Aeration
C:N ratio
Moisture
Particle size
pH
Bin size

Carbon to nitrogen ratio

Carbon and nitrogen are both necessary for microbial growth. Organic carbon (which makes up about 50 percent of the mass of microbial cells) provides both an energy source and a basic cellular building block. Nitrogen is a crucial component of the proteins, nucleic acids, amino acids and enzymes necessary for cell growth and function.

The ideal C/N ratio for composting is generally considered to be around 30:1, or 30 parts carbon for each part nitrogen by weight. Why 30:1? At lower ratios, nitrogen will be supplied in excess and will be lost as ammonia gas, causing undesirable odors. Higher ratios mean that there is not sufficient nitrogen for optimal growth of the microbial populations, so the compost will remain relatively cool and degradation will proceed at a slow rate.

As composting proceeds, the C/N ratio gradually decreases from 30:1 to 10:1-15:1 for the finished product. This occurs because each time that organic compounds are consumed by microorganisms, two-thirds of the carbon is given off as carbon dioxide. The remaining third is incorporated along with nitrogen into microbial cells.

In general, materials that are green and moist tend to be high in nitrogen, and those that are brown and dry are high in carbon.

Since drying and weighing the materials that you put in your pile is not practical, you can use the simple rule that compost needs to be about half "browns" and half "greens" by volume. You can adjust this ratio up or down depending on the quantity and quality of the materials you have at hand. Composting soon becomes a matter of instinct, like the cook who bakes without a recipe. If the pile doesn't heat up, you know there's not enough "greens" in the mix, while an ammonia smell means it needs more "browns."