Compost turns chunky mulch into rich, crumbly soil. It’s one of the best ways to cycle nutrients in your garden—weeds, kitchen scraps, and garden clippings decompose into valuable inputs for the soil. The decomposition process invites worms, bacteria, and fungi that add disease resistance, water retention, and nutrients to the dirt.
A hot compost pile is the quickest way to turn raw materials into black, crumbly soil. You can have compost in as little as two weeks! The microorganisms generate heat as they work to eat your scraps. The heat helps speed up mulch breakdown and kill diseases or weed seeds.
If you recently made a pile it may take a few days to heat up. There are a few tricks you can use to boost heat levels and effectively manage your compost. The question remains, how hot should our compost piles be? Let’s dive into hot and cold compost, and how to keep your pile performing at its best.
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Keep track of soil and worm bin temperature accurately with the Urban Worm Thermometer. The stainless steel stem penetrates up to 5-inches into soil to give you the most accurate reading. And the green, yellow, and red tick marks let you know when you’re in the optimal zone for your soil microbes and worms.
The Short Answer
Compost should have different temperatures depending on its life cycle. Fresh piles are the hottest, while mature, curing ones cool off. As microorganisms, worms, and nematodes eat up raw waste, they produce tons of heat. It dissipates after they consume all the waste, and then you can harvest the pile and use its compost in your garden.
Decomposition occurs at its greatest between 104-140°F (40-60°C). As waste runs out, temperatures slowly decline below 84°F (29°C). Use a soil thermometer to measure the piles, or watch for visible steam curling out of freshly turned compost.
The Long Answer
Compost is complex! How hot your pile gets depends on particle size, aeration, moisture levels, and browns-to-greens ratios. Let’s dive in.
Hot Compost Is Hot!
It’s magical that you can stack dead leaves, banana peels, and plant clippings to create fresh soil. Hot compost piles are the best way to do this, as they create hospitable conditions for the good microorganisms you want in your soil. Take proper care of your pile, and it’ll take excellent care of your plants.
Hot piles need a proper ratio of browns to greens, usually around 2:1 or 3:1—this causes a 30:1 ratio of carbon to nitrogen in most cases, which is ideal for the method. Browns are dry plant material like dead leaves, thin twigs, chemical-free paper waste, and straw. Greens are fresh material like kitchen scraps, fleshy plants, grass clippings, and farm animal manure.
Once you’ve mixed the materials in a pile, water it so it’s 50% moist. Grasp a clump and squeeze it; it should feel like a wrung-out sponge. If no water comes out, the pile needs more irrigation. If water comes out without squeezing, it’s wet enough and should dry. Turning piles helps encourage quicker drying if they’re too wet.
Weed Seeds and Pathogens
Keeping compost hot ensures weed seeds and plant pathogens die during decomposition. Healthy bacteria and fungi outcompete them and create a rich home for bugs, worms, and algae. Weed seeds can’t survive the heat, and they die, too.
Certain temperatures destroy these unwanted additions more easily than others. Ensure your pile stays above 104°F (40°C) for five days. During these five days, you’ll want your inputs to exceed 131°F (55°C) for four hours. Most diseases, pests, and seeds wither at these temperatures.
Certain weeds like mint, bamboo, kudzu, or English ivy survive these temperatures. Keep noxious invasive plants out, and they won’t get the chance to take over. Most annual or perennial weeds can’t survive, and you can put them in worry-free. If you’re curious whether or not a weed will die, test a small piece of one in a hot pile. If it dies, you can safely assume it’s valuable green material.
Cold Compost Is Cool
If you’re making cold (passive) compost, you needn’t worry about its temperature. This method utilizes gradual decomposition over many months with little oxygen. It takes advantage of similar natural processes as hot compost, albeit at a much slower rate.
The one disadvantage is weed seeds and pathogens don’t die in cold piles. There’s a bright side though; allowing these things may not be all that harmful. When weeds sprout, you can pull them up and lay them on the soil. They’ll decompose with time, adding cover and nutrients where weeds used to grow.
Pathogens are a bit different—cold piles may not kill them. I still put some diseased leaves in them as an immunity boost. The good bacteria, fungi, and archaea compete with diseases for space and nutrients. Then, you spread the decomposed material, and it helps your plants resist the pathogens that died in it. However, more rampant diseases may proliferate. Use caution with this method if you have problems with garden diseases.
Boost The Heat
A cold pile heats up quickly with a few techniques. Before you make your pile, chop all the mulch pieces into smaller sizes. Small particles are easier for composting organisms to eat, and they facilitate faster decomposition.
You can boost heat in existing piles by manipulating water and airflow levels or by adding a protein-rich amendment. If piles are cold and dry, they need water and proper turning. Using a broad fork or pitchfork, stab the decaying mulch and turn it. Bring particles on the outside to the inside, then rake up the area to keep it tidy.
Slow-working piles may need a nutrient boost. Amendments like grass clippings, oatmeal, soy meal, and alfalfa pellets provide a quick decaying source of nutrients like protein and nitrogen. Incorporate them throughout, and avoid adding new waste for a few weeks.
Can It Get Too Warm?
Extreme heat kills good and bad microbes, meaning your amendment will be devoid of life when you add it to your garden beds. Temperatures above 140-149°F (60-65°C) are too high for sensitive bacteria, fungi, and archaea.
The best way to lower temperatures is by turning. Daily turning with a pitchfork ensures the piles stay hot but not too hot. It also keeps a steady flow of oxygen reaching the beneficial microbes, allowing them to eat rapidly the raw mulch and kitchen scraps.
Interestingly enough, turning piles also can boost temperatures. It allows microbes to reach new, undecayed particles they haven’t touched, and they feed in a frenzy. When you cycle a pile, you introduce airflow and boost microbial activity; these actions balance each other out, resulting in an optimum composting temperature.
Seasons Affect Temperature
The final uncontrollable factor is the weather. Cool, wet weather initiates cold composting to occur. Decay still happens, just at a much slower rate than when sunshine and warm temperatures are present.
Keep piles at least three feet wide, long, and tall. Larger ones trap heat inside, and they decay better during winter weather than small ones. Even in the best of situations, most piles won’t readily decompose until the following spring through summer.
Start fresh piles in early spring for a midsummer harvest or in early summer for a fall harvest. You may also keep a pile going continuously, adding to it as you generate waste. Then, when you want to harvest, dig to the bottom of the pile and use the crumbly amendment.
No matter the method or style of composting, you can’t go wrong when generating upcycling waste. Compost is an invaluable natural amendment. When you make your own, you funnel waste away from landfills. This budget-conscious action also helps your ornamentals and crops thrive with valuable microbes, nutrients, and bugs.
