Composting In Schools

Getting the Right Mix

Calculations for Thermophilic Composting

Tom L. Richard and Nancy M. Trautmann

One of the first tasks in developing a successful composting program is getting the right combination of ingredients. Two parameters are particularly important in this regard: moisture content and the carbon to nitrogen (C/N) ratio.

Moisture is essential to all living organisms, and most microorganisms, lacking mechanisms for moisture retention (like our skin), are particularly sensitive in this regard. Below a moisture content of 35 to 40%, decomposition rates are greatly reduced; below 30% they virtually stop. Too much moisture, however, is one of the most common factors leading to anaerobic conditions and resulting odor complaints. The upper limit of moisture varies with different materials, and is a function of their particle sizes and structural characteristics, both of which affect their porosity. For most compost mixtures, 55 to 60% is the recommended upper limit for moisture content. Because composting is usually a drying process (through evaporation due to microbially generated heat), starting moisture contents are usually in this upper range.

Of the many elements required for microbial decomposition, carbon and nitrogen are both the most important and the most commonly limiting (occasionally phosphorous can also be limiting). Carbon is both an energy source (note the root in our word for high energy food: carbohydrate), and the basic building block making up about 50 percent of the mass of microbial cells.

Nitrogen is a crucial component of proteins, and bacteria, whose biomass is over 50% protein, need plenty of nitrogen for rapid growth. When there is too little nitrogen, the microbial population will not grow to its optimum size, and composting will slow down. In contrast, too much nitrogen allows rapid microbial growth and accelerates decomposition, but this can create serious odor problems as oxygen is used up and anaerobic conditions occur. In addition, some of this excess nitrogen will be given off as ammonia gas that generates odors while allowing valuable nitrogen to escape. Therefore, materials with a high nitrogen content, such as fresh grass clippings, require more careful management to insure adequate oxygen transport , as well as thorough blending with a high carbon waste. For most materials, a C/N ratio of about 30 to 1 (by weight) will keep these elements in approximate balance, although several other factors can also come into play.

So, if you have several materials you want to compost, how do you figure out the appropriate mix to achieve moisture and C/N goals? The theory behind calculating mix ratios is relatively simple - high school algebra is the only prerequisite. To help you understand these equations, and calculate the right mix for your situation, we developed a set of informative pages, on-line calculations, and spreadsheets you can download and operate anytime with software on your own computer. You can access this material directly from the Cornell Composting Science and Engineering page, or by clicking on one of the items below:

Moisture Content
Carbon/Nitrogen Ratios


Science &

in Schools


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