microbial world, a material with a carbon:nitrogen ratio of ~24:1 is the balance point between that material decomposing and tying up nitrogen (immobilization) or releasing nitrogen (mineralization) while providing enough
energy for microbial respiration. Materials such as wood chips or
straw have rather high carbon:nitrogen ratios and so will immobilize nitrogen and decompose slowly. Green vegetation has a fairly low carbon:nitrogen ratio, so will mineralize nitrogen and decompose quickly. Allowing a mixture of materials (i.e. wood chips + manure) to compost will bring the resulting finished compost down closer to the carbon:nitrogen ratio of the soil itself at ~10-15:1 and thus neither immobilize or mineralize nitrogen, but provide an army of soil microorganisms that can associate with, and
feed plants. In this regard, compost
should not be thought of as a "fertilizer" but rather as the introduction of a wide array of microorganisms suited to acquire and supply nutrients to growing plants, giving a similar response as applying what would commonly be regarded as "fertilizer".