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

Rotting food in landfills is a major contributor to climate change. Composting works differently. Here's the science behind composting and some great reasons to get started.

Sabrina Stierwalt, PhD
5-minute read
Episode #365
The Quick And Dirty
  • In compost, microorganisms breakdown complex organic molecules in food scraps and convert them into nutrient-rich soil for future plant growth
  • Rotting food waste in landfills is a major contributor to climate change—composting is far better for the environment
  • It's easy to get started composting on your own

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 in our episode on sugar. It’s the main building block for the cellular walls in plants. This high-temperature phase can last anywhere from a few days to several months depending on what you’re composting and how much of it you’ve got. 

Once the thermophilic organisms power through their available sources of food, the temperature in the compost eventually decreases again so that the mesophiles can chime back in. This third and final step is called the curing or maturation phase. The mesophilic organisms break down the remaining, more resistant compounds, like lignin, the polymer that makes certain plants rigid, over the next several months. What’s left is a dark material, rich in nutrients like nitrogen, that can be mixed with soil to help the next generation of plants.

There is a sweet spot in temperature for all of this to happen. The compost needs to get above 40 degrees Celsius for the thermophilic phase. Small, indoor compost systems reach temperatures as high as 40-50 degrees Celsius. But larger or commercial compost systems will aim for temperatures above 55 degrees, which are lethal to disease-carrying organisms. Too much higher though—around 60-65 degrees Celsius—and even the thermophilic organisms can’t survive. 

What should you compost?

According to the US Department of Agriculture, fruits and vegetable scraps, eggshells, coffee grounds and filters, tea bags, and your dryer lint are all great fodder for compost. You can even throw in shredded paper, yard trimmings, house plants, and cotton rags depending on the size of your compost bin. 

Throwing in your pets’ waste is not recommended for the DIY home composter.

You don’t want to add dairy products, eggs, fats, or oils. These materials attract pests (although some commercial composters can handle them). You also don’t want to throw in stuff that's harmful to plants like certain types of tree leaves or coal. Commercial sites may add manure, but throwing in your pets’ waste is not recommended for the DIY home composter. 

Compost and climate change

According to the Food and Agriculture Organization, a branch of the United Nations, 30 percent of our food is wasted globally, which accounts for eight percent of total global greenhouse gas emissions. Eight percent! That’s third place after the largest emitters, the US and China. And not only were greenhouse gasses like carbon dioxide and methane emitted in the making of that food, not to mention hydrofluorocarbons used in its refrigeration, but a lot of wasted plastic was used in its transport and storage.

In poorer countries, 40 percent of that food waste happens just after harvest, in some cases due to lack of an ability to store it properly. But in wealthier countries, 40 percent of the food waste happens in supermarkets and with consumers.

Food waste in landfills breaks down without the presence of oxygen, a process called anaerobic decomposition. That causes the release of methane, one of the worst greenhouse gases. On the other hand, composted food decomposes aerobically—with oxygen—and mainly produces carbon dioxide, a far less potent gas. Compost is also beneficial because it can be used as fertilizer for future plant growth. 

Reducing food waste could reduce greenhouse gas emissions over the next 30 years by 70 billion tons—that’s almost as much as onshore wind turbines.

Not wasting food in the first place is certainly the best solution. But taking action at the consumer level to dispose of food scraps through composting is a start. Scientists at Project Drawdown, a research organization dedicated to climate change solutions, found that reducing food waste could reduce greenhouse gas emissions over the next 30 years by 70 billion tons—that’s almost as much as onshore wind turbines. The same group found that composting can reduce emissions by 2.3 billion tons in the same time period. So composting offers a way to make a real difference with the choices we make in our homes.

How to get started with composting

More and more groups are looking to composting to solve their waste management needs. It isn’t just for farmers anymore. In Haiti, composting toilets are being used to support agriculture and reforestation efforts while also tackling the country’s issues with access to safe sanitation.

So how can you get started with composting? 

On their webpages, both the USDA and the EPA give helpful steps for setting up a compost pile in your yard. If you don’t have the space for an outdoor setup, you can purchase a compost bin at your local hardware store or garden center. 

Composting is like a recipe. Is nothing happening? Add a little more nitrogen or a little more water. It may take a few tries to get it right. So if reading government webpages isn’t really your thing, I also recommend asking people in your community—at hardware stores, plant nurseries, farmer’s markets, or your local zero-waste shop. There’s bound to be someone there who's already a composter. I put out a call for help on my neighborhood message board and immediately got a wealth of knowledge specific to my local climate. 

But I’m not a gardener, you might say. Neither am I. Once my kids were born, I couldn't keep any plant life alive. But despite my best efforts, those kids produce a lot of food waste. So if you want to make an improvement, but aren’t much of a gardener, like me, check if your neighborhood has a compost drop-off site. Happy composting!

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.