Feedstock Characteristics: Compostable Materials

Some optimum characteristics of composting are given below. This informative summary was taken from The Association for Organics Recycling as well as the U.S. EPA’s website. There are many guides, including CalRecycle’s Comprehensive Compost Odor Response Project (C-CORP) report, that provide details on the composting process. Some of this information may be accessed through the Resources web pages.

When a feedstock meets the optimum conditions given below, it is an indication that composting is likely to be successful. Generally, a single feedstock will not meet all of these requirements and will have to be blended with one or more other materials and/or chipped/ground to reduce the particle size. The optimum ranges for composting are given below. This does not mean that anything outside these ranges will not work, however, to reduce odors and increase the likelihood of success, aerobic composting should meet the conditions below.

Carbon to Nitrogen Ratio (C:N ratio)

A range of 25:1-40:1 provides the necessary proportions of carbon and nitrogen for metabolism by the composting microbes. If there is too little nitrogen, (a high C:N ratio) the composting process will be slow; if there is too much nitrogen (a low C:N ratio) it is likely that the compost will emit odors (ammonia) and can pollute runoff water.

Controlled decomposition requires a proper balance of “green” organic materials (grass clippings, etc), which contain large amounts of nitrogen, and “brown” organic materials (wood chips, branches, etc), which contain large amounts of carbon but little nitrogen. The compost microorganisms get their energy from carbon sources. Nitrogen is required to increase the numbers of microorganisms. If high nitrogen materials such as grass clippings are used, they must be thoroughly mixed with a carbon source, such as wood chips or branches

Moisture Content

This is vital for microbial activity as it acts as a medium for the many biochemical reactions that occur during composting. A range of between 40 – 60 percent (mass / mass) water is generally acceptable.

Bulk Density

This gives an indication of the particle size of the feedstocks and therefore how much free air space there will be between particles. Feedstocks with high bulk densities are likely to have little free air space available between particles; anaerobic (oxygen free) conditions are more likely to occur under these conditions. Feedstocks with low densities may have difficulties in retaining heat if open windrows are used. Bulking agents, such as wood chips, may be added to increase porosity. The best range is between 500 to 750 kg/m3.

Particle Size

This relates to the available surface area on which the composting micro-organisms can work. The larger the particle size, the smaller the surface area, therefore the slower the composting process will tend to be. Particle size diameters of between 25 – 50 mm are generally considered optimal. There is also a relationship between particle size and bulk density: smaller particles may limit airflow, while large particles may result in high bulk densities. Chipping and grinding is typically carried out to reduce particle size.


This is the scale that indicates the acidity or alkalinity of a substance. A pH of less than 7 is acidic, a pH greater than 7 is alkaline, pH 7 is neutral. Feedstocks with a pH value of between 6 and 8 are desirable to provide optimal conditions for microbial growth. The release of by-products as a result of the composting process will change the pH during the process.

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