Some compost systems are able to maintain adequate oxygen passively, through natural diffusion and convection. Oxygen concentrations greater than 10% are considered optimal for maintaining aerobic composting. So, how much oxygen is sufficient to maintain aerobic conditions? Although the atmosphere is 21% oxygen, aerobic microbes can survive at concentrations as low as 5%. Without sufficient oxygen, the process will become anaerobic and produce undesirable odors, including the rotten-egg smell of hydrogen sulfide gas. As microorganisms oxidize carbon for energy, oxygen is used up and carbon dioxide is produced. Particle size also is a relevant consideration although the same amount of carbon is contained in comparable masses of wood chips and sawdust, the larger surface area in the sawdust makes its carbon more readily available for microbial use.Īnother essential ingredient for successful composting is oxygen.

Although all of these materials can still be composted, their relatively slow rates of decomposition mean that not all of their carbon will be readily available to microorganisms, so a higher initial C/N ratio can be planned. Corn stalks and straw are similarly slow to break down because they are made up of a resistant form of cellulose. Newspaper, for example, is slower than other types of paper to break down because it is made up of cellulose fibers sheathed in lignin, a highly resistant compound found in wood. Some of the carbon, however, may be bound up in compounds that are highly resistant to biological degradation. Most of the nitrogen in compostable materials is readily available. The remaining third is incorporated along with nitrogen into microbialĬells, then later released for further use once those cells die.Īlthough attaining a C/N ratio of roughly 30:1 is a useful goal in planning composting operations, this ratio may need to be adjusted according to the bioavailability of the materials in question. This occurs because each time that organic compoundsĪre consumed by microorganisms, two-thirds of the carbon is given off as carbonĭioxide. Also available for purchase from the Northeast Regional Agricultural Engineering Service, (PALS) Cornell University.Īs composting proceeds, the C/N ratio gradually decreases from 30:1 to 10-15:1įor the finished product. Composting to Reduce the Waste Stream: A Guide to Small Scale Food and Yard Waste Composting. You can calculate the C/N ratio of your compost mixture, or you can estimate optimal conditions simply by using a combination of materials that are high in carbon and others that are high in nitrogen. Brown or woody materials such as autumn leaves, wood chips, sawdust, and shredded paper are high in carbon.

High nitrogen materials include grass clippings, plant cuttings, and fruit and vegetable scraps. 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. Typical C/N ratios for common compost materials can be looked up in published tables such as Appendix A (page 106), On-Farm Composting Handbook. 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. Why 30:1? At lower ratios, nitrogen will be supplied in excess and will be lost as ammonia gas, causing undesirable odors. 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. To provide optimal amounts of these two crucial elements, you can use the carbon-to-nitrogen (C/N) ratio for each of your compost ingredients. Nitrogen is a crucial component of the proteins, nucleic acids, amino acids, enzymes and co-enzymes necessary for cell growth and function. Carbon provides both an energy source and and the basic building block making up about 50 percent of the mass of microbial cells. Of the many elements required for microbial decomposition, carbon and nitrogen are the most important.

Compost grass clippings