The Science of Composting (and Why It's Good for the Planet)

If you leave food scraps sitting on your counter or in your trashcan, they'll get slimy and smelly and might even attract flies or other pests. But if you turn those scraps into compost, they become nutrient-rich soil for future plants. That's a big improvement on the unrecognizable bag of goo in your refrigerator's crisper that might have once been zucchini. 

So, how does composting work? What's different about composting that gives such different results? The answer lies in the science of composting, which involves a little chemistry, a little biology, and, my favorite, a little physics. 

The science of composting

Composting speeds up the normal decay process of our food scraps by setting up the ideal conditions for microorganisms that like to eat those scraps. Those ideal conditions are warm temperatures, moisture, nutrients to consume, and lots of oxygen.

These bacteria and fungi—that’s the biology part—secrete enzymes that work to break down more complex organic compounds while their cells absorb simpler compounds. Sugars, starches, and proteins are turned into water, energy, and carbon dioxide, while nutrients like nitrogen and phosphorus are released. That’s the chemistry. To make sure the compost stays at the right temperature and to keep the oxygen levels up, it has to be mixed and aerated. That’s the physics. 

In thermophilic or hot composting—that's composting with thermophiles or organisms that thrive at high temperatures—intense microbial activity creates high temperatures that then lead to fast decomposition of the food scraps. Those high temperatures serve a double purpose as they can also kill weeds or disease-carrying organisms that would otherwise infiltrate the compost pile.  

Composting speeds up the normal decay process of our food scraps by setting up the ideal conditions for microorganisms that like to eat those scraps.

The first phase happens before the compost temperatures get above 40 degrees or so. Microorganisms that are happier at moderate temperatures start to grow rapidly. These microorganisms, called mesophiles, include certain types of bacteria and fungi. They start quickly breaking down the most easily degraded compounds, including sugars and starches. The heat produced by their activity causes temperatures in the compost to rise.

Once the temperatures are above about 40 degrees, which only takes a few days, conditions are no longer ideal for the mesophilic organisms and the thermophilic organisms—again, think bacteria and fungi—take over. The higher temperatures in this second phase allow for the breakdown of proteins, fats, and complex carbohydrates like cellulose. If cellulose sounds familiar, we talked about it...

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