THE CARBON/NITROGEN RATIO
CARBON / NITROGEN RATIOS
|Sources: Gotaas, Harold B. (1956). Composting - Sanitary Disposal and Reclamation of Organic Wastes (p.44). World Health Organization, Monograph Series Number 31. Geneva. and Rynk, Robert, ed. (1992). On-Farm Composting Handbook. Northeast Regional Agricultural Engineering Service. Ph: (607) 255-7654. pp. 106-113. Some data from Biocycle, Journal of Composting and Recycling, July 1998, p.18, 61, 62; and January 1998, p.20.|
One way to understand the blend of ingredients in your compost pile is by using the C/N ratio (carbon/nitrogen ratio). Quite frankly, the chance of the average person measuring and monitoring the carbon and nitrogen quantities of their organic material is almost nil. This is like making wine the "foolproof" way. If composting requires this sort of drudgery, no one would do it.
However, I've found that by using all of the organic refuse my family produces, including humanure, urine, food refuse, weeds from our garden, rotting sawdust (which is hauled in), grass clippings, and maybe a little straw or hay now and then, we get the right mix of carbon and nitrogen for successful thermophilic composting. We do not compost newspapers or other burnable materials, we recycle them or burn them in our woodstove.
Nevertheless, no discussion of composting is complete without a review of the subject of the carbon/nitrogen ratio. A good C/N ratio for a compost pile is between 20/1 and 35/1.16 That's 20 parts of carbon to one part of nitrogen, up to 35 parts of carbon to one part of nitrogen. Or, for simplicity, you can figure on shooting for an optimum 30/1 ratio.
For microorganisms, carbon is the basic building block of life and is a source of energy, but nitrogen is also necessary for such things as proteins, genetic material, and cell structure. Microorganisms that digest compost need about 30 parts of carbon for every part of nitrogen they consume. That's a balanced diet for them. If there's too much nitrogen, the microorganisms can't use it all and the excess is lost in the form of smelly ammonia gas. Nitrogen loss due to excess nitrogen in the pile (a low C/N ratio) can be over 60%. At a C/N ratio of 30 or 35 to 1, only one half of one percent of the nitrogen will be lost (see Table 3.1). That's why you don't want too much nitrogen (manure, for example) in your compost: the nitrogen will be lost in the air in the form of ammonia gas, and nitrogen is too valuable for plants to allow it to escape into the atmosphere.17
That's also why humanure and urine alone will not compost. They contain too much nitrogen and not enough carbon, and microorganisms, like humans, gag at the thought of eating it. Since there's nothing worse than several billion gagging microorganisms, a carbon-based material must be added to the humanure in order to make it appealing. Plant cellulose is a carbon-based material, and therefore plant by-products such as hay, straw, weeds, or even paper products if ground to the proper consistency, will provide the needed carbon. Kitchen food scraps are generally C/N balanced, and they can readily be added to humanure compost. Sawdust (preferably not kiln-dried) is a good carbon material for balancing the nitrogen of humanure. Sawmill sawdust has a moisture content of 40-65%, which is good for compost.18 Lumber yard sawdust, on the other hand, is kiln-dried and is biologically inert due to the dehydration. Therefore, it is not as desirable in compost unless rehydrated with water (or urine) before being added to the compost pile. Also, lumber yard sawdust nowadays can often be contaminated with wood preservatives such as chromated copper arsenate (from "pressure treated lumber"). Both chromium and arsenic are human carcinogens, so it would be wise to avoid such materials.
The C/N ratio of humanure is between five and ten, averaging eight parts of carbon to one part of nitrogen. Therefore, you need to add a fair amount of carbon to humanure to get a 30/1 ratio (see Tables 3.2 and 3.3). I've found that the proper balance is obtained by putting all the organic refuse of my household (excluding printed material and burnable paper packaging) in the same compost pile, layered with weeds, straw, hay, leaves, or whatever organic material happens to be within reach. The humanure, when collected in the toilet, is covered with clean, partially rotted, hardwood or softwood sawdust, or another carbon-based material such as peat moss or rice hulls. This carbonaceous "cover material" not only balances the nitrogen, but also prevents odors remarkably well.
It has recently become popular for backyard composters to refer to organic materials as "browns" and "greens." The browns (such as dried leaves) supply carbon, and the greens (such as fresh grass clippings) supply nitrogen. It's recommended that two to three volumes of browns be mixed with one volume of greens in order to produce a mix with the correct C/N ratio for composting.19 However, since most backyard composters are not humanure composters, many backyard composters have a pile of material sitting in their compost bin showing very little activity. What is usually missing is nitrogen as well as moisture, two critical ingredients to any compost pile. Both of these are provided by humanure when collected with urine and a carbon cover material. The humanure mix can be quite brown, but is also quite high in nitrogen. So the "brown/green" approach doesn't really work, nor is it necessary, when composting humanure along with other household organic material. Let's face it, humanure composters are in a class by themselves.
Source: The Humanure Handbook. Jenkins Publishing,
PO Box 607, Grove City, PA 16127. To order, phone: 1-800-639-4099.