Decomposition is a dynamic process that naturally occurs in every ecosystem. Given enough time everything organic will break down. However, when the variables are ideally combined, thermophilic, or “heat loving”, composting will occur. This is the most efficient way to break down organic materials. How does it happen? MICROBES!

Microorganisms

Microbes, including bacteria and fungi, are the key to decomposition. Some types of composting also leverage the decomposing abilities of invertebrates such as worms (vermiculture), mites, sowbugs, beetles, and other arthropods (any outdoor compost pile). However, microorganisms are present in any type of composting and are the key drivers of the processes taking place. These microbes have several basic requirements - primarily oxygen for respiration, carbon as a source of energy, nitrogen to build proteins and reproduce, and adequate moisture to survive and reproduce. Let’s dive deeper into each of these variables and how they impact the composting process.

Oxygen

Oxygen is necessary for the metabolism and respiration of the aerobic microbes that inhabit a compost pile. If oxygen levels become depleted anaerobic respiration takes over and the decomposition process slows considerably and byproducts are produced including methane gas and undesirable odors. Aeration is necessary to promote continual aerobic respiration within the pile throughout the composting process. There are many ways to promote aeration in a system, from turning the pile to break up clumps of organic matter (to expose new surfaces to air), to mechanized aeration in the form of piping, to passive holes drilled into the top of a worm bin.

Moisture

Most microbes live in the thin films of water on the surface of organic matter particles. The compost pile must remain sufficiently moist to provide them this habitat, but not too saturated that air pores are water logged and prevent aerobic respiration. It is commonly recommended that the moisture level of a compost pile  be akin to that of a wrung out sponge. 

Carbon and Nitrogen

There are many elements necessary for microbial decomposition. Carbon and Nitrogen are the most important, and often the most limiting. As previously mentioned, Carbon is important as an energy source that fuels microbial metabolism and respiration. It is also important as a building block of cells. Nitrogen is an important component of the proteins, DNA, and enzymes that allow for cellular growth and function. Bacterial cells are often greater than 50% protein by biomass, and thus need lots of nitrogen for rapid growth.

Based on these two elemental needs, the ideal ratio of C:N in the composting process has been found to be approximately 30:1. Ratios lower than this allow for rapid microbial growth but excess N is lost as ammonia gas. Ratios higher than this will not provide the microbes with enough protein to reproduce; smaller population sizes mean slower decomposition.

Putting it All Together - a Thermophilic Process

In thermophilic composting, any soluble sugars in the original mixture are almost immediately taken up by bacteria and other microorganisms. This easily accessed carbon provides energy that results in explosive microbial growth, which causes the temperature to rise. During the thermophilic phase, more complex compounds such as proteins, fats, and cellulose get broken down by heat-tolerant microbes. Eventually, these compounds become depleted, the temperature drops, and the long process of maturation begins. During this final phase, complex polymers continue to break down slowly, often by fungi. Those most resistant to decay become incorporated into humus - the end product of composting.