Why Won’t My Compost Get Hot?

A compost pile that won’t heat up is the most common disappointment to the new composter.

The leaves are raked. The food waste is collected. The pitchfork is ready.

After mixing them together, our intrepid composter impatiently waits a few days before opening the tumbler or heading out to the leaf pile. Expecting temps of 130°F, she instead gets….nothing.

No rise in temperature. No activity. And a greatly reduced expectation of finished compost by spring.

This article will cover the basics of composting and will explore the most common reasons why compost will not heat up. 

The 4 Requirements for a Hot Compost Pile

A compost pile needs oxygen, water, materials with the correct C:N ratio and enough volume to trap heat

I am not a gear head, but someone once explained to me that a motor won’t start because it’s lacking fuel, air, or a spark.

A fire needs fuel because it needs an energy source with locked up energy to release in a heat form.

It needs air because the fuel source needs to interact with oxygen in order to burn. It also needs a spark or an ignition source to get the combustion party started.

Troubleshooting the lawnmower became a little less daunting after this lesson.

Composting is similar.

It needs air, heat, and a proper fuel source. It also needs water.

1) Air: Oxygen is Key to Composting

As composting guru – and my friend! – Peter Moon writes in this wonderful post at O2Compost.com, the secret of composting is oxygen.

Now Peter’s wheelhouse is aerated static pile composting which I am using here at the Urban Worm Company. We compost spent brewer’s grain and wood chips to prepare the material for our Michigan SoilWorks CFT vermicomposter.

With aerated static pile or “ASP” composting, air is forced into the pile by way of a perforated pipe to deliver the air. It’s a pretty nifty, labor-saving setup.

Turning a Compost Pile

But most composters turn their piles for the purpose of introducing – and reintroducing – oxygen to their material. Composting should be an aerobic process and the microbes responsible for heating up a pile consume it very quickly.

If your pile is not heating up, it may be that it is lacking oxygen.

Time to get out the pitchfork and give it a turn!

2) Heat: Somebody Has To Start the Party

Hot composting is a process that should build upon itself. The heat initially created by mesophilic microbes – which prefer moderate temperatures – attracts the proliferation of thermophilic – or heat-loving – microbes.

Reotemp Compost Thermometer
Temperatures above 130°F in our 2-cubic yard composter

The boost in activity of thermophilic microbes creates more heat, which makes conditions even more ripe for heat-loving microbes. This thermophilic activity takes temperatures from a moderate but active 100°F to a too-hot-to-touch 160°F.

It’s one thing to create this heat, but quite another to trap it.

A hot composting pile needs to be at least 3 feet x 3 feet x 3 feet in order to trap enough heat in its core to get the thermophilic party started. Otherwise, the heat simply escapes too easily.

And larger is even better as long as it’s within your physical capacity to keep turning it.

A compost pile that won’t heat up is often too shallow or otherwise lacking in volume to trap heat. This explains the frequent frustration with small compost tumblers.

It also explains why tiny countertop composters are not actually composters; they are just stylish ways for your food to rot in your kitchen before you run it out to the pile or tumbler.

3) Carbon-to-Nitrogen Ratio: The Fuel for Compost

If you’ve researched composting to any extent, you’ve surely come across the term carbon-to-nitrogen ratio, or “C:N.”

This is a measure of the mass of carbon to the mass of nitrogen in a given substrate.

Microbes, like most organisms, consume these two elements in a 30-to-1 ratio, so you’ll want to create a pile with 30 parts of a carbon-rich brown material to one part nitrogen-rich green material.

Too much carbon or brown stuff, the pile will not heat up and decomposition will be prolonged.

But if you have too little carbon, your pile will may get hot for awhile but will release too much nitrogen in the form of ammonia which will not be a pleasant smell.

This next part is hugely important to understanding C:N.

Greens are Still Mostly Carbon!

Every organic substance is mostly carbon. Even something like chicken manure, which has about the lowest relative carbon content any material, has a C:N of 6:1. In other words, this nitrogen-rich green material has six times more carbon than nitrogen.

With this in mind, breezy guidance like “add one part brown, one part green” is not sufficient.

Such guidance typically refers to volume, not mass. And it totally disregards bulk density which is the amount of mass per unit of volume. So someone adding a 20 gallons of a “brown” like hardwood chips at 550:1 C:N is way different than someone adding the same volume of dry loose leaves with a measly 50:1 C:N.

The volume is the same, but the amount of carbon you’re adding to the mix is wildly different because of the different C:N ratios and bulk density.

So I would use one of the many compost calculators on the web and I’d be thrilled if you started with mine!

It allows you to choose from a range of dozens of greens and browns and is super simple to use, giving you a running C:N as you add ingredients to the mix.

4) Moisture: Without It, There’s No Composting

A compost pile needs to be moist without being wet.

At moisture levels above 65%, liquid begins to occupy the pore spaces where oxygen used to live. Once the oxygen is consumed in the liquid, there is no path for new oxygen to enter the mixture. The result is an anaerobic and likely odorous pile.

At moisture levels below 50%, composting begins to stop and you’ll need to add water to the mix.

If your pile gets so dry that it becomes hydrophobic, it will actually not absorb moisture at all. Your pile will then need to be torn down, wetted down, and built back up again.

This article will help you understand how to measure the moisture content in your compost! It’s meant more for worm composters but the principles are still the same!

If you’ve recently turned your pile and you know your carbon-to-nitrogen ratio is good, then first thing I would check is the moisture level.

If your fresh material falls apart in your hand and there’s no way you could squeeze a drop of moisture out of it, you may need to add water.

Summary: Tips to Maintaining a Hot Compost Pile

Composting is cooking, not baking. It’s not so demanding in terms of precision that it requires minute attention to detail.

So while the guidance above should help you keep a hot pile, here are a few more tips to keep your pile temps elevated.

Keep Exposed to Sunlight

If composting at home, do what you can to keep your pile or tumbler in maximum sunlight, namely by keeping it in a south-facing location for you Northern Hemisphere dwellers.

I realize you’ve got to be a good neighbor so if the front lawn in your gated community gets more sunlight than anywhere else on your property, I’d understand if you didn’t choose to put it there!

Add Compost Tea

A healthy addition of compost tea can give your pile a boost of microbial activity. The beneficial bacteria that exist in the billions in compost tea can get the microbial party started in your pile!

Keep Exposed to Rain

A compost pile is thirsty, so let nature help you. Keeping your compost pile in an area where rain can get to it easily will provide a low-cost way to keep your pile moistened.

Placing your pile under a tree will protect it from both rain and sunlight. The is protection we want to avoid!

Keep Adding Fresh Material

You know the old adage. If at first you don’t succeed, add a crap ton of coffee grounds! A normal household compost pile is unlikely to have enough green material to get a 1 cubic yard pile fired up.

You may benefit from outside help like manure from a local farmer. If you’re in the ‘burbs and don’t care to put a bin full of manure in the family truckster, you might find a copious supply of spent – but nitrogen-rich! – coffee grounds from Starbucks or your local coffee shop.

Higher end convenience stores (like WaWa) in the northeast that feature 19 different kinds of coffee are also excellent sources.

Only Change One Variable At a Time

When we get a temperature crash in our aerated static pile composter, we are careful to only change one variable.

In other words, if we think we need more water but maybe less air in the pile, we make only one change and then let it sit for a few days to see if we get a temperature rise. This helps us dial in on the true cause of our issues.

9 thoughts on “Why Won’t My Compost Get Hot?

  1. I have compost that I am turning regularly, it seems to be breaking down, no odors, volume has decreased, but it still never gets hot. I am not adding to it but have a second bin I’ve started. The bins are 4x4x4. The first bin was almost full when I quit adding and now is less than half. If it looks good and earthy and it never got hot enough, what are the downsides to adding it into my garden (raised beds and around fruit trees and in planting holes).

    1. Hi Janna! Sounds like you’ve succeeded in slower, cold composting. I don’t think there’s any issue with using this material in your garden! If the pile didn’t heat up, then you may find “volunteers” sprouting if you didn’t acheove high enough temperatures to kill the seeds.

  2. Pingback: To Infinitea and Beyond: 12 Alternate Uses For Compost Tea
  3. Thank you for a great article!
    I bought one of the small rotary composters mentioned in the article (19-gallon capacity in each of the two tumbling vessels), and it has never gotten hot. After a year, there are still recognizable leaves, stems and small sticks in the rotary composter. I balanced the C:N, applied two different inoculants, used pH’d water treated for chloramines and maintained the one drop moisture squeezed from the fist, and it just never took off. I was concerned that there were manufacturing chemicals left on the plastic panels. After reading this article, I don’t think it possible to obtain a hot compost in a small rotary composter. Do you agree?

  4. Aww, Wawa! I grew up in the Philly area but have since moved South, and I admit that although I love where we live now, I do miss that Wawa coffee. Thanks for the great tips and the reminder of home 🙂

  5. I have a 3’ x 3’ x 4’ bin that doesn’t get hot. It’s composed of loose leaves and various vegetables. I’ve added coffee grounds, and I grind lemon and orange peels before I add them. I turn it twice a week and periodically add water but still no heat. I cover it with a large 10’ x 12’ tarp. Could that cover be the reason it’s not heating up even though I turn it?

    1. Hi Robert,
      The tarp is not why it’s not heating it up. If I had to guess, it’s likely a lack of nitrogen-rich green materials. You need more than you might think.

    2. Hey Roger. The lack of heat is due to low microbial action. If you want the pile to compost thermophilically it likely needs more nitrogen. Try adding some manure (cow, rabbit, alpaca, chicken) for a high nitrogen source. Add 10% of the volume of what you currently have.

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