If you think about it, that compost pile you built so carefully in the back yard is actually a teeming mass of bacteria, fungi, and other microorganisms.  That may sound a little disgusting, but it’s really a good thing.  The organic material that you put into the compost pile contains elements that plants need to grow.  But those elements are locked up, unusable until bacteria and fungi break down the material.  Let’s take a closer look at the microscopic life forms that cause decomposition to happen.

The microorganisms involved in making compost need just four things to live.  They need an energy source.  This comes from carbon-rich materials such as dried leaves and straw.  They need a protein source.  This comes from nitrogen-rich materials such as kitchen waste or manure.  And they need oxygen and water.  If we expect to get compost, it’s up to us to provide all of these elements.  Many composting problems can be attributed to a deficiency in one of these things.

Thousands of different bacteria are at work in a compost pile, but for this discussion we are interested in three main categories.  Each group works within a different temperature range.  When bacteria digest organic matter in the compost pile, they create heat.  As the temperature in the pile rises, either from bacterial activity or from rising outside temperatures, new types of bacteria appear that are comfortable in the new climate.  In fact, any change in the conditions within the compost pile means that different microbes are going to show up.

Psychrophiles

These cool-temperature bacteria are usually the first to appear in the compost pile.  They work best at a temperature around 55 degrees Fahrenheit, but they remain active at temperatures as low as 28 degrees Fahrenheit.  (This explains why a compost pile will continue to decompose during the cold winter months, though much more slowly than it does between spring and fall.)  After the psychrophilic bacteria have been digesting organic matter for a while, the compost warms up enough to be hospitable to mesophilic bacteria.

Mesophiles

Most of the decomposition that happens in a compost pile is done by mesophiles.  They thrive in temperatures around 70 to 90 degrees Fahrenheit.  This category of bacteria is more efficient at decomposing organic matter than the psychrophiles, so rotting occurs at a faster pace once these guys move in.

If these bacteria generate a lot of heat, the temperature inside the compost pile continues to rise.  At around 100 degrees Fahrenheit, another type of bacteria takes over as the mesophiles expire from the heat.

Thermophiles

Thermophiles flourish in high temperatures.  When they become active and start generating heat in the compost pile, the interior temperature rises to about 160 degrees Fahrenheit.  This peak lasts for three to five days.  During this time the heat kills off weed seeds and some disease-causing organisms.  Eventually the pile begins to cool down and the mesophilic bacteria take over again, finishing up the decomposition process.

Gardeners need to understand that plant matter can be turned into good compost even when the pile stays at a moderate temperature.  However, seeds and harmful bacteria may not be killed if the pile never gets really hot.

Other organisms

Bacteria are not the only microorganisms at work in the compost heap.  Fungi and actinomycetes play important roles in breaking down raw material.  Larger organisms such as earthworms, mites, grubs, insects, spiders, and nematodes also assist in the rotting process by chewing, digesting, and mixing materials.

The earthworm in particular is every gardener’s friend.  Earthworms eat organic matter and excrete what is referred to as castings, which are rich in plant nutrients.  When you see earthworms in compost (or in the soil), you know that there is plenty of beneficial microbial activity going on.

It’s amazing to realize that a humble pile of leaves, plant debris, grass clippings, and other materials is actually a complex ecosystem.  Industrious microorganisms will keep working as long as the conditions are favorable.  Your very simple job is to provide the basic requirements and adjust the moisture and oxygen content of the pile once in a while.  In return, you’ll get loads of rich, crumbly compost for your garden.