Types of Composting Process

For organic material to break down into humus, bacteria needs to consume it. Because bacteria need water to digest their food, very dry things, like mummies and Dead Sea scrolls, stay preserved. The kind of composting that takes place depends on the type of bacteria doing the job, which are largely functions of temperature and the availability of air.

Cold Composting

Anything that was once alive can rot, even in temperatures as low as 0 degrees F/-20 degrees C, due to the action of psychrophilic bacteria. Like all composting bacteria, in consuming organic materials, these bacteria produce enzymes that penetrate the cell walls of their food, and split the contents of the cell into smaller pieces. Water is needed for this process, which is called hydrolysis. Once the pieces are small enough, they reach the bacteria through either osmosis or a not-entirely understood complex process called active transport. The waste products of aerobic bacteria are carbon dioxide, water, heat and humus. Psychrophilic bacteria do their work very slowly. Making compost at these temperatures could take years.

Mid-Temperature Composting

If a pile of dead plants is left to its own devices, mesophilic bacteria will do most of the work of breaking it down. The pile will range in temperature from 32-104 degrees F/0-40 degrees C, which is the preferred comfort level for this bacteria. The process will take months, but eventually, you'll get compost.

Hot Composting

If the pile has a good ratio of 25-30 parts carbon to one of nitrogen, and is turned frequently to give the bacteria fresh food and air, the mesophilic bacteria will produce enough heat for the thermophilic bacteria to kick in. Now the pile will heat up to 104-170 degrees F/40-65 degrees C, and aptly-named hot compost pile. Not only does the pile at this temperature range get hot enough to kill weeds, seeds and fungus, but it'll even get hot enough to kill the bacteria themselves. The entire composting process can take as little as two weeks with this method. Small wonder that this is considered the optimal composting process for gardeners to use.

Anaerobic Composting

Anaerobic means "without air." If the wet pile is put in an airtight container, the bacteria will quickly consume all the available oxygen, and then anaerobic bacteria will take over. Whereas the other types of composting carbon dioxide, composting without oxygen releases volatile ammonia and methane, which is a fire hazard. Biogas facilities that use this method create and collect the methane under carefully controlled conditions, but it's not the best method for garden composting. Of all the ways to compost, the one that gets hottest is the best because it's fast and less nitrogen is lost as ammonia. To get a hot pile, chop the inputs into small pieces, turn it frequently, and maintain a good carbon-nitrogen ratio.