Carbon and Nitrogen Content in Materials
Combining "green" and "brown" materials can create a mix with the optimal carbon to nitrogen ratio. The optimum mix for simple backyard composting is half grass clippings and half leaves.
- Green plant material such as grass clippings typically are relatively rich in nitrogen.
- Brown plant materials such as fall leaves and branches are relatively high in carbon.
Decomposers need carbon for energy and nitrogen for growth and it is the composter’s job to supply both kinds of materials in roughly the proportions the microorganisms prefer.
The ideal C:N ratio for composting is between 25:1 and 30:1, with carbon being the higher number. Precision is unnecessary - with a little experience you will acquire a feel for the best combinations.
Carbonaceous materials are generally brown or yellow, dry, coarse and bulky compared with nitrogenous materials, which tend to be green, succulent, gooey and dense.
High-carbon materials are almost always plant materials such as straw, cornstalks, sawdust and leaves. Nitrogenous materials more often include animal by-products, although it is quite possible to make compost without using any materials derived from animals.
Examples of high-nitrogen materials are grass clippings, alfalfa meal, blood meal and poultry manure. A few materials, such as fresh clover, most kitchen garbage and manure mixed with bedding, already have C:N ratios in the ideal range.
The carbon materials contribute mass to the pile and give rise to the organic gums abundant in humus. Nitrogen is necessary to stimulate microbes to reproduce as rapidly as possible. However, even materials that contain very little nitrogen will break down over time, but they will never reach the temperatures needed for hot composting. If there is too much nitrogen in relation to carbon, nitrogen will be lost as ammonia, easily detected by its smell. This generally lasts only a day or two, until the material stabilizes. In the worst case, excess nitrogen may cause the pile to become putrefied and anaerobic, usually because carbonaceous materials also contribute to proper aeration.
Some composting guides recommend adding synthetic nitrogen carriers such as urea or sodium nitrate as activators. This is never necessary and is a bad idea because these materials can disrupt microbe populations. Moreover, their manufacture consumes vast quantities of natural gas. If you need a concentrated nitrogen source, there are many naturally derived alternatives available commercially - cottonseed meal is recommended most often and can be purchased inexpensively at local feed stores.