Factors that affect composting

Moisture

Water is necessary for the microbes in organic material to work efficiently, as it acts as a medium for chemical reactions, the transport of nutrients and the movement of the micro-organisms. The moisture content of the composting material should be maintained between 40% and 50% (wet basis). Too much moisture will cause conditions to become anaerobic (lack of oxygen) and unpleasant odour may result. The compost is too wet if water can be squeezed out by hand, or too dry if it is not moist to the touch. The composting material generally dries out with time. In turned systems, water should be added regularly. In no-turn systems, moisture can be prevented from escaping using a thick layer of finished compost or bulking agent.

Carbon to nitrogen ratio (C:N)

Micro-organisms require a balance of carbon and nitrogen for healthy cell growth. It is important to provide carbon and nitrogen in the right proportions to encourage microbial activity. A carbon to nitrogen ratio (C:N) between 15:1 (15 parts carbon to 1 part nitrogen) and 30:1 is required for good composting results. While the C:N largely determines the blend of materials to be used, the rate at which carbon compounds decompose should also be considered.

Table 1 shows a variety of agricultural materials that are used for composting, and their suitability for composting. Virtually any plant or animal product can be used.

Table 1. Characteristics of some organic material used for composting

Aeration (oxygen addition)

The composting pile should contain sufficient oxygen to maintain adequate microbial activity. While the initial mixing of the materials will introduce oxygen into the pile, this small supply of oxygen will be rapidly exhausted. To maintain sufficient oxygen levels, some form of aeration will be required.

Turning the piles regularly (weekly) provides good aeration, as the pores created allow air to move easily through the pile. However, this is known to increase methane emissions into the atmosphere.

No-turn systems introduce air into the material via passive or forced aeration of windrows. A bulking agent such as sawdust or woodchips can be used to increase the air pockets within the composting material. Such systems need to be carefully constructed and require additional monitoring to ensure their success.

Particle size

A small particle size will ensure the microbes have ready access to the organic material to help speed up the composting process. Smaller particles also allow oxygen to penetrate through the pile more evenly and be available to the microbes. If the material is clumped into large particles, the material will be compacted, allowing less oxygen into the pore spaces.

Temperature

The natural composting processes produce heat, and the micro-organisms grow best within a temperature range between 55°C and 65°C. Operation at this temperature also assists the destruction of pathogens and weed seeds. However, if the temperature exceeds 65°C, the composting organisms will die and the process will slow down substantially. Provided the oxygen and water supplies are within the optimum ranges, the composting material should maintain suitable operating temperatures. In Queensland, composting can be successfully achieved throughout the year. However due to the higher temperatures during summer, more moisture addition may be necessary in drier areas.

Time required for composting

The length of time required to transform manure into compost depends on moisture, temperature, aeration, C:N, and the physical structure of the raw materials. If optimal conditions are maintained, the composting process can be completed in around 8 weeks, with 4 weeks additional curing time following the active composting stage. Curing involves the further aerobic decomposition of some compounds, organic acids and large particles that remain after composting. Less oxygen and water are required during curing than during active composting. However, compost that has had insufficient curing may damage some horticultural crops. The test of completion of the compost process is if the temperature of the material does not rise after turning and watering.

References

• Misra, RV, Roy, RN & Hiraoka, H 2003, ‘Large scale composting’ in On-farm Composting Methods, FAO, Rome, p:21-28.
• Potts, J & Casey, KD 1999, Co-Composting Timber Residues and Feedlot Manure Project – Final Report, Department of Primary Industries and Fisheries, Toowoomba.
• Rynk R. (ed) 1992, On-Farm Composting Handbook, Northeast Regional Agricultural Engineering Service, New York.