The Composting Process

Bulletin 792

When are conditions right for composting?

The composting process is essentially a biological one that compares to the raising of plants or animals. The rate of composting, like the rate of plant or animal growth, can be affected by many factors.

Four key ones are:

1. nutrient balance,
2. moisture content,
3. temperature, and
4. aeration.

Nutrient balance is determined primarily by the ratio of carbon to nitrogen in the compost mix (C/N ratio). It is like balancing carbohydrates and protein in a diet. Bacteria, actinomycetes, and fungi also require carbon and nitrogen for growth. These microbes use 30 parts of carbon to 1 part of nitrogen. Composting is usually successful when the mixture of organic materials consists of 20 to 40 parts of carbon to 1 part of nitrogen. However, as the ratio exceeds 30, the rate of composting decreases. As the ratio decreases below 25, excess nitrogen is converted to ammonia. This is wasted into the atmosphere and results in undesirable odors.

Phosphorus is another principal element required by compost microbes. Trace elements are needed in very small quantities. Normally, these elements are available in satisfactory amounts in compost when the C/N ratio is properly established.

The moisture content of compost should ideally be 60 percent after organic wastes have been mixed. Depending on the components of the mixture, initial moisture content can range from 55 to 70 percent. However, as the moisture content exceeds 60 percent, the structural strength of the compost deteriorates, oxygen movement is inhibited, and the process tends to become anaerobic. Low C/N ratio materials (e.g. meat wastes) putrefy when anaerobic. High ratio materials ferment. Both processes produce odors and must be avoided. As the moisture content decreases below 50 percent, the rate of decomposition decreases rapidly. As a rule of thumb, a mixture of organic wastes that contains 50 percent moisture feels damp to the touch but is not soggy.

The temperature increase that occurs during composting results from the breakdown of organic material by bacteria, actinomycetes, fungi and protozoa. The temperature can range from near freezing to 160 degrees F. Starting at ambient temperature when the components are mixed, the compost can reach 150 degrees F in less than two days. Applying heat to compost from an external source serves no purpose unless ambient temperatures are far below freezing. Heat is generated from within the compost medium.

The hundreds of types of microorganisms involved with composting are generally classified into three categories according to temperatures most favorable to their metabolism and growth:

As the microorganisms decompose (oxidize) organic matter, heat is generated and the temperature of the compost is raised a few degrees as a result. In composting, as in the decomposition of any complex substance, the breakdown is a dynamic process accomplished by a succession of microorganisms with each group reaching its peak population when conditions have become optimum for its activity. One group of microorganisms dies and another group thrives until the next incremental change in nutrition and temperature occurs, etc. Composting rate is generally measured by rate of carbon dioxide production. The maximum rate occurs when compost temperatures range from 110-150 degrees F. As the temperature exceeds 150 degrees F, the composting rate drops rapidly and becomes negligible at temperatures higher than 160 degrees F.

Most composting should include temperatures in the thermophilic range (100-150 degrees F). At these temperatures the rate of organic matter decomposition is maximum, and weed seeds and most microbes of pathogenic significance cannot survive. It takes three days at 131 degrees F (55 degrees C) to kill parasites, fecal and plant pathogens. It is important that piles are turned frequently to ensure that all parts are exposed to high temperatures.

Aeration is a key element in composting. Proper aeration is needed to control the environment required for biological processes to thrive with optimum efficiency. A number of controllable factors are involved.

1. Temperature should be controlled to a 140-150 degrees F upper limit. Above these temperatures even thermophilic microbes either self-destruct or become less effective.

2. Moisture is removed naturally from the compost medium. Over two-thirds of the original water content of compost may be lost during decomposition. Often water must be added during the process to maintain activity.

3. Carbon dioxide is a product of the biochemical reactions that are part of composting. This gas must be removed from the compost microenvironment to avoid toxic concentrations that inhibit the process.

4. Oxygen must be available to microbes in sufficient quantities to ensure vitality of the aerobic types and to minimize odors. The amount and type of bulking agent added during preparation of the compost determines the free air space in the pile. Pore space should range from 35-50% to maintain adequate aeration. Air may be forced through the pile to speed up the process. However, forced aeration adds complexity to the process.