Compost Pile Microbes

The process of decomposition is complex but natural. Many organisms work to break down organic matter. Most are not seen by the human eye. Others that are large enough to see are usually associated with the later breakdown stages. A succession of microbes and insects combine efforts to turn feedstocks, such as leaves, grass clippings, yard prunings and food waste, into the fully decomposed finished product known as compost.

The Compost Food Web

If you are composting for the first time, you may be surprised by the size and complexity of the community of small organisms that take up residence in your compost pile. These organisms, which include many insects, bugs, slugs, bacteria, and fungi, form what is called a “food web.” In the food web, each organism has a job to do in turning your organic waste into dark, crumbly finished compost.

The food web decomposition process is divided into three levels:

  • Level One (primary consumers) is comprised of the organisms that shred organic matter and the microscopic organisms that eat the shredded organic residues.
  • Level Two (secondary consumers) is comprised of the organisms that eat level one organisms.
  • Level Three (tertiary consumers) is comprised of the organisms that eat level two organisms.

Organic Material Decomposition

The macro-organisms you can see in or around your compost pile, such as mites, centipedes, sow bugs, snails, beetles, ants and earthworms, are physical decomposers; they grind, tear, and chew materials into smaller pieces. However, micro-organisms such as bacteria, fungi, and actinomycetes–even though they go unnoticed in your compost pile–are responsible for most of the organic material breakdown. They are chemical decomposers because they use chemicals in their bodies to break down organic matter. The most abundant type of chemical decomposer in a compost pile is aerobic bacteria. When they break down organic material, they give off heat. Billions of aerobic bacteria working to decompose the organic matter in a compost pile causes the pile to warm up.

As the temperature rises, different organisms thrive. Psychrophilic bacteria are most active at around 55oF. Mesophilic bacteria take over around 70oF up to 100oF. When the compost pile temperature goes over 100oF, the heat loving thermophilic bacteria take over. Thermophilic bacteria prefer a temperature between 113oF and 160oF. If the pile heats to more than 160oF, bacteria begin to die off and decomposition slows down. This is why experts recommend turning your pile before the temperature exceeds 160oF.

As thermophilic bacteria run out of food, the pile will cool and the makeup of the microbial community will shift back towards cooler-temperature bacteria. This is when fungi become more active in the compost pile. Fungi prefer a temperature range between 70oF and 75oF and do a great job breaking down cellulose and lignin, the woody materials in your pile. The difficult process of breaking down these more complex materials can take many weeks.
Actinomycetes, a fungi-like bacteria
Actinomycetes are fungi-like bacteria that are light greyish in color and credited with creating the Earthy aroma of good compost. Along with fungi, Actinomycetes play a critical role in degrading the more complex woody materials in your compost pile, such as lignin, chitin, cellulose and proteins. These bacteria prefer a high pH and generally work in moderate temperatures. In mature compost, Actinomycetes can be seen as long threadlike filaments stretching through the compost. Some say Actinomycetes in compost resembles cobwebs.

(Photo by Matt Cotton, Integrated Waste Management Consulting.)

Avoiding odors

Research by Cecilia Sundberg, from the Swedish University of Ag Sciences, shows that a low pH is correlated with stronger odors and an increase in the emissions of volatile organic compounds (VOCs). This can be a concern when adding food to your pile, as some foods are acidic and can drop the pH. Large amounts of wet grass clippings can also cause strong odors if they become clumped or matted.

Odors may upset your neighbors, and VOC are bad for air quality; they react with engine emissions and sunlight to make ground-level ozone. The study recommended incorporating recycled finished compost into the compost pile and maintaining high aeration rates during the initial compost process. Both of these suggestions are intended to keep oxygen throughout the pile. This will help to keep the pile from becoming too hot, becoming anaerobic and to prevent the compost from developing an acidic or low pH. The recycled compost works as a bulking agent creating space for air to enter the pile and gives a boost of microbes to get the decomposition process moving. For a large compost pile, high aeration rates can be achieved by increasing the airflow though the compost with pressurized air pushed or pulled through the pile.

For a small compost pile, high aeration rates can be achieved by turning the pile more often, and especially whenever the temperature approaches 160oF. Placing perforated aeration pipes through the pile can also help. These are typically placed horizontally in the lower part of the pile, and rely on heat convection or wind to move the air. Some passive designs may use perforated pipes placed vertically. Whether it’s food or piles of wet grass causing the odors, the remedy is often the same: more oxygen.

Maintaining the Balance

All members of the compost food web are very beneficial to a compost pile and should be left alone to do their work. They need each other to survive. Removing any of the member organisms through the use of insecticides will interfere with their natural cycle as well as contaminate your compost with insecticide residues. While the organisms are busy recovering from the imbalance created in their food web, your compost pile will decompose slower. For best results, don’t worry about the insects, they know what to do. Instead, focus on turning your compost regularly. Regular turning will distribute moisture evenly, prevent your compost from compacting, and will allow oxygen to infiltrate the entire pile. All of these conditions help speed up the natural process, lead to complete decomposition, and enhance the production of the finished product we call compost.

The “Great Escape” Issue

Home composters sometimes fear that the hungry organisms in the compost pile may escape to the garden or lawn and eat everything in sight. Fortunately, this is not the case. The compost pile is their preferred environment. In fact, other organisms from the garden or lawn may leave their homes and go into the compost pile!

However, if you are concerned about the food web in your compost pile “breaking out,” try the following tips:

  1. Create a barrier by spreading a line of wood ash (not barbecue ash because of fat residues) or crushed egg shells around your compost pile. This will keep the activity contained within the pile.
  2. A similar, but more lethal technique, is to sink small margarine containers full of stale beer, molasses and water, or yeast and water in the ground around the compost pile. Unsuspecting slugs, sow bugs, and earwigs will be attracted to the liquid, crawl inside, and drown.

Of course, some bugs and insects may overpopulate parts of your garden and have nothing to do with your compost pile. Use integrated pest management and non-lethal means whenever possible to keep them under control


Compost piles made entirely from yard trimmings do not usually attract large numbers of flies. So, large numbers of flies–particularly fruit flies—may mean you’re food waste is too close to the surface in your pile. Flies are attracted by meat or dairy products, and even animal manure. Reduce flies by keeping meat and dairy out of the pile. Bury all the plant food waste deep in the pile and cover the food waste with several inches of soil, finished compost or at least a foot of dry leaves. Ensure that the pile is not too damp or too acidic by maintaining a balance of materials. You can also compost your food waste in a worm bin.

Home Composting

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