Carbon: Nitrogen Relationships

There are two chemical elements that are of utmost importance in all phases of organic matter – Carbon (C) and Nitrogen (N). Of even greater importance is their proportion or ratio to each other. This relationship is called the C:N ratio. In order to better understand this ratio suppose that a material is made up of 60 parts C and 2 parts of N by weight. It would have a C:N ratio of 30:1. This means that the material has 30 times as much C as N. If you are trying to compost leaves with an 60:1 C:N ratio for every 60 pounds of leaves, you willneed 2 pounds of N. It is important to remember that a small percentage of the carbon and nitrogen may not actually be available for decomposition. The decomposition of organic matter is brought about by living organisms which utilize the carbon as a source of energy and the nitrogen for building cell structure. More carbon than nitrogen is needed. However, the amount of nitrogen needed varies with the type of organic materials and the type of micro-organisms present. When the nitrogen content is too low for the amount of carbon (for example, 80:1, C:N), organisms will have to recycle the nitrogen through many generations in order to breakdown the carbon containing material. As organisms die, their stored nitrogen is then used by other organisms to form new cell material. (The average C:N ratio of the bodies of bacteria and fungi falls between 4:1 and 10:1.) In the process more carbon is used. Thus the amount of carbon is reduced to a more suitable level while the nitrogen is recycled. More time is required for the process, however, when the initial C:N ratio climbs above 30:1.

Carbon/Nitrogen Ratios for Composting Organics

Sandy loam (fine)7:1
Food scraps18:1
Alfalfa hay10:1
Grass clippings12-25:1
Coffee grounds20:1
Vegetable trimming12-20:1
Cow manure20:1
Horse manure25:1
Horse manure with litter60:1
Rotted manure20:1
Poultry manure (fresh)10:1
Poultry manure with litter18:1
Sandy loam (coarse)25:1
Oak leaves (green)26:1
Leaves, varies35-85:1
Peat moss58:1
Corn stalks60:1
Pine needles60-110:1
Farm manure90:1
Douglas fir bark491:1
Sawdust, weathered 2 months625:1

The presence of nitrogen in the soil can cause a slightly different process when soil and composting materials mix, as in sheet or trench composting or in mulching. When the C:N ratio is too great, living microbial cells make maximum use of the available carbon by drawing on any available soil nitrogen in theproper proportion. This condition is known as “robbing” the soil of nitrogen and has the effect of delaying the availability of nitrogen as a fertilizer for growing plants, until some later season when it is no longer being used in the lifecycles of soil bacteria. (As a result of this process, additional nitrogen may be needed when partially decomposed compost around plants as a mulch or soil amendment.) On the other hand, when the energy source, carbon, is less than that required for converting available nitrogen into protein, organisms make full use of the available carbon and get rid of the excess nitrogen as ammonia. This release of ammonia can produce a loss of nitrogen from the compost pile if the ammonia escapes to the atmosphere.

A C:N ratio of 20:1 (also known as a ratio of 20), when C and N are available, has been widely accepted as the upper limit at which there is no danger of robbing the soil of nitrogen. If a considerable amount of carbon is in the form of lignins or other resistant materials, the actual C:N ratio could be larger than 20. Because of the potential for robbing the soil of nitrogen and the need for conserving maximum nitrogen in the compost, the C:N ratio is obviously a critical factor in composting.

Since living organisms utilize about 30 parts carbon for each part of nitrogen, an initial C:N (available quantity) ratio of 30 would seem most favorable for rapid composting and would provide some nitrogen in an immediately available form in the finished compost. Some research workers have reported optimum values from 20 to 31. A majority of investigators believe that for C:N ratios above 30 there will be little loss of nitrogen. The University of California studies on materials with a initial C:N ratio varying from 20 to 78 and with nitrogen contents varying from 0.52% to 1.74% indicated that initial C:N ratio of 30 to 35 was optimum. These reported optimum C:N ratios may include some carbon which was not available. The composting time will increase considerably with increases in the C:N ratio above the range 30. If the unavailable carbon is small, the C:N ratio can be reducedby bacteria to as low a value as 10. Fourteen to twenty are common C:N ratios depending upon the original material from which the humus was formed. These studies showed that composting a material with a higher C:N ratio would not be harmful to the soil, however, because the remaining carbon is so slowly available that nitrogen robbery would not be significant.

Materials should be proportioned on a 30:1 ratio by weight in order for optimum decomposition to occur. However, C:N of 25:1-35:1 will result in proficient decomposition. If the compost pile is maintained at a steady ratio of 30:1 microorganisms will reach their optimum decomposition rates and materials will quickly decompose. When the C:N ratio becomes too high (too much C) decomposition slows down. When the C:N ratio becomes too low (too much nitrogen), N will be lost to the atmosphere as ammonia gas and can lead to odor problems.

The C:N ratio of 30:1 is an ideal. However, this ratio is usually given for materials on a dry weight basis, but composting materials are usually not completely dry. Though composting is not exact, the composter must use judgement in combining compostable materials. Generally, the dry, coarse materials such as straw, wood chips, etc. are high in C and low in N. Opposite to this, the “green” materials such as grass clippings, fresh plant material, kitchen scraps and manures, are high in N and lower in C.


The proper blending of these materials will come about over time as you try recipes and add to others and as materials become available for your use. Achieving the exact ratio of 30:1 may never happen, but composting will take place in and around this magical number. Over time and with experience, you will develop your own mixes that work and compost well.

The C:N ratio of plant material is dependent on its growth stage, conditions under which it has been grown, plant parts (leaves vs. limbs), and dryness of materials.

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