People have used compost to amend soils for 2,000 years.

Role of Bacteria in Composting

by Casandra Maier

Rich, organic, well-aged compost improves the nutrition and fertility of your soil and increases the health of your plants. All organic matter breaks down eventually, but composting speeds up this process by using organisms and microorganisms to break organic material down at a faster rate. Beneficial bacteria, fungi and other organisms take on the job of breaking down organic materials in your compost pile. As they work, they create carbon dioxide, water and heat, which all play a part in creating a rich humus.

Breaking Down Organic Matter

Each gram of decaying compost contains millions of beneficial microorganisms that work to break down organic matter. Of the microorganisms present, 80 to 90 percent are bacteria, including actinomycetes and aerobic bacteria. They are some of the most important workers in your compost pile.

Larger organisms like spiders, worms, beetles, bugs, nematodes, mites and snails start the decomposition process by grinding larger pieces of organic matter into smaller pieces. Bacteria then go to work on these smaller pieces, consuming, digesting and recycling them. These bacteria are also versatile, extracting carbon and nitrogen from virtually any organic substance as a source of energy. They use this energy to create protein and reproduce. As they work, these bacteria also generate heat, increasing the temperature of your compost pile, which further speeds the decomposition process.

Excreting Nutrients

As the bacteria in your compost pile consume and digest organic matter, they excrete and free up important minerals and nutrients, including nitrogen, carbon, ammonia, magnesium and phosphorus. Once the compost is mature enough to add to your garden soil, your plants can absorb these nutrients. These nutrients are essential for the health and vigor of your plants. Adding compost to nutrient-poor soil may make it able to support plants.

Aerobic Bacteria

Aerobic bacteria are the main type of bacteria in your compost pile. Of these workers, there are three different varieties, each of which is active at different phases of the decomposition process.

Psychrophilic bacteria go to work on the initial organic matter in the compost pile, thriving at temperatures around 55 degrees Fahrenheit. These bacteria raise the temperature of your compost pile to 70 F, at which time, the mesophilic bacteria take over. These bacteria work at moderate to warm temperatures between 70 and 100 F. At 100 F, the thermophilic bacteria take over, raising the temperature of the compost pile to as much as 160 F. Once your pile reaches 160 F, it is important to turn the soil and add new materials, which cools the pile and starts the process over again.

Actinomycete Bacteria

Another kind of bacteria present, Actinomycete bacteria, closely resemble fungi and appear during the late stages of composting to clean up remaining materials that are difficult for aerobic bacteria to break down. Actinomycete bacteria are responsible for breaking down cellulose, proteins, lignin and starches. Along with aerobic bacteria, actinomycetes change the chemical structure of the compost pile to create a rich soil amendment.


The bacteria in your compost pile are fragile microorganisms that are easily killed or become inactive if the proper conditions are not met. It is important that your compost pile not become too hot or too dry. This is dangerous for your yard, can be a fire hazard and also kills off many of the beneficial bacteria.

Oxygen also supports bacteria in your compost pile. Bacteria require levels of oxygen at 5 percent or more to survive, otherwise decomposition is greatly reduced. Anaerobic bacteria also begin to develop in low-oxygen compost, which creates an environment that is acidic, smelly and bad for your plants. Keep the temperatures down and oxygen levels high by sticking to a regular schedule of turning your compost pile.

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