Function

• Composting is a process that takes months and occurs in stages tied to the temperature inside the pile. As microorganisms eat waste, they use the carbon it contains for energy and the nitrogen to build their bodies. The decomposition process releases carbon dioxide, organic acids and heat, while the microorganisms excrete nitrogen, phosphorus and magnesium--the nutrients plants crave. As the temperatures rise inside the pile, strains of bacteria die off and new strains that thrive at the higher temperature replace them. This occurs until the pile reaches around 170 degrees F, at which point the pile becomes too hot to support life and becomes sterile.
  
  

Psychrophilic Bacteria and Mesophilic Bacteria

• Macroorganisms such as insects and worms begin the process by breaking down the waste into smaller pieces that are easier for microorganisms to consume. Psychrophilic bacteria start the job, working in a temperature range of between 55 and 70 degrees F. They do not produce a great deal of heat, but they do produce enough to raise the temperature to the point where mesophilic bacteria begin to activate. These bacteria work rapidly and do most of the work of decomposition in the pile. Mesophilic bacteria thrive between 70 and 100 degrees F. They will continue to work for as long as the pile stays at this temperature, whether through outside temperatures or through turning of the pile.
  
  

Thermophilic Bacteria

• When the temperature reaches over 100 degrees F, the mesophilic bacteria begin to die off and thermophilic bacteria take over the decomposition process. Typically, this phase is short lived--no more than a few days--because these bacteria go through too much material too quickly to sustain life for any extended period. Temperatures in the pile range from 113 to 160 degrees F. In the meantime, the intense heat the bacteria produce kills any weed seeds or pathogens that may be present in the compost. After this burst of intense activity, the thermophilic bacteria die off and the temperature cools back down to the preferred range of mesophilic bacteria. With most of the easily digested materials broken down, this phase stays in place for quite a while.
  
  

Actinomycetes and Fungi

• As the mesophilic bacteria continue with their work, actinomycetes begin to work on some of the harder to digest material such as cellulose, starches, lignin and protein. These compounds are too complex for the mesophilic bacteria to break down. As the pile cools down even more as the process continues, fungi continue on the work started by the actinomycetes in breaking down the tough materials. They are involved in the final steps of the process.
  
  

Oxygen

• All of these bacteria are aerobic bacteria and need oxygen to do their work. Venting the pile or turning it regularly will help to keep a minimum 5 percent oxygen concentration in the pile. Without this oxygen, the aerobic bacteria will die. Anaerobic bacteria will take their place and decompose the waste, but they will not create any of the beneficial plant nutrients of aerobic bacteria. Anaerobic bacteria also produce noxious gases such as methane and hydrogen sulfide as byproducts of their activity.