Have you ever heard the expression something is more of an art than a science? I think this is particularly true of composting. There are so many factors that affect the temperature of a compost pile that unless you are doing it in a lab, there will be some fluctuations.
Sometimes these variations in compost piles result in a bad smell, while other times, it results in pest or weed issues. Occasionally, a deviation from the intended compost temperature can result in a very hot pile. So hot, it just might burn!
What Makes A Compost Pile Hot?
There are many ways to find out what makes a compost pile hot. From the minute you build your pile, millions of bacteria are beginning to wake up and eat. As nitrogen off-gases with the carbon in your compost, even more heat accumulates.
Add a direct hit from the hottest light in the universe, the sun, and you can begin to guess what makes a compost hot. Let’s look at each of the different elements that affect the heat of the compost and how we can help them do their jobs better!
Nitrogen and carbon
When composting, materials high in both nitrogen and carbon are added. Once the pile is the appropriate size, you should have a ratio of 30:1 with 30 parts carbon to 1 part nitrogen.
At the end of a composting cycle, the carbon to nitrogen ratio is 10:1. The 20 parts of carbon are broken down and often released as heat and, more specifically, as water vapor. This occurs most frequently during the thermophilic stage of composting when temperatures reach 105℉ or higher.
Microorganisms
Most of the activity in a compost pile, and therefore the heat generation, comes from the hordes of microorganisms that eat decay. These creatures occupy every stage and zone of a compost pile.
Some microorganisms thrive in the oxygen-rich aerobic outside of the pile as it quickly heats up. Others live exclusively in the deep anaerobic areas at the center of the pile. As the inside of the pile heats up, new hungrier bacteria come to life and they generate even more heat.
Finally, as the compost is turned and cools mesophilic microbes take over and consume the pile until it returns to ambient temperature. Throughout the entire composting process, microorganisms are causing the pile to heat up.
Sunlight
It shouldn’t come as a surprise to know that one of the critical elements that can cause a hot pile is the sun. Sunlight, and the heat it emits, can be used to activate a pile quicker or help one heat up after it has been flipped and moistened. Using sunlight can help jump-start thermophilic reactions that allow the fastest breakdown of organic materials.
Sunlight can also cause dangers if it generates too much heat during a time when the compost’s materials are already near their heat limit due to internal temp spikes. Older mature piles might benefit from indirect sunlight to allow for slow aging. This is especially true when composting manure.
How Long Can a Compost Pile Stay Hot?
The duration for which a compost pile can stay hot depends on two main factors. The size of a pile will determine how much fuel is available to be released as heat during the composting process. The bigger a pile is the more material there is to be broken down, and the more heat can be generated. A smaller compost pile will not stay hot for as long.
The other factor that greatly affects how long a compost pile can stay hot is the moisture content of the materials used to create the pile. If materials have too much moisture, it will take a long time for the microbes in the pile to be able to multiply and consume the waste. This often results in bad odors.
Conversely, composts can be created with garden materials or house waste that has far too little moisture. If this happens there will be very little for the bacteria to feed on, and decomposition will not happen. Moisture levels between 30% and 65% are ideal for healthy, active piles.
To get the ideal moisture level, you can either add materials that are the right percentage of moisture or add your own using irrigation methods. Below is a chart with some common compostable materials and their moisture percentages:
| Material | Moisture |
| Newspaper | 5% |
| Kibble | 10% |
| Peaches | 80% |
| Lettuce | 87% |
Can A Hot Compost Pile Catch Fire?
The decomposition of organic matter can cause a lot of heat. But can a hot compost pile catch fire? While it is technically possible, it is extremely rare for a compost pile to spontaneously combust. The pile would need to be seriously dry, way overheated, and even then still need a spark or catalyst of fire.
While a fire is rare, smoke or rather steam trailing out from the top of the pile can be a much more common occurrence. This is actually water vapor that is being released as the microorganisms break down the materials.
Carbon breaking down also results in a heat exchange. When combined with ample moisture, there is a release of vapor that could resemble smoke to a wary bystander.
Although it is not likely for a properly built pile with adequate moisture to burst into flames, excessive heat can be a serious problem for compost piles.
What Happens When A Compost Pile Gets Too Hot?
It is very important when building a compost pile, especially one larger than a couple of feet in diameter, to restrict the maximum temperature of the pile to below 140℉. Anywhere above this temperature and the microbes in your pile will die off.
If the temperature in a pile is either too low or too high, you will not have the bacteria needed to break down the materials. When the temperature reaches above 140℉, you will need to flip the pile and add aeration to reduce the temperatures to a more stable level.
The safest way to manage a compost pile is to build it with the correct amount of carbon to nitrogen ratio and adequate moisture. You will want to achieve a temperature of 105℉ within the first few days. After that, it is important to hold the temperature of your pile above that and below 140℉ for at least 4 days.
Depending on the size of your pile, you can maintain the range of 105℉ to 140℉ for up to 20 days before letting the temperatures fall, and the mesophilic microbes begin to take over. A compost managed in this way should yield high-quality, weed, and pathogen-free soil amendments for your yard!