Chapter 10

Compost Management on Organic Farms

Compost Pile Management

Composting is most efficient when the major parameters—carbon-to-nitrogen (C:N) ratio, moisture content, temperature, oxygen content, pH of materials, and substrate physical properties, which affect the composting process are properly managed. Each of these factors has the potential to significantly affect the composting process.

Carbon-to-nitrogen (C:N) Ratio

An initial C:N ratio of approximately 30:1 (dry weight basis) is recommended for most efficient composting. This is achieved by combining various raw materials for which concentrations of carbon and nitrogen are known. Care must be taken in establishing the mix, however, because materials vary not only in forms and concentrations of C and N, but in bulk density (weight per unit volume) and particle size, as well.

Material Characteristics

In addition to the C:N ratio, the quality of the substrate in terms of chemical and physical composition is important. For example, carbon in compounds resistant to microbial attack (such as lignin—the chief component of wood) will be composted at a much slower rate than carbon of simple sugars.

Moisture Content

For aerobic composting (taking place in the presence of oxygen), the maximum moisture content should be kept at a level that allows the whole composting process to be aerobic. Too low of a moisture content will deprive microbes of water needed for their metabolism and inhibit their activity, resulting in slower composting. Drier piles tend to heat up and cool down more rapidly than wetter piles, and excessive dryness makes piles susceptible to spontaneous combustion. Very dry, dusty composts become populated by molds instead of the beneficial organisms. If the pile becomes too wet oxygen will then quickly become limited, and microbial activity will decrease, as reflected by the decreasing temperature. Without sufficient oxygen, the pile will become anaerobic. Anaerobes break down materials at a much slower rate than aerobic microorganisms. Slow decomposition produces many undesirable byproducts, among them noxious odors that have been compared to the rotten-egg smell of hydrogen sulfide gas. It also creates organic acids that can inhibit plant growth.

Temperature

Temperature is a very good indicator of the process occurring within the composting material. The temperature increases due to the microbial activity is noticeable within a few hours of forming a pile as easily degradable compounds are consumed. The temperature usually increases rapidly to 122 to 140 degrees F (50 to 60°C) where it is maintained for several weeks. This is called the active composting stage. When temperatures reach the 150 to 160 degrees F (66 to 71°C) range, thermophilic organisms begin to die and composting slows.

Oxygen Content

Composting is an aerobic process because microbes involved require oxygen to live. Thus, it is necessary to provide an adequate supply of oxygen either passively or actively. A well-aerated compost pile has at least five percent oxygen content during the active phase of composting (ideally closer to 10%). As microbial activity increases in the compost pile, more oxygen will be consumed. If the oxygen supply is not replenished, composting can shift to anaerobic decomposition, slowing of the composting process, and emitting foul odors (e.g., rotten eggs).

Pile Porosity

Good aeration can also be achieved by managing pile porosity. Porosity is defined as the volume of pores divided by the total volume of compost. Some of those pores will be filled with water and the rest with air. The initial pile mix should have between 45 to 60 percent air-filled porosity and during the active phase of composting, it shouldn't drop below 35 percent. Particles that are too small will pack tightly and reduce porosity in the pile. However, smaller sized particles will provide more exposed surface area than larger ones and accelerate the composting process.

pH of Materials

The optimal pH range is 6.0 to 7.5 for bacteria and 5.5 to 8.0 for fungi. When the compost pH exceeds 7.5, gaseous losses of ammonia are more likely to occur. The pH of most animal manures is approximately 6.8 to 7.4. Composting alone leads to major changes in materials and their pH as decomposition occurs.

Substrate Physical Properties Influence on Composting

Substrate physical properties that affect composting include porosity, structure, texture, and particle size. These physical characteristics affect composting through their effects on oxygen availability and surface area for microbes.

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