Why Does Compost Get Hot?

The Basics

Compost heaps consume oxygen and produce carbon dioxide and heat.
Compost is a natural fertilizer that recycles plant and vegetable matter to create an economical soil additive that improves texture and fertility and introduces beneficial bacteria and organisms. Plant residues like grass clippings, weeds and shrub and tree clippings are composed of cellulose, proteins, sugars and starches, some of which decompose quickly and some of which break down slowly, providing a constant process of decomposition in a compost bin. A compost "heap" collects confines and converts all of these, using oxygen and water. Organic matter is composed primarily of carbon and nitrogen. The carbon provides the fuel for the composting furnace and the nitrogen is the chief product of the process. The balance between carbon and nitrogen in the contents of the pile determine how long it takes to "ripen"---and how hot it gets in the course of the decay.

This mesophylic bacteria is called "E. coli."
Sawdust, newspaper and tree bark have a large ration of carbon to nitrogen. Pine needles, corn stalks and leaves have a smaller ratio, and grass clippings, hay and food scraps have the smallest ratio. Although small items like grass clippings and pine needles may begin to break down earlier because of their smaller size and more accessible surfaces, it is the high-carbon-to-nitrogen ratio materials that provide the fuel for the initial oxidation.
The process of oxidation produces carbon dioxide and heat. As the pile begins to decay, microbes work their way into the material, eating and eliminating material and throwing off heat in the process. As more oxygen and water is worked into the heap, the oxidation process speeds up, raising the temperature of the heap and inviting in warmth-loving microbes. These "mesophylic" bacteria thrive in summer temperatures---between 70 and 90 degrees F---and also exude heat as they consume matter. Water from rain or from the heap-keeper helps depress the temperature of the pile as it cooks.

A fungus spore
When the high-carbon-to-nitrogen materials start decomposing, a new burst of heat welcomes the next dinner guests---"thermophilic" or heat-loving bacteria chomp away and radiate heat as the oxidation rate increases. Together, the bacteria and rapid oxidation can drive the temperature up as high as 160 degrees, depending on how often the heap is "turned" or mixed and how much water is added. A successful heap-keeper knows that achieving useful, nitrogen-rich compost depends on starting with the right carbon-to-nitrogen balance of organic matter and constantly introducing enough oxygen to keep the pile "cooking." The high temperatures of a successful pile will kill germs and organisms that may be harmful and will render sterile the seeds of wild plants. As the end of the composting process approaches, materials that break down more slowly---cellulose and more complex proteins called lignins---are attacked by fungi and yet another type of bacteria called actinomycetes. The temperature of the heap cools as these slow-working organisms complete their jobs.