Most of the temperature rise within the composting material is a result of the activity of the microbes in the bin. As the microorganisms work to decompose the compost, their metabolic activity generates heat, which raises
the temperature of the composting material.
Temperatures between 32°C (90°F) and 60°C (140°F) normally indicate rapid decomposition is taking place in the compost heap and this is most common shortly after
material has been added.
Lower temperatures signal a slowing in the composting process. However, high temperatures, in excess of 60°C, result in a reduction in the activity of the majority of microorganisms.
The microbes involved in the aerobic composting process ( Compost Microbes) fall into two groups:
- Mesophilic those that are active in temperatures of (10 °C to 45 °C) and
- Thermophilic active at 45°C to 70°C.
The psychrophiles a third group
of microorganisms functioning at lower temperatures, in the range 10°C to 20°C (14°F to 68°F), do not play a significant role in hot composting but will be involved in keeping cold composting ticking over during in the winter
in the UK and areas with a similar climate.
During the hot thermophlic stage efforts should be made to ensure that the composting material is kept aerated and moist, additional moisture should be added in the form
of water or compost comfrey tea. Providing air and moisture are at levels which ensure biological activity is necessary to hold the high temperatures required to destroy weed seeds, pathogenic microorganisms, the eggs of
parasites and fruit flies, some of which, have been shown to survive in composting material at temperatures as high as 57°C for several days.
If checked regularly the temperature of the
decomposing material provides an indication as to when the pile should be turned to aerate to restore the microbial activity necessary to maintain the required temperature of for as long a period as possible.
It has been found that during the thermophilic stage up to 87% of the organisms present in the sampled heaps were in the spore forming Bacillus species, with one species Bacillus stearothermophilus predominant at temperatures of
over 65°C. However, there are a wide range of thermophilic bacteria and fungi that have been isolated from compost during the thermophilic stage. Although the heap will be repopulated when it cools it is desirable avoid temperatures of over 65°C to
maintain a broad population of microbes to ensure decomposition of the composting material.
The estimated optimum “hot” temperature varies in the published works between 45°-55°C and 55°-59°C
while others favour 60°- 65°C.
For the purposes of organic certification, the National
Organic Program rule (United States Department of Agriculture [USDA], 2000) requires that compost produced through a process that combines plant and animal materials, using an in-vessel or static aerated pile system, must maintain the
composting materials at a temperature between 131 °F (55°C) and 170 °F ( 76°C) for a minimum of three days. If using a windrow system these temperatures must be held for 15 days, during which time, the materials must be turned
a minimum of five times.
When hot composting at home the keen composter should maintain the temperature for 10 to 15 days. Larger and deeper heaps result in higher temperatures and better temperature distribution throughout
the pile and therefore expose more material to the higher temperature required for speedy composting.
As recommended elsewhere on this site three wooden compost bins of 1m x1m x 1m (approx 3ft x 3ft x 3ft) provide an adequate
size to retain the heat produced by the microbial activity and make turning to aerate relatively easy as the material can be moved from bin to bin. However, at home, particularly with a single bin where the C:N ratio and the moisture contented
may vary it may not be possible to maintain temperatures of over 50°C after the initial surge even with regular turning. The good news is that temperatures of 40°C if held still produce sanitised compost relatively rapidly.
Alternatively, an insulated commercially available freestanding bin, such as the Hotbin, which operates at temperatures of 40-60°C, or tumbler bin can be used.
While some people argue that if
handled correctly the composting process can be achieved by turning the heap once, two or three times should be regarded as a minimum with the target being least five times during the initial two week period.
method of maintaining higher temperatures is to base the frequency of turning the heap on the heap temperature. The temperature is recorded regularly and compost turned whenever the temperature falls below 55°C or rises above 72°C. During this
stage of active decomposition by the thermophiles the temperature will fall a few degrees immediately after turning but will rise again reaching its original temperature within two or three hours.
that if you are monitoring the temperature of the composting process it is useful to take additional readings during the three-hour period after turning. The recording and turning being continued until there is no marked change in temperature following the
If temperature monitoring has been undertaken irregularly and the heap has been allowed to overheat to such an extent as to inhibit bacterial activity i.e. reaching temperatures of over 76°C. turning
will be less effective in reducing the temperature. Watering the heap is sometimes suggested as a means of reducing the temperature but this should be attempted with care, as there is a danger of the heap becoming waterlogged.
a fall in temperature, during what should be the thermophilic stage, occur if parts of the material are becoming anaerobic. This is usually accompanied by a fall in pH. It can often be rectified by increased aeration and the addition of more “Browns”
This temperature drop can occur in quite localised areas, which have become anaerobic, with the temperature falling to below that of the surrounding aerobic compost. I suspect that this might be the case with material
being composted in sealed compostable cornstarch bags and would recommend opening (and emptying these bags, if they are used.
The thermophilic stage is followed by a second mesophilic stage as the heap cools; during
this, stage different microbes come to dominate the compost population. Some workers record a relatively quick fall in temperature to ambient temperature of stable and mature compost while others record a more gradual fall in temperature. The temperature will
vary within different parts of the compost heap, which is why a number of temperature samples should be taken from different locations when recoding temperatures.