I finally got out in the garden this past weekend, and besides the bed rebuilds, I turned and watered one of the compost piles. I dumped out it onto a tarp, gave it a thorough turning, scooped it back into the wire bin, adding a little water.

I took the temperature and it was about 60 Degrees Fahrenheit. The next day, I took the temperature again, 100 degrees!

Now the compost thermometer looks like a really long meat thermometer because well, it’s a really long thermometer. About a foot long.  Anyway, when I got home tonight, I approached the pile, thermometer in hand.

“Excuse me Mr. Heap, you’re going to feel some discomfort.”

With that, I stuck it in the heap and watched as the needle swept, not crept, past 100, 110, 120, finally coming to a rest just north of 140 Fahrenheit!

Wow! It’s amazing what a little air and water can do to a winter stagnant compost pile.

The fact that it is heating up is pretty cool, but what’s happening in the pile that causes this dramatic change?  So here is an explanation of what’s going on in the pile. It’s an excerpt of a much longer article on Composting I’m working on, but it seemed right to post it here as I am so impressed by the dramatic reaction right outside the kitchen door. It’s a little geeky as there is science involved, but I dig that. Pun intended.

Put simply, composting works because bacteria break down organic material in multiple stages. I’ll compare it to a multi stage rocket. During the process,   the bacteria are activated by the carbon, and they use oxygen to break it down into usable forms – leaving heat (an exothermic reaction)and carbon dioxide (bacterial flatulence?) in their wake.  Bacteria, like plants are not created equal. Just like plants, different bacteria have conditions that are most favorable for them to thrive. Composting takes advantage of these differences to break down your organic waste to usable humus.

In the first stage, mesophilic bacteria (literally “middle lovers” as they survive and thrive at a temperature between 70F – 100F (21C –  38C))  work to break down the easily digested materials. Their reactions are the ignition that causes the compost pile to heat up before handing off to the second stage bacteria, the heat-lovers.

After the first stage has achieved lift-off, the second stage begins. Thermophilic bacteria thrive at hotter temperatures ranging from 113 to 160 degrees Fahrenheit (45C  – 71C). This temperature range (above 130 F), kills many unwanted seeds and disease-causing beasties. Fats, proteins, and cellulose ( the fibrous component in plants) break down at this temperature as well.

Just as rapidly as a pile may heat up, once the food supply or conditions are less than optimal, a compost pile will begin to cool.  By turning, watering or activating you can spur spikes in activity (measured by heat) but eventually the pile will no longer get hot. It may stay warm for some time indicating there is some activity, but it will not get hot again. At this point, you can screen the compost using what you need, and return the bigger bits to the pile or you can let it sit and it will continue to break down slowly.

The third stage then begins. Think of this as the rocket coasting through space. It may not be as exciting as the first two stages but that doesn’t mean you aren’t going somewhere! Just because the thermophilic bacteria are no longer heating and thriving in the pile, doesn’t mean the pile is not active. In fact, the next wave of bacteria (and fungi) prefers cooler (70-75F or N to N C) temperatures. They arrive and hang around breaking down the most stubborn materials. This coasting stage can stage can last months, even years.

But I usually don’t wait that long. I screen out the dark compost and add the rest back to the pile.

So, when your compost pile is steaming in the morning, now you know why!