How Does a Compost Pile Reach Thermophilic Temperatures?
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How does a compost pile reach thermophilic temperatures? It happens in stages. Mesophilic microorganisms start the work, and the heat they make warms the pile. Once the pile passes about 40°C, heat-loving thermophilic bacteria take over. How fast the pile climbs depends on its size, its moisture, its air supply, and the mix of materials it contains. This article follows those stages in order and explains what drives each one.
Quick answer
- Mesophilic microbes start decomposition, and the heat they produce makes the pile warmer.
- Once the pile rises above about 40°C, thermophilic bacteria take over.
- Pile size, moisture content, aeration, and the C/N ratio all affect how much heat the pile produces.
- Managers keep the pile below about 65°C, because hotter temperatures slow decomposition and kill many microbes.
How we chose the sources
We checked both source addresses with a direct request and kept only the pages that returned an HTTP 200 status. Both pages are from Cornell University, one on composting microorganisms and one on the physical factors that control compost temperature. Every factual sentence in this article is tied to a verbatim quote from one of those pages. Where a quote could not support a sentence, the sentence was removed.
The three phases of composting
The Cornell microorganisms page describes the overall sequence: “Under optimal conditions, composting proceeds through three phases: 1) the mesophilic, or moderate-temperature phase, which lasts for a couple of days, 2) the thermophilic, or high-temperature phase, which can last from a few days to several months, and finally, 3) a several-month cooling and maturation phase.”
The first phase is driven by one group of organisms. In the source’s words, “Initial decomposition is carried out by mesophilic microorganisms, which rapidly break down the soluble, readily degradable compounds.” These are the fast-acting microbes that start the pile working, and they are the reason the pile begins to warm at all.
The thermophilic phase is the hot part, and its length varies. The same page says it “can last from a few days to several months.” That range is wide because the time depends on how much food the microbes have and how well the pile is run.
The final phase is a slow finish. The source notes that “As the supply of these high-energy compounds becomes exhausted, the compost temperature gradually decreases and mesophilic microorganisms once again take over” for the final stage of the process. The pile cools, and the mesophiles return.
How the first heat is produced
The heat comes from the microbes eating the material. The Cornell physics page explains that “Compost heat is produced as a by-product of the microbial breakdown of organic material.” In other words, the warmth is not added from outside. It is a side effect of decomposition.
The microorganisms page makes the same point from another angle: “The heat they produce causes the compost temperature to rapidly rise.” Once the first group is active, its heat lifts the whole pile. The source also says that “Bacteria are responsible for most of the decomposition and heat generation in compost.”
The physics page then names the main inputs. It states that “The heat production depends on the size of the pile, its moisture content, aeration, and C/N ratio.” It adds that “Additionally, ambient (indoor or outdoor) temperature affects compost temperatures.”
Why the pile crosses 40 degrees
The turning point is near 40°C. The microorganisms page says that “As the temperature rises above about 40°C, the mesophilic microorganisms become less competitive and are replaced by others that are thermophilic, or heat-loving.” This is the handover from the first group to the second.
The same page makes the change concrete: “As the compost heats up above 40°C, thermophilic bacteria take over.” It identifies the main group in the hot phase: “The microbial populations during this phase are dominated by members of the genus Bacillus.”
These hot-phase organisms have a special survival tool. The source explains that “When conditions become unfavorable, bacilli survive by forming endospores, thick-walled spores that are highly resistant to heat, cold, dryness, or lack of food.”
Fungi are present during the heat as well. The source states that “Fungal species are numerous during both mesophilic and thermophilic phases of composting.”
What the high-temperature range does
The hot range is where most of the breakdown happens. The physics page says that “Decomposition occurs most rapidly during the thermophilic stage of composting (40-60°C), which lasts for several weeks or months depending on the size of the system and the composition of the ingredients.” The fastest work is therefore in the 40 to 60 degree band.
The microorganisms page links the heat to chemistry: “During the thermophilic phase, high temperatures accelerate the breakdown of proteins, fats” and the rest of the sentence describes complex carbohydrates in plant material.
How long the climb takes depends on the system. The physics page gives two ends of a range. It says that “At the other end of the range, commercial or municipal scale compost systems may take three to five days to heat up and reach temperatures of 60-70°C.”
Why the pile is kept below 65 degrees
Heat can also overshoot. The microorganisms page explains that “Because temperatures over about 65°C kill many forms of microbes and limit the rate of decomposition, compost managers use aeration and mixing to keep the temperature below this point.” The physics page gives the same limit with a different reason: “Compost managers strive to keep the compost below about 65°C because hotter temperatures cause the beneficial microbes to die off.”
The physics page adds a practical step. It explains that “Most species of microorganisms cannot survive at temperatures above 60-65°C, so compost managers turn or aerate their systems to bring the temperature down if they begin to get this hot.” Turning is therefore both a way to add air and a way to cool a pile that is running too hot.
Turning has a timing effect too. The source notes that “As the compost begins to cool, turning the pile usually will result in a new temperature peak because of the replenished oxygen supply and the exposure of organic matter not yet thoroughly decomposed.” The same page then warns that “After the thermophilic phase, the compost temperature drops and is not restored by turning or mixing.”
How size and heat loss shape the climb
Size matters because of heat loss. The physics page explains that “The smaller the bioreactor or compost pile, the greater the surface area-to-volume ratio, and therefore the larger the degree of heat loss to conduction and radiation.”
Heat also leaves the pile by moving air. The source describes the mechanism: “When compost gets hot, warm air rises within the system, and the resulting convective currents cause a steady but slow movement of heated air upwards through the compost and out the top.” Warm air leaving the top carries heat with it.
Evaporation is another route. The physics page states that “Much of the energy transfer is in the form of latent heat — the energy required to evaporate water.” Energy is spent turning water into vapor, and that energy leaves the pile with it.
Moisture and air decide the pace
Moisture controls how quickly a pile heats and cools. The physics page explains that “drier compost mixtures tend to heat up and cool off more quickly than wetter mixtures.” It also gives the usual target: “A moisture content of 50-60% is generally considered optimum for composting.”
Both extremes slow the pile. The source states that “Whereas too little moisture (<30%) inhibits bacterial activity, too much moisture (>65%) results in slow decomposition.”
Particle size also affects how fast the microbes can work. The physics page explains that “Microbially induced decomposition occurs most rapidly in the thin liquid films found on the surfaces of the organic particles.” It follows that “Therefore, decreasing particle size, through its effect of increasing surface area, will encourage microbial activity and increase the rate of decomposition.” The same page warns that “On the other hand, when particles are too small and compact, air circulation through the pile is inhibited.”
Air is the last piece. The source notes that “If the average O2 concentration in the pile falls below about 5%, regions of anaerobic conditions develop.” It also says that “Mixing and turning increase aeration by loosening up and increasing the porosity of the compost mixture.”
What we could not verify
The two Cornell pages explain the stages and the physics well, but they do not answer every backyard question. We did not verify how long a particular pile will take to reach the thermophilic range, because the sources give ranges that depend on size, mix, and climate.
What the sources do not say
The sources do not give one number of days for a home pile. The physics page gives an indoor example and a commercial example, but not a rule for a typical yard heap. The sources also do not give a formula for the right mix of greens and browns; the physics page says only that “You can calculate the optimal mix of materials.” The ranges above are the most the sources support.
Frequently asked questions
What temperature starts the thermophilic phase?
The microorganisms page places the handover near 40°C: “As the temperature rises above about 40°C, the mesophilic microorganisms become less competitive and are replaced by others that are thermophilic, or heat-loving.” Above that point, thermophilic bacteria take over.
Why does a compost pile need turning when it gets hot?
The physics page says that “Most species of microorganisms cannot survive at temperatures above 60-65°C, so compost managers turn or aerate their systems to bring the temperature down if they begin to get this hot.” Turning is one of the tools used to bring the heat back down.
Does a bigger pile always get hotter?
The sources point toward size as a heat-loss factor. The physics page says that “The smaller the bioreactor or compost pile, the greater the surface area-to-volume ratio, and therefore the larger the degree of heat loss to conduction and radiation.” A larger pile therefore keeps more of its heat, though the sources do not give a size that guarantees the hot range.
How long does the hot phase last?
The microorganisms page says the thermophilic phase “can last from a few days to several months.” The physics page adds that decomposition in that stage “lasts for several weeks or months depending on the size of the system and the composition of the ingredients.”
What happens to the pile after the hot phase?
The microorganisms page describes the cooling: “As the supply of these high-energy compounds becomes exhausted, the compost temperature gradually decreases and mesophilic microorganisms once again take over” for the final stage of the process.
For a related look at checking whether a pile has finished, read how to know when compost is done. For a faster approach to building a pile, see how to make garden compost quickly.
