Composting is the natural process of recycling organic materials like food scraps, leaves, grass clippings, and more into a rich, fertile soil amendment. When done correctly, composting relies on beneficial microorganisms to break down organic matter and produce usable compost. One of the most important factors for successful composting is maintaining the ideal temperature range. This allows the microbes to thrive and do their work efficiently.
What is the ideal temperature for composting? Experts recommend keeping your compost pile between 135-160°F (57-71°C). This “thermophilic” temperature range creates the optimal environment for heat-loving bacteria and fungi to rapidly decompose the materials. Lower temperatures under 135°F favor mesophilic organisms that break down materials much more slowly. Temperatures over 160°F will kill off too much of the microbial life.
The Science of Compost Temperatures
Composting can be described as a controlled version of the natural process of organic matter decomposition. Leaves fall in the forest, food scraps get buried, plants die and breakdown, all helping to sustain the circle of life. The key to fast, efficient composting is managing environmental conditions to maximize biological activity.
The main drivers of the composting process are oxygen, moisture, particle size, and temperature. These factors determine how quickly and thoroughly the organic materials are broken down. Getting the recipe right supports a boom in the compost microbial population and their metabolism.
As microorganisms digest and decompose the carbon and nitrogen-rich materials, their biological activity generates heat. This heat buildup drives compost temperatures up.
Heat is both a byproduct of the composting process and an essential ingredient for speeding it up. Higher temperatures accelerate the chemical reactions and metabolic rates of the microbes. Heat also sterilizes the pile by killing weed seeds, insect eggs and larvae, and plant or animal pathogens. The temperature trends in a compost pile provide important clues about what’s happening on the inside.
Compost goes through four primary phases during the decompose process:
- Mesophilic Phase – Initial decomposition carried out by mesophilic organisms at ambient temperatures up to 105°F. Materials begin to heat up.
- Thermophilic Phase – Temperatures climb to the ideal range of 135-160°F. Heat-loving thermophilic bacteria take over the process. Most active composting occurs here.
- Cooling Phase – The readily digestible materials are consumed, temperatures start to decline, and microbial activity drops off.
- Curing Phase – Finished compost rests and matures at ambient temperatures. Beneficial fungi may continue to break down resistant organic matter like wood chips or corn cobs over longer periods.
Monitoring temperatures helps determine what phase the compost is in and if any adjustments need to be made to maintain optimal conditions.
The Ideal Compost Temperature Range
Most experts agree that 135-160°F is the “sweet spot” for rapid, efficient composting. Some key benefits of maintaining pile temperatures in this zone:
- Kills most weed seeds, fly larvae, and plant pathogens – Most are destroyed at sustained temperatures above 130°F. Avoiding recontamination is critical if using compost near sensitive crops.
- Optimal microbial activity – Mesophilic bacteria thrive up to 105°F. Thermophilic bacteria and fungi take over at 135-160°F to aggressively decompose materials. Temperatures above 160°F kill off too much life.
- Fastest decomposition – Chemical reaction rates double with each 18°F increase in temperature. Heat-loving microbes generate more metabolic heat to rapidly break down organic matter.
- Reduced pile odor and flies – The high temperatures destroy smelly anaerobic bacteria and fly breeding sites. However, ammonia odors can still persist.
- Reduced moisture loss – Higher temperatures mean faster evaporation. But a hot compost pile will retain more moisture than one left cold.
- Kill weed seeds and plant pathogens – Temperatures above 130°F kill most plant disease organisms and weed seeds that could germinate after being spread with finished compost. Using thermophilic heat is an important sanitization step.
- Reduced pile turnover time – Active hot compost can breakdown materials into usable finished compost in as little as 3-6 weeks with the right conditions. Cold piles may take 6 months to over a year.
Maintaining your compost pile in the 135-160°F thermophilic temperature range will provide the fastest, most efficient composting results.
How to Monitor Compost Temperatures
The only way to know if your compost pile is heating up properly is to monitor temperatures. You have two main options:
Compost thermometer
A compost thermometer is specially designed with an extra-long probe to easily insert deep into the center of the pile. It allows you to get accurate temperature readings without disturbing the contents. Look for a large dial display or digital readout to make it easy to read.
To use, simply plunge the probe into the center of pile in several locations. Don’t just take a single reading, as you want to find the hottest spot. Maximum pile temperatures can exceed 160°F in the very center while the outer layers are still under 100°F. Monitor temperatures daily when first building a new pile. Once it heats up, record readings weekly or as needed to track progress.
Multi-purpose food thermometer
Any high-temperature food thermometer designed for meat, candy, bread, cheese making etc. can also work for compost duty. The probe needs to be long enough to reach at least 12 inches into a pile or bin. Make sure it can read up to 220°F.
The probe tip is more fragile than a rugged compost thermometer so you’ll need to be careful inserting it. But it can provide accurate enough temperature measurements. Disinfect the probe with rubbing alcohol before using with food again.
Digital thermometers are much easier to read quickly versus analog dial displays. Look for a large backlit screen and auto-off to conserve battery life.
How Hot is Hot Enough?
As mentioned, you want to maintain your compost pile in the 135-160°F optimal range. Some key temperatures guidelines:
- Above 160°F – Danger zone. Beneficial microbes start to die off. No need to keep pile this hot. Turn or moisten to cool it down.
- 150-160°F – Maximal microbial activity range. Watch temperatures closely so they don’t climb higher.
- 135-150°F – Ideal zone for fast, efficient composting. Most experts recommend targeting this range.
- 120-135°F – Still Beneficial. Process is slowed but composting will happen.
- Under 120°F – Pile likely needs adjustment. Won’t compost efficiently without more heat.
- Under 100°F – Pile is too cold and composting stalled. Mesophilic bacteria work too slowly. Intervention required.
- Over 175°F – Danger zone. Most microbial life is destroyed. The pile is sterile and no longer decomposing.
Use these target numbers to gauge your pile temperatures and make any needed corrections.
How to Increase Compost Temperatures
If your compost pile is staying under 120°F and won’t heat up, it needs a booster shot. Here are some ways to increase temperatures:
- Add nitrogen-rich green materials – Fresh grass clippings, fruit and vegetable scraps, green plant prunings, livestock manure, blood meal, and high-nitrogen bat or bird guano will help drive up temperatures. Don’t overload greens, as too much nitrogen can also hinder the process.
- Reduce pile size – A larger compost volume will have more insulation to trap heat. Scale down piles to 3 cubic feet (1 cubic meter) or less to help them heat up. Smaller batches will also compost faster.
- Cover and insulate the pile – Use a tarp, wool blanket, burlap bags or thick layer of straw to retain heat. The more a pile is insulated, the easier it will be to get up to temperature.
- Turn and aerate the pile – Turning mixes in fresh oxygen to fuel microbial activity. Use a pitchfork or compost turner to gently lift and blend the pile, taking care not to over-disturb the contents. Turn every 3-5 days to boost airflow.
- Add moisture – Microbes function best with about 50-60% moisture, similar to a wrung-out sponge. Use a moisture probe or squeeze test to check for proper wetness. Add water while turning if needed, but avoid soaking the pile.
- Monitor temperatures consistently – Check the core temperatures daily and make adjustments as soon as you see it dropping under 120°F. The sooner you intervene, the faster the compost will reheat.
With the right inputs and active management, you can get your compost pile heating back up into the ideal range within a few days. Then adjust factors like moisture, aeration, and pile size to maintain optimal conditions.
How to Cool Down a Hot Compost Pile
While the typical challenge is keeping compost hot enough, sometimes piles can overheat. If temperatures climb above 165°F, it’s time to take action to cool things down. Here’s how:
- Turn or mix the hot pile – Turning folds in cooler outer layers and releases trapped heat. Mix thoroughly but gently to redistribute contents and temperature zones.
- Add coarse dry materials – Folding in dried leaves, straw, corn stalks, sawdust, or wood chips will provide aeration and absorb excess moisture that may be driving the heat.
- Reduce pile size – Compost generates more heat in larger volumes. Divide into smaller mounds to encourage cooling. This also exposes more surface area.
- Allow heat to radiate – Removing any covers will let heat dissipate into the air more quickly. You can also use a compost thermometer to poke vent holes.
- Light misting of water – A very light sprinkling of water while mixing can create an evaporative cooling effect without over-wetting.
- Monitor temperatures diligently – Check the core temp at least daily to determine if cooling efforts are working or if more modification is needed.
Once temperatures drop back down closer to 150°F, you can stop managing so intensively. But monitor regularly to ensure it doesn’t climb back up. The goal is keeping the “sweet spot” of 135-160°F.
Getting Proper Insulation & Airflow
Two key factors for building and maintaining a hot compost pile are proper insulation to contain heat and sufficient airflow to feed oxygen to the microbes. This requires finding the right balance.
To hold in heat, the pile or compost bin needs cladding with insulating materials like:
- Straw or dried grass clippings
- Leaves or shredded newspaper/cardboard
- Old rugs, blankets, or towels
- Woodchips or sawdust
- Insulating foam sheets
- Soil or earthen materials
You don’t want to cut off airflow completely, so only cover loosely or insulate the bin walls. Turning the pile also brings in fresh oxygen while letting out excess heat.
For aeration, you can place:
- Wood chips, twigs, corn cobs – Anything that doesn’t compact down.
- Hardware cloth or chicken wire tubes in the center of the pile.
- PVC pipes with holes drilled in them.
- Aeration tubes available for compost bins.
Turning with a shovel or pitchfork is the easiest way to mix in air. How often you turn depends on how hot the pile gets. For maximum heat, turn every 2-3 days. For slower composting, every 7-10 days is sufficient. Monitor with a compost thermometer and adjust as needed.
Troubleshooting Temperature Problems
Here are some common compost temperature issues and how to fix them:
Problem: Pile under 100°F and won’t heat up.
Solutions:
- Add fresh green materials – nitrogen sources like grass clippings provide energy.
- Chop ingredients into smaller pieces – gives microbes more surface area to attack.
- Add manure or nitrogen fertilizer – provides missing nutrient balance.
- Turn and fluff the pile – improves airflow for oxygen.
- Moisten dry materials – aim for 40-60% moisture.
- Reduce pile size – smaller amounts retain heat better.
- Insulate with straw or a tarp – prevents heat escaping.
- Check pH – acidic conditions below 6.5 can inhibit microbial growth.
- Be patient – it can take 1-2 weeks to start heating up.
Problem: Pile temperature spikes over 170°F.
Solutions:
- Turn the pile to release heat and moisture.
- Add coarse, dry materials like leaves or straw to absorb excess moisture.
- Divide into smaller piles to dissipate heat.
- Stop adding nitrogen-rich greens that fuel heating.
- Leave cover off to let heat radiate into the air.
- Light water misting can help evaporative cooling.
- Monitor temperatures daily until back in ideal range.
Problem: Temperature fluctuates up and down.
Solutions:
- Ensure proper moisture levels – too wet or dry inhibits activity.
- Turn and fluff regularly – every 3-5 days to maintain airflow.
- Eliminate pile dead zones – mix thoroughly when turning.
- Protect pile from heavy rain – prevents oversaturation.
- Increase insulation around bin – contain heat and moisture.
- Check ingredients ratio – too many greens or browns can cause issues.
- Monitor temperatures diligently – keep pile optimized.
With close observation and quick corrective actions, you can troubleshoot any compost temperature issues that arise.
Conclusion
Maintaining your compost pile at the ideal temperature range of 135-160°F is key to fast, efficient decomposing of organic waste. This thermophilic zone supports peak microbial activity to rapidly break down materials while killing pathogens and weed seeds. Monitor temperatures with a long-stem thermometer and make adjustments as needed to airflow, moisture, pile size, insulation, and nitrogen balance. Turning the compost and testing internal temperatures is vital for keeping your compost hot and actively processing. While a properly constructed compost pile will generate its heat, you need to manage conditions to sustain temperatures in the optimal zone. With the right balance of science and art, you can produce finished compost more quickly. Keep an eye on the temperature trends and enjoy the heat!