Proper temperature is an important factor, particular in the aerobic composting process. Considerable amounts of heat are released by aerobic decomposition. Since composting material has relatively good insulation properties, a sufficiently large composting mass will retain the heat of the exthermo-biological reaction and high temperatures will develop.
High temperatures are essential for destruction of pathogenic organisms and undesirable weed seeds. Decomposition also proceeds much more rapidly in the thermophilic temperature range. The optimum temperature range is 135º -160º Fahrenheit, around 150º Fahrenheit usually being the best. Since only a few of the thermophilic organisms actively carry on decomposition above 160º Fahrenheit, it is undesirable to have temperatures above this for extended periods.
Eggs of parasites, cysts and flies have survived in compost stacks for days when the temperature in the interior of the stack is above 135º Fahrenheit. Since a factor of safety is necessary, and since a higher temperature can be readily maintained during a large part of the active composting period, all the material should be subjected to a temperature of at least 150º Fahrenheit.
In some instances compost operators have avoided prolonged high temperatures because the nitrogen loss tends to be greater at high temperatures owing to the vaporization of ammonia, which takes place when the C:N ratio is low. But there are other ways of minimizing nitrogen loss than operating at a lower temperature. The advantages of destroying pathogenic organisms and weed seeds, controlling flies, and providing better decomposition outweigh any small nitrogen loss due to high temperatures.
A drop in temperature in the compost pile before material is stabilized indicates that the pile is becoming anaerobic and should be aerated. High temperatures do not persist when the pile becomes anaerobic. The temperature curve for different parts of the pile varies somewhat with the size of the pile, the ambient (surrounding) temperature, the moisture content, the degree of aeration, and the character of the composting material. The provision of aerobic conditions, however, is the important factor in maintaining high temperatures during decomposition. The size of the compost pile or windrow may be increased to provide higher temperatures in cold weather or decreased to keep the temperatures from becoming too high in warm weather. Experience shows that turning to release the heat of compost piles, which have become so hot (170º-180º F.) that bacterial activity is inhibited, is not very effective. When the material is actively decomposing, the temperature, which falls slightly during turning, will return to the previous level in two or three hours. Also, it is impossible to bring about any significant drop in temperature by watering the material without waterlogging the mass.
Variations in the moisture content between 30% and 75% have little effect on the maximum temperature in the interior of the pile. The initial temperature rises a little more rapidly when the moisture content is 30% to 50% than when it is 70%. Studies, however, did show an important and significant correlation between the moisture content and the temperature distribution within the pile. When moisture content is high, temperatures near the surface will be higher, and the high temperature zone will extend nearer to the surface than when the moisture content is low. For example, in experiments at University of California during mild weather when the air temperature fluctuated between 50º and 80º Fahrenheit, the zone of maximum temperature in a pile with a moisture content of 61% extended to within about one inch of the surface while the maximum temperature zone in a pile containing 40% moisture began 6 inches below the surface.
Deeper piles caused higher temperatures and better temperature distribution, and subject more material to a high temperature at any one time. Hence, the actual mass of the material evolving heat is important in providing adequately high temperatures.
Shredding or pulverizing the material also provides better temperature distribution and less heat loss.
Materials with a high C:N ratio or containing large amounts of ash or mineral matter usually attains high temperatures more slowly in the compost pile.
Aeration to maintain aerobic conditions in the compost pile is essential for high temperatures. When the compost pile becomes anaerobic, the temperature drops rapidly. Even small areas, which have become anaerobic, will often exhibit a lower temperature than the surrounding aerobic material.