How hot does compost get?

How to Make Hot Compost

Hot compost is a quick and easy way to get through your waste.

A hot compost pile can reach temperatures of 49-77 degrees Celsius (120-170 degrees Fahrenheit) in just a few days, and if you get it right, you can compost your organic matter in around four weeks. You can even use these high temperatures to heat your water, home or greenhouse. The temperature of the pile must be monitored because remaining at a temperature of 65 degrees Celsius or higher for even just a few hours, will kill beneficial microorganisms that add to your compost.

What you will need

Equal parts green and brown materials, all shredded to a small size. Fresh grass clippings and dried shredded leaves work great for your first effort because they’re already in small pieces, and the grass clippings are full of moisture.

A fork or shovel for turning

A compost thermometer or a meat thermometer attached to the end of a stick

A tarp (optional)

A bin/container (optional)


Make sure all organic matter is chopped into small pieces and mix together the green and brown materials well. Add in around a shovelful of already made compost or soil, which will be full of microorganisms to jumpstart the process.

As you build the pile, sprinkle with some water to keep the organic matter moist. It needs to be the consistency of a wrung-out sponge.

Your compost pile needs to be around one cubic meter in size. Any larger than one and a half cubic metres, will mean moisture and heat levels are wrong for speedy decomposition. At this stage you could cover the pile with a breathable tarp to maintain moisture, but this isn’t necessary.

Over the next month you will need to monitor and record the daily temperarture of the pile with a compost thermometer. Between one and five days, the temperature should rise, between 49 and 77 degrees Celsius (120 to 170 degrees Fahrenheit). The temperature will depend on moisture levels, the size of your organic matter and the size of your pile.

Once the temperature cools to below 43 degrees Celsius (110 degrees Fahrenheit), which is usually between four and seven days, you will need to turn over the organic matter to introduce oxygen. This will then heat the pile back up.

Everytime you turn the compost, make sure you bring the pile’s exterior material into the interior. This enables all material to be evenly broken down. Water can be added to maintain the correct moisture levels, but be liberal, as the pile will cool if it is too moist.

Continue monitoring and recording temperatures everyday, and the turn the compost every four to five days, when the temperature drops below 43 degrees Celsius (110 degrees Fahrenheit). Continue moistening if needed. After around 14 days, the compost will no longer be recognisable. After one month, you should have turned the pile four times.

By this point, most of the pile will be a dark, crumbly compost and the temperature will decrease below 29 degrees Celsius (85 degrees Fahrenheit).

Now you must let the compost ‘cure’ for a couple of weeks before you can use it.

Rozie Apps is assistant editor at Permaculture magazine and Permanent Publications.

Further resources

Make a low cost, heat-retaining DIY compost tumbler

How I made really great compost

The many benefits of Hugelkultur

500 showers heated from one small compost pile: a how to tutorial

Read How to Make & Use Compost

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How to Manage the Temperature of Your Compost Pile

By Cathy Cromell, The National Gardening Association

Understanding temperature phases and managing a hot compost pile help you produce useable compost quickly. All materials eventually break down in cool, unmanaged compost piles as well, but decomposition occurs faster with higher temperatures. Also, if you need to destroy weed seeds or plant pathogens during the composting process, creating a hot pile is essential.

Following are tips for working with a hot pile:

  • Take its temperature: Use a compost thermometer to take your pile’s temperature daily. Record it in a notebook or spreadsheet, and over time you’ll get a feel for how long different phases take with your composting ingredients and methods.

  • Size it right: Compost piles require mass to self-insulate and maintain high temperatures during thermophilic composting. The minimum size is 3 x 3 x 3 feet (1 cubic yard or 1 cubic meter) up to 5 x 5 x 5 feet (1.5 cubic meters). This size allows the material to self-insulate and is easy to turn for a typical gardener. Larger sizes inhibit airflow to the center of the pile.

  • Turn, mix, and water: Temperatures drop as supplies of food, air, and water are exhausted. Turning the pile to aerate, mixing undecomposed ingredients on the outside into the center, and/or adding moisture may encourage temperatures to rise and promote more rapid decomposition. At some point, food supplies are exhausted, and turning the pile no longer boosts temperatures.

  • Destroy pathogens and weed seeds: Most plant pathogens are destroyed if temperatures remain between 130 and 140 degrees Fahrenheit (54 to 60 degrees Celsius) for 72 hours. Most weed seeds are destroyed if exposed to temperatures above 131 degrees Fahrenheit (55 degrees Celsius) for 72 hours.

  • Don’t overheat: Heating your pile above 170 degrees Fahrenheit (77 degrees Celsius) for more than a few hours is not recommended because it inhibits most microbial activity and shuts down the decomposition process.

If the pile is too hot, turn it to aerate the core and release heat build-up.

Temperature is an essential component of any quality compost pile. As organic matter decomposes energy is released, which results in an increase in heat. This heat provides an environment wherein bacteria (good bacteria) can work to break down waste.

Just like the food we eat, there are target temperatures for compost that make it both safe and productive. A compost that’s too cool is at risk of contamination. If we allow compost to get too hot we hinder bacterial growth and stop the effective breakdown of organic matter.

Monitoring these temperatures, controlling the release of unwanted heat or adding organic matter to increase heat is key to ensuring your compost is free from potentially harmful pathogens and produces quality compost.

Below 135°F, compost is considered too cool. In fact, eggs of parasites, cysts and flies have been known to survive for days in this relatively cool environment. At these temps bacteria works more slowly, which can result in the transformation of an aerobic (oxygen rich) compost into an anaerobic (oxygen depleted) breakdown.

When oxygen is removed from the decomposition equation the breakdown of matter happens at a much slower rate. This means it may take upwards of a year before you’re able to harvest any quality compost. If cool temperatures persist, you need to get bacteria active – stat! Their activity will release heat into the pile and induce continued breakdown.

Start by adding nitrogen-rich “green” organic material. Your brown-to-green ratio should be about 4-to-1. That’s four parts dried leaves to one part grass clippings or vegetable scraps. Next, add moisture to the pile to induce activity. Studies have shown a 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.

Above 160°F, compost is too hot. Few thermophilic organisms are able to get much done when their environment is too hot. If left above 160°F, bacteria will cease active decomposition, which in turn will render the pile inert.

To cool off a pile, give it a few turns with a pitch fork to allow heat to escape. If after a few hours temperatures start to creep back up, check your ratios of green to brown matter. Add more dried leaves to slow down the bacteria.

If you had to set a target temperature for your compost pile it would be 150°F. Above 135°F where pathogenic organisms struggle for survival and below 160°F where temperatures become too hot for bacteria to work properly.

Perhaps the best tool for monitoring the temperature of a compost pile is a long stem, T-handle thermometer like the RT610B, which comes in 12 and 24 inch lengths. The T-handle allows for easy insertion and removal, while the display is large and easy to read when the thermometer is inserted in the pile. The waterproof housing protects against rain and compost moisture so the instrument can be left in place over an extended period of time without fear of breakdown.

Suspicions of high or low temperatures can be quelled, or confirmed using the Min/Max feature that recalls the highest and lowest temperatures reached since the last reset. Leave the thermometer in place and come back to find out what happened while you stepped away. With a 5,000 hour battery life you can be confident that the thermometer won’t quit on you.

When maintained at your target temperature you should be able to harvest your compost in only 4-6 months – just in time for Spring. Good, quality compost will energize the soil and add rich nutrients to your garden. With a compost pile you’ll be able to enhance your garden’s ability to grown healthy plants while reducing your volume of trash.

Washington State University – Compost Fundamentals
Bonnie Plants: Composting 101 – What is Compost?
Gardening Know-How: Heat and Compost – Heating Up Compost Piles

My Compost Is Too Hot: What To Do About Overheated Compost Piles

The optimum temperature for compost to process is 160 degrees Fahrenheit (71 C). In sunny, hot climates where the pile hasn’t been turned recently, even higher temperatures can occur. Can compost get too hot? Read on to learn more.

Can Compost Get Too Hot?

If compost is too hot, it can kill beneficial microbes. Overheated compost piles pose no fire danger if they are properly moist but some of the organic properties will be compromised.

Excessive temperatures in compost can cause a spontaneous combustion, but this is very rare even among over-heated compost piles. Properly aerated and moist compost piles, no matter how hot, are not dangerous. Even hot compost bins that are fairly enclosed will not catch fire if they are tumbled and kept moist.

However, the problem is what excessive heat does to the living creatures that break down that organic waste. Overheated compost piles will likely kill many of these beneficial creatures.

High temperatures are necessary to destroy pathogens and weed seeds in compost piles. Heat is released in the aerobic process that takes place as organic matter rots. However, excessively high temperatures remove some of the nitrogen in the compost.

The high temperatures will persist as long as the pile is turned and oxygen introduced. Anaerobic conditions occur when the pile is not turned. These drop the temperature and slow the decomposition process. Can compost get too hot? Of course it can, but in rare instances. Temperatures that exceed 200 degrees Fahrenheit (93 C.) are likely damaging to the organisms that live and work in the compost.

What Causes Overheated Compost Piles To Catch Fire?

A rare combination of events can cause a compost pile to catch fire. These all must be met before the occasion arises.

  • The first is dry, unattended material with pockets of debris mixed throughout that aren’t uniform.
  • Next, the pile must be large and insulated with limited air flow.
  • And, finally, improper moisture distribution throughout the pile.

Only the largest piles, like those in commercial composting operations, are really in any danger if they are mismanaged. Key to preventing any issues is proper maintenance of your organic matter to prevent hot compost bins or piles.

How to Tell if Your Compost is Too Hot

It doesn’t matter if you have a bin, tumbler or just a pile on the ground; compost needs to be in sun and heat. It also releases heat. The key to managing the heat level is to make sure there is introduction of oxygen and moisture to all parts of the compost.

You also need the right balance of carbon and nitrogen materials. Compost is too hot often with too much nitrogen. The proper mix is 25 to 30 parts carbon to one part nitrogen. With these practices in place, your compost bin will likely keep at just the right temperature to create some organic goodness for your garden.

Compost Pile Hazards

by Nick Gromicko, CMI® Compost is an accumulation of degrading food scraps, plants and other nutrient-rich organic matter. It is an easy and environmentally responsible way to dispose of biodegradable kitchen waste, which can then be returned to the soil as fertilizer for vegetable and flower gardens.

Composting Is Good

  • Composting helps to reduce the volume of material in landfills.
  • Compost is used to improve soil structure and provide nutrients for growing plants.

So, what’s wrong with composting? The benefits of the practice are generally well-known, but few people are actually aware of the potential hazards and dangers composting can pose. Diseases Contracted from Handling Compost Compost can be a breeding ground for dangerous pathogens, some of which have killed or seriously harmed unsuspecting gardeners. Inspectors should familiarize themselves with these illnesses, some of which can be contracted in other parts of the house. Listed below are some of the more common physical ailments that can result from unprotected contact with compost:

  • Aspergillosis is a fungal infection of the lungs that is caused after the inhalation of a fungus commonly found in rotting plant matter. While normally not life-threatening, aspergillosis can be extremely dangerous if enough spores are inhaled. The disease killed a 47-year-old British man after he was engulfed in clouds of dust from the compost he had intended to use in his garden.
  • The symptoms of Farmer’s Lung resemble pneumonia, and may result from respiratory exposure to certain fungal and bacterial pathogens present in rotting organic materials, such as mushrooms, hay and sugar cane. Beware of dusty white patches, as they are a sign that dangerous spores are present. Farmer’s Lung can usually be treated with antibiotics.
  • Histoplasmosis is caused by fungus that grows in guano and bird droppings. Healthy immune systems can usually fight off histoplasmosis, although infections can become serious if large amounts of the toxin are inhaled, or if the infected person has a weakened immune system.
  • Legionnaire’s Disease is a respiratory infection that’s caused by the inhalation of L. Longbeachae.
  • Paronychia is a local infection that occurs in the tissue around the fingernails and toenails. Prolonged moisture and the abrasive effects of soil can create openings in the skin that allow the infection to occur, producing pain and throbbing.
  • Tetanus is a disease of the central nervous system that’s caused by bacteria that is very common in soil. While even a minor cut can allow the bacteria to enter the bloodstream, immunizations against tetanus are quite common.

How to Avoid Potential Hazards of Composting The following general safety precautions should be followed in order to avoid transmission of dangerous fungi, bacteria and other pathogens found in compost:

  • Always wear dry, breathable gloves to avoid direct contact with the skin, and to protect yourself from injury while using gardening tools and implements.
  • Wear protective footwear that covers your skin adequately to avoid direct contact with compost. Do not wear them anywhere except outdoors.
  • When stirring and tilling the compost, which is required on a regular basis in order for it to process and break down, always wear a nose and mouth guard or dust mask to avoid inhaling the various spores that will become airborne during tilling and turning.
  • Avoid tilling on windy days.
  • Do not store compost in fully closed or airtight containers. Without any air, it can actually become combustible.
  • Wash your hands after dealing with compost. While this suggestion may sound obvious, many garden enthusiasts get so absorbed with their activities that they forget the potential dangers from poisoning.
  • If you develop a severe cough or infection of the skin (especially if there is an open sore or puncture wound), seek medical attention immediately. You may require antibiotics or a tetanus shot.

Compost Fires Surprisingly, a great deal of heat is created by the microbial activity, which is occasionally enough to cause a fire. In August 2009, acompost pile spontaneously combusted at the Saginaw Compost Facility in Saginaw, Michigan. However, these fires are extremely rare, as they occur only under a limited set of circumstances that would ordinarily be avoided using common sense.
According to the Alberta, Canada’s Department of Agriculture, the following key conditions must be met in order for a compost pile to light itself on fire:

  • dry materials that go unattended;
  • biological activity;
  • dry pockets of debris among a non-uniform mix of materials;
  • large, well-insulated piles;
  • limited air flow;
  • poor moisture distribution due to neglect or oversight in monitoring; and
  • unknown temperature within the pile, and time for the temperature to build up.

WARNING: While self-incineration of compost is possible, compost piles probably catch fire more often from ordinary sources, such as lit cigarettes or electrical mishaps. Also, gardeners who use ash from incinerated trash or the fireplace sometimes neglect to make sure that the ash has cooled sufficiently before adding it to the compost pile.

Inspectors can offer their clients the following tips to help avoid compost fires:

  • Assure adequate ventilation of the pile to release heat. Turn the pile or use a mechanical aeration system to ensure ventilation. Narrow, short piles generally have adequate ventilation.
  • Do not turn a pile that is smoldering, as the sudden infusion of oxygen can cause the pile to erupt into flames.
  • Do not let the pile get too dry. The University of Missouri states, “Organic material can ignite spontaneously due to biological activity at moisture contents between 26 to 46% moisture, if the temperature exceeds 200° F.”
  • Monitor the pile’s temperature, focusing on the hottest spot in the pile. Use a thermometer long enough to reach the center of the pile. Do not let the pile get too hot. If the temperature of the pile exceeds 160° F, reduce the temperature through the following methods:
    • reduce the size of the pile;
    • add water to 55% moisture;
    • mix in coarse, bulky material, such as wood chips; and
    • do not pile compost next to buildings or any flammable structures, as fire can spread easily.

Compost-Friendly Pests Worms are often added to compost piles to aid in the breakdown of organic matter. But if the compost piles are not constructed and maintained properly, they have the tendency to attract unwanted pests. Flies, termites and beetles are attracted to the smell of decay, and they, in turn, will attract larger predatory critters to the pile. Use the following pest-control tips:

  • Do not compost eggs, meat, oils, bones, cheese or fats. Compost piles should be “vegetarian.”
  • Bury the compost with soil or leaves to contain the smell and to aid with the biodegrading process.
  • If using a portable composter, make sure it has a cover that will discourage the entry of pests and animals.
  • Beware that enclosed compost piles can overheat and create high levels of dangerous gasses, such as methane, so be sure to rotate the container or till the pile daily.
  • Do not place compost near a building. In addition to the fire concerns, compost placed adjacent to buildings can promote infestation.

NOTE: These practices can also mitigate the foul smells that can plague compost piles. In summary, the benefits of compost piles can be quickly eclipsed by health hazards and nuisances if they are not designed correctly and maintained properly.

Fire Prevention 52: Spontaneous Combustion–Fact or Fiction?

A pile of dirty rags spontaneously catches fire in someone’s garage. Round hay bales stored in a farmer’s field go up in flames. Smoke can be seen coming from a city’s large compost pile. All three scenarios occur without the addition of flame or heat. Are these stories fact or fiction?

Spontaneous combustion or spontaneous ignition, as it is often called, is the occurrence of fire without the application of an external heat source. Due to chemical, biological, or physical processes, combustible materials self-heat to a temperature high enough for ignition to occur. According to the National Fire Protection Association (NFPA), an estimated 14,070 fires occur annually from spontaneous combustion.

Think something like this cannot occur in your park or home? Think again. A few months ago, an NPS fire crew in the San Francisco Bay area was conducting routine maintenance of their hand tools. One of their tasks involved applying linseed oil with a rag to their tool handles. Even though the crew took precautions with the rag, placing it outside to dry flat on a workbench at the end of the day, the rag caught fire, started the workbench on fire, and spread up the wall of an attached shed. Fortunately, an employee saw the smoke and the fire was extinguished before significant damage occurred.

Rags and towels soaked with oils, including cooking oils; hot laundry left in piles; large compost, mulch, manure, and leaf piles; and moist baled hay can spontaneously combust in the right conditions. Avoid this type of fire by following a few simple and proven tips:

  • Store piles of hay, compost, mulch, manure, and leaves away from buildings, in case a fire occurs, and keep the piles small to allow for the circulation of air and the dissipation of heat.
  • Work groups or businesses using large quantities of oily rags should dispose of those rags in an OSHA-approved container to await pickup by an industrial cleaning company.
  • If you’re working on a project at home, spread the soiled rags in a single layer on concrete to prevent the buildup of heat and allow the rags to become hard and brittle. Place the rags out of direct sunlight and secure the corners to prevent movement by wind.
  • Hay should be completely dry before baling and moving to a storage facility. Ensure that the facility is well ventilated.
  • Dial 911 or your local fire emergency number if your hay bales or mulch, leaf, manure, or compost pile is emitting smoke. The combustible material will need to be spread out to dissipate the rising heat, but the introduction of oxygen can result in an immediate fire. Firefighters should be standing by onsite.

Fire Info for You

Learn more about spontaneous combustion by viewing an ABC News video with Diane Sawyer, which demonstrates how a linseed oil-soaked rag spontaneously combusted.

Discover more about the NPS fire mentioned above by reading a Facilitated Learning Analysis of the incident.

Increase your knowledge of spontaneous combustion and its occurrences by reading the NFPA report titled, “Fires Caused by Spontaneous Combustion or Chemical Reaction Fact Sheet.”

Park Structural Fire Coordinators
Keep watch for unsafe storage and disposal of oily rags in concession facilities and maintenance buildings.

Park Leadership
Prepare standard operating procedures (SOPs) that outline the proper disposal of oily rags and ensure that the proper storage containers are available for use.

Take Action

Walk through your home or workplace and identify spontaneous combustion risks. Immediately remove the risk by taking appropriate measures, such as removing the pile of rags in your garage, or inform management of the fire hazard. Contact the park’s safety officer or park structural fire coordinator for approved disposal options.

NPS Fire Facts

According to the U.S. Fire Administration, spontaneous fires are one of the leading causes of fires in agricultural storage facilities, such as barns, silos, and stables. Many NPS sites interpret cultural resources and operate historic farming operations that use or generate materials that can spontaneously combust, such as manure piles or soiled straw. Structures storing hay and grain are at risk for fire, so proper storage of these materials is essential.

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