In organic matter, carbon (C) is the energy source and the basic building block for microbial cells. Nitrogen (N), also important, and along with C, is the most commonly limiting element.
Microorganisms need about 25–30 parts of C by weight for each part of N used for protein production. A C:N ratio of 20–40:1 is often suitable to start composting, depending on the makeup of the raw material. As composting proceeds, the C:N ratio gradually decreases to between 10 and 20:1 (depending on the C:N starting point).
Preparing and mixing the raw materials to an optimum C:N ratio results in the fastest rate of decomposition – provided that other factors are not limiting. In general, a high C:N ratio slows the decomposition rate so that the temperature increase is limited, and the total length of composting is extended. Raw materials of low C:N ratios (<15:1) may decompose rapidly, but odour can become a problem because of the complete and rapid usage of oxygen without replenishment. Furthermore, under these conditions, N can be lost as gaseous emissions of ammonia, reducing the nutrient value of the compost. Ammonia is also a highly odorous gas.
