Anyone that has been exposed to a learning process relative to composting, has surely heard of the Carbon Nitrogen Ratio, C:N. So without more details than is needed, here are the principals behind the C:N ratio concept:

Carbon Nitrogen Ratio | Composts

Microorganisms are behind decompositions of organic matter that creates our nutrient packed compost. These little guys need a certain ratio of carbon and nitrogen to satisfy their metabolic processes. Carbon found in carbohydrates and or sugars are metabolized to produce energy for the microbes work. The nitrogen is used to create proteins for building cell structure. The C:N ratio refers to the relative proportions on C and N. A ratio of 25 to 30 parts carbon to 1 part nitrogen is what we are looking for to make the process optimum for a compost used for fertilizing plants such as herbs and vegetables.  This is a 30:1 ratio or more simple stated, a C:N of 30. If the ratio is far from this mark the decomposition slows as more complex metabolic processes are needed to continue decomposition and a growth curve for the microbe.

For example if there is excess carbon in the compost due to a lack of nitrogen , when the compost is use for gardening , microbes will kick in and rob the surrounding substrate, soil or fertilizer of nitrogen that was meant for the plant. If there is not enough carbon in the compost mix, excess nitrogen ends up as ammonia that can not be used to complete the Nitrogen Cycle, and evaporates into the atmosphere. What a wast of nitrogen that would be.

The University of California did an intensive study not to long ago with C:N ratios from 20 to 78 and found that a ratio of 30 to 35 was optimum. They also found that even though the optimum for compost is a C:N ratio of 30, a variance will not effect the quality of the compost significantly. The California research showed that a composting of feedstock material with a greater C:N ratio would not be that harmful to the soil. The extra  C is so slowly available that nitrogen robbery is minimal.

Carbon Nitrogen Ratio | Soils

The optimum ratio in soil or substrate organic matter is different than when you are beginning to creating a compost mix with raw matterials. A 10 carbons to 1 nitrogen ratio, or a C:N ratio of 10:1 is optimal.

Following are some sample C:N ratios of organic matter:

Sandy loam (fine)




Food scraps


Alfalfa hay


Grass clippings


Rotted manure


Sandy loam (coarse)


Vegetable trimmings


Oak leaves


Leaves, varies from

35:1 to 85:1

Peat moss


Corn stalks




Pine needles

60:1 to 110:1

Farm manure


Alder sawdust


Sawdust weathered 3 years




Douglas fir bark


Sawdust weathered 2 months