Compost Pile

Compost is a gardener's best friend. It provides nutrients and a healthy soil for plants. How is compost made? With the help of microbes! How do microbes make compost? They eat grass and leaves and kitchen scraps and virtually any other organic material and convert these into humus by a process known as biodegradation.

Composting is a process that microbes have been doing for billions of years, long before humans began gathering leaves into piles. Forest floors are major composting centers every fall when the leaves pile up.

Have you ever noticed that some compost piles get so hot that they give off steam? The microbes that live in hot compost piles are heat lovers called "Thermophiles."

Bacillus stearothermophilus spore
This is the resting stage of a bacterium that lives in hot compost.

Some compost piles are hot, and others are cool. Both hot piles and cool piles will create compost. However, hot compost piles degrade material more quickly than cool piles. Compost piles heat up for the same reason that you heat up when you do lots of exercise. Your metabolism speeds up. Like your muscles, the cells of microbes in a compost heap are working hard and are using lots of fuel (banana skins, grass, leaves, etc.) When they use lots of fuel, heat is given off as a by product. This heat kills many microbes, (just like too high a fever will kill you). However, some microbes like the heat. These thermophiles can live at temperatures above 45 degrees centigrade. Some thermophiles like it really, really hot. They are called hyperthermophiles. Some hyperthermophiles can live at temperatures above the boiling point! Ouch!

Fungi are important in the composting process. Fungi live on leaves and wood in the compost pile. Many fungi can break down (degrade) the cellulose in leaves. They can also degrade wood, which is made of large, complex molecules called lignins. Wood also contains the long molecules hemicellulose and cellulose, the stuff that paper is made of. Fungi are the most efficient organisms at breaking down wood. This fungus is growing on a rotting wood stump.

Conidiospores of Mold on Banana

Styrofoam is a type of plastic. Will Styrofoam cups turn to compost? Can any plastic items be turned into compost? It depends on the type of plastic.

Styrofoam Eating Microbes
No microbes are known that can degrade styrofoam. That's why we have no image here.

Alcaligenes eutrophus
This microbes makes a plastic which will degrade if put on a compost heap. This plastic is called polyhydroxybutyrate, or PHB for short.

If you put plastic on your compost pile, will it disappear? The answer is: it depends on the kind of plastic. Some plastics are biodegradable. Biodegradable means that organisms can degrade the material. Most human made (synthetic) plastics are NOT biodegradable. This is why when you put a Styrofoam cup on your compost pile it does not turn into compost, but instead just sits there.

Try This!

Build a compost pile in your backyard. Observe it every couple of weeks and record your observations. What is happening? Is is warm or cool? Does it stink? Is it getting smaller? What color are the grass and leaves that were placed on the pile?