Composting Temperatures

Hot composting:

In-vessel composters, such as the Compost Drum that is manufactured by Actium Composting Equipment, creates compost quickly by using the hot-composting (also known as active-composting) technique. The key to successful hot-composting is to create a composting environment that has ample oxygen, and achieve temperatures that reach 40°C – 65°C (104°F – 149°F). This environment promotes the activity of aerobic (oxygen-loving) microorganism that rapidly break-down organic matter with temperatures that most pathogens cannot survive. Alternatively, cold-composting (also known as passive-composting) technique utilizes different microorganisms that decompose organic materials at a significantly slower rate than hot composting. Cold-composting occurs when organic materials are added to a pile and left without frequent turning – mature compost using this technique can be ready between one to two years. Cold composting typically follows the day to day air temperatures – potentially reaching higher temperatures in the center of the pile. Both aerobic and anaerobic microorganisms exist within a cold compost pile. Anaerobic microorganisms are not as efficient at breaking down organic materials compared to aerobic microorganisms. Anaerobic microorganisms flourish where there is limited to no oxygen available. Low oxygen supply occurs when compost is not turned regularly and/or there is a high supply of moisture. This article details the benefits of using the hot-composting technique, along with how temperature influences decomposition.

Benefits and drawbacks of hot-composting:

The main benefits of hot-composting include:

  • producing mature compost quickly;
  • generating a greater amount of compost in less time than a cold-compost system;
  • reaching high temperatures (> 55°C, > 131°F) within the compost that can effectively kill most weed seeds and larvae;
  • reaching high temperatures (> 55°C, > 131°F) that kill most pathogens when composting; and
  • decomposing organic materials that are difficult to decay (i.e. bones and corn cobs) at a quicker rate than when cold-composted.

The main drawback of hot-composting compared to cold-composting is that monitoring and some maintenance is needed to warrant a favourable environment for rapid aerobic decomposition throughout the composting process; which includes:

  • diversifying and mixing different organic materials to ensure a carbon to nitrogen (C:N) ratio remains within the range of 20:1 to 30:1;
  • maintaining moisture levels in the range of 40-70%;
  • ensuring adequate oxygen levels (for example, turning the organic materials regularly (after materials are added or at least once a week)
  • monitoring compost temperature to make sure it stays within the range of 40°C – 65°C (104°F – 149°F), and to observe stages of decomposer activity; and
  • monitoring compost temperature to make sure it reaches > 55°C (131°F) for at least three days for the sterilization of weed seeds, larvae, and human pathogens.

Temperature ranges for composting:

Different types of microorganisms thrive at different compost temperature ranges. The three types discussed in this article are the psychrophiles, mesophiles, and thermophiles.


Psychrophilic microorganisms live within the cool temperature range of -10°C to 21°C (14°F to 70°F), and they actively decompose organic materials at temperatures ranging from 13°C to 21°C (55°F to 70°F). Psychrophiles are utilized when cold composting, as they decompose material at ambient temperatures. In a hot-composting environment, psychrophiles will exist only briefly – during the beginning of Spring when air temperatures become warmer, or when the very first materials are added to start composting. When the psychrophiles decompose organic materials, the
microorganisms will release heat, raising the temperature of the compost. When the compost temperature surpasses 21°C (70°F), it is no longer a viable environment for the psychrophiles, and the mesophiles begin to populate the compost. When temperatures get too warm for the psychrophiles, they either become dormant, die-off, or move to a section of the pile that is cooler.


Mesophiles will inhabit both cold-compost and hot-compost types. They live within environments that have moderately warm temperatures, ranging from 20°C to 45°C (68°F to 113°F). They are most active at decomposing materials when temperatures are between 21°C to 32°C (70°F to 90°F). Though mesophilic decomposers thrive in this temperature range, so do human pathogens (such as Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, and Salmonella sp). For these pathogens to die-off, temperatures need to increase to a point where they cannot survive.


The decomposing activity of mesophiles heat-up the compost to the point that surpasses their habitable range, and then thermophiles take over. Thermophiles can live in temperatures that destroy most pathogens. The temperature range for thermophiles is 40°C to 71°C (104°F to 160°F), and the temperature for pathogen sterilization is > 55°C (> 131°F). At high temperatures, both sterilization and decomposition are occurring. Thermophiles are very efficient decomposers − they accelerate the breakdown of complex carbohydrates, fats and proteins, and convert organics into humus – a complex material that is nutrient-rich. In a cold-compost, thermophiles are less likely to be present, but if they are, it will be most likely at the center of the pile. For hot-composting, fostering thermophiles is key to killing pathogens and decomposing organic matter rapidly. Having the ability to turn a batch or in-vessel composter is beneficial for hot composting because:

  • turning aerates the compost, providing oxygen supply to the aerobic thermophiles;
  • mixing organic matter allows thermophiles to have greater access to materials in need of sterilization or further decomposition, and;
  • turning permits equal temperature distribution, ensuring all compost materials have been sterilized and underwent thermophilic decomposition.

Achieving high temperatures is crucial for some compost operations for them to meet specific quality standards. For instance, to ensure pathogen removal, the Ontario Compost Quality Standards require temperatures to be monitored daily until > 55°C (131°F) occur for at least three consecutive days when composting most organic materials. In-vessel composters by Actium Composting Equipment are designed to help sustain temperatures where thermophiles can
thrive, and sterilization can occur; they are made with an insulated outer-layer to minimize heat escape and an optional electrical heater for maintaining a warm compost environment when operated in colder climates. As well, long-stem thermometers (Reotemp Instruments, USA) come with Actium Composters to measure compost temperatures, and to monitor when sterilizing has occurred.

Compost curing:

After the thermophiles have completed their cycle, the compost gradually cools down, and the mesophilic and psychrophilic microorganisms return. The decomposing process slows as less material is available for microorganisms to
consume. The compost is not yet complete or ready to be added to the garden. At this later stage, the compost is stabilizing as it cures. Temperatures will continue to decrease until it is near ambient temperatures, and decomposing microbial activity will eventually stop. The curing process can last a minimum of 21 days. When composting using a drum or vessel, the compost can be removed during the curing stage and placed in a secondary compost area or bin, protected from the rain.