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I went to stir my compost pile the other day, and to my surprise, when I dug into the center of the pile, I found that it was hot. As in, really hot. Once I removed the top layer steam was literally rising off of it. I didn't have a thermometer handy, but I'm estimating that the temperature was at least 50 C (120 F) based on how hot it was to the touch.

Is this normal? Is it good/bad? How much hotter could my compost heap get?

(Note: I have a big compost heap, due to having a big yard that creates a lot of yard waste.)

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3 Answers 3

up vote 27 down vote accepted

In sum of the below; you will want to hang around 140 - 160° F; that way you will be above the minimum range of thermophobic parasites and below the temperature that affects thermophilic bacteria (who are doing all the work). How hot it can get depends on your set up and climate and other variables, but managing it is a pretty active and important aspect of the process.

From some information from Washington State University-

I am not sure if you are worried about the temperature, however in terms of safety bear in mind the following:

Eggs of parasites, cysts and flies have survived in compost stacks for days when the temperature in the interior of the stack is around 135° F. Since a higher temperature can be readily maintained during a large part of the active composting period, all the material should be subjected to a temperature of at least 150° F for safety.

In terms of the temperature dynamics of the compost:

Experience shows that turning to release the heat of compost piles, which have become so hot (170°-180° F.) that bacterial activity is inhibited, is not very effective. When the material is actively decomposing, the temperature, which falls slightly during turning, will return to the previous level in two or three hours. Also, it is impossible to bring about any significant drop in temperature by watering the material without water logging the mass... Variations in moisture content between 30% and 75% have little effect on the maximum temperature in the interior of the pile. The initial temperature rises a little more rapidly when the moisture content is 30% to 50% than when it is 70%... Deeper piles caused higher temperatures and better temperature distribution, and subject more material to a high temperature at any one time. Hence, the actual mass of the material evolving heat is important in providing adequately high temperatures*.

[*See the link for an explanation for C:N Ratio (Carbon:Nitrogen) and it's impact on temperature]

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I know this is an old post, but no one has answered the title question. The answer is, too hot; as in under certain circumstances organic material can auto ignite, almost always found in hay/straw piles. The limiting factors for compost temp are: The Net r-value from the most central part of the pile, availability of oxygen and water.

A garden compost pile will generally never get too hot, as they are generally modest in size (less than 2m^3)

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A woman caught her hanging planters on fire here a week or so ago. She'd achieved the right mix of fertilizer, bagged potting soil and peat-moss for them to spontaneously combust. Fortunately, somebody saw the smoke and flame and so the worst damage was charred planting boxes and a scorched porch ceiling. Keep your compost damp... And your hay dry... I remember a few barn burners during my childhood. –  Fiasco Labs Jun 16 '13 at 15:07
    
wow that is amazing, pretty tough to do in a small area. I saw a barn burn, maybe 10 years ago, scary stuff. –  Grady Player Jun 16 '13 at 22:56
    
Yep, that's what I thought. The off-duty police officer saw it as he was driving home and thought it was a first for the area. Then as a mechanic acquaintance once commented, there are people who can break a ball-bearing with a broomstick. –  Fiasco Labs Jun 16 '13 at 23:21
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on a total tangent, my mom called me 2 days ago because her car was "on fire", when I got there her makeup mirror had focused the sun on he glove box, burning a hole in the plastic... so there are just strange combinations of normally innocuous things than can conspire to get you. –  Grady Player Jun 16 '13 at 23:54
    
We had an experimental airplane with a bubble canopy burn a track across the upholstery once. Engineering failure when you don't take lensing into account. On the makeup mirror, with my luck there would have been something really flammable in the glove box. –  Fiasco Labs Jun 17 '13 at 0:39

Also make sure your pile is not too big. Average size of 4x4x4 foot (1 cubic meter) is preferred. This makes it easier to turn over. It also makes it easier to dry when too wet.

You can make multiple piles next to each other in different stages. 2 Piles is good, 3 is better.

You should be quite happy with the temperature. Try to keep it up for a couple of days. After that split the most and mix with new green and brown material.

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