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How Does Composting Work?

Composting utilizes the same decomposition process as occurs naturally - however, the process is accelerated and improved. Microorganisms and invertebrates are used to decompose yard waste and food. They require water and oxygen to do so. They also use nitrogen found in the yard waste and food to help with the decomposition process. While these organisms are breaking down the yard waste and food, heat is produced. The compost pile can reach temperatures as high as 160° F. Decomposition can take anywhere from a month to a year. As the composting process nears completion, temperatures in the composting pile will come down to match the surrounding temperature. As the yard waste and food is decomposed, carbon contained within that waste is converted to carbon dioxide, which reduces the size and weight of the composting pile.

How composting works

Compost Organisms

A variety of organisms participate in the composting process, but they are generally a mixture of microorganisms and invertebrates. These can all be separated into three groups:

  • 1st Level Consumers: Bacteria, Mold (Fungi), Actinomycetes, Potworms, Fly Larva, Flies, Earthworms, Roundworms (Nematodes), Snales/Slugs, Millipedes, and Sow bugs
  • 2nd Level Consumers: Protozoa, Rotifera, Roundworms (Nematodes), Spring Tails, Mold Mites, Beetle Mites, Feather-Winged Beetles, Soil Flatworms
  • 3rd Level Consumers: Ground Beetles, Pseudo-Scorpions, Centipedes, Predatory Mites, Ants, Rove Beetles

1st level consumer organisms appear first. They attract and become food for 2nd level consumers. 2nd level consumers then attract and become food for 3rd level consumers. The most important organisms in starting the decomposition process and generating heat needed for other organisms to appear are aerobic bacteria.

Bacteria are single-celled organisms. While they cannot make their own food, they can eat most any form of organic matter. They are capable of reproducing through a process called fission at a very fast rate. Though they may only live for 20 or 30 minutes, they can produce billions of offspring in a short period of time.

When a compost pile is first created, psychophilic bacteria are active. This is especially true in the fall when the weather is cool, since they are most active in around 55° F (though they can remain active in temperatures as low as 0° F). The decomposition activity of the psychophilic bacteria raises the temperature of the pile, creating an environment where the most efficient decomposers, mesophilic bacteria can thrive.

Mesophilic bacteria are most active when the temperature of the compost pile is between 70° and 100° F. This form of bacteria causes rapid decomposition of the compost pile and generates a lot of heat. As the temperature rises, thermophilic bacteria become active.

Thermophilic bacteria are usually active while the pile is between 113° and 155° F. The heat at those temperatures kills most weed seeds and disease-causing organisms that might exist in the yard waste. The thermophilic bacteria will typically decompose quickly and become inactive after 2 or 3 days, unless the compost pile is turned and fed new material. Once this happens, the temperature of the pile will drop back down to where mesophilic bacteria are active.

Because the mesophilic bacteria are the most efficient decomposers, it is often advantageous to simply get the pile to moderate temperatures. Unless the plant waste is known to have contained diseased material or weed seeds, it is not important to achieve the higher temperatures where thermophilic bacteria thrive. High temperatures can also be a problem, because they kill or deactivate many of the other types of decomposers in the pile if it reaches temperatures above 140° F.

Other microorganisms in addition to bacteria play an important role in the composting process. These include actinomycetes and fungi (mold). All these microorganisms give the pile an earthy odor. As the composting process nears its end, these can appear on the outer 4 to 6 inches of compost as a blue-gray or light green powdery or cobweb type of substance.

In the portions of the pile that remain cool, nematodes feed on bacteria, protozoa, and spores from the fungi. Springtails, mold mites, beetle mites, and feather-winged beetles feed on the fungi. Predatory mites and pseudo-scorpions feed on smaller mites and nematodes. Complex invertebrates like ground beetles, centipedes, ants, and rove beetles feed on lower life forms found in the pile. As the plant life in the pile continues to decay, many invertebrates like potworms, fly larva, sowbugs, snails, slugs, millipedes, and earthworms are attracted to the pile.

While bacteria are the most efficient microorganism that assists with decomposition, earthworms are the best invertebrate decomposer. They spend their days eating their way through soil and organic matter. As they digest the soil and organic matter, the earthworms break down and neutralize it using digestive secretions like calcium carbonate. The material ingested is further broken down by the earthworm’s gizzard which crushes the material into finely ground particles. The earthworms’ intestines continue to break down the material and enrich it with enzymes, hormones, and other digestive substances. Then they deposit the resulting waste material as casts which contain concentrations of bacteria, nitrogen, calcium, magnesium, phosphorus, and potassium higher than those of the soil itself. Earthworm casts are the highest quality and richest form of humus material.

As the inner portion of the pile gets hot, most of the invertebrates move to cooler portions of the pile or die. As you can see, composting piles have a complex lifecycle.