Role of Bacteria in Compost

Decomposition

• Compost begins as plant material such as grass clippings, leaves and fruit and vegetable scraps. Through the process of decomposition, bacteria and other organisms break down plant material and reduce it to small particles. These particles of decomposed plants make a natural fertilizer for flower and vegetable gardens. The process of decomposition releases heat energy and converts organic materials into inorganic nutrients. Decomposers such as bacteria, fungi, worms and insects break down organic materials from dead organisms. Composts that contain large amounts of leafy plant material become dominated by bacteria during decomposition. Bacteria consume almost any type of organic material in compost.

Aerobic Bacteria

• Aerobic bacteria colonize and feed on dead plant material in compost. Macroinvertebrates such as mites, millipedes and earthworms first break apart the plant material, making it available for bacteria to colonize and feed on. Bacteria are chemical decomposers. They produce enzymes that catalyze chemical reactions. Bacteria cause oxidizing reactions during decomposition that release heat. These chemical reactions cause organic matter to decompose. Bacteria populations are highest during the initial stages of decomposition and decline as plant biomass decreases.

Types of Bacteria

• Three kinds of bacteria are active during different stages of decomposition. Psychrophilic bacteria are first on the scene. They begin breaking down plant matter at temperatures between 55 and 70 degrees F. Psychrophilic bacteria give off a small amount of heat, raising the temperature of the compost to the 70 degree threshold. At 70 degrees, mesophilic bacteria colonize and decompose the organic material at a more rapid pace. They remain active until the heat released raises the temperature of the pile to 100 degrees. Thermophilic, or "heat-loving" bacteria take over decomposition when the temperature of the compost pile rises above 100 degrees. Thermophiles quickly consume the decomposing material, increasing the compost temperature. However, thermophiles begin to die off when the temperature reaches 160 degrees, causing the pile to cool down. The number of thermophiles also decreases when there is not enough plant biomass remaining to support the colony.

Temperature

• Temperature and air circulation affect the bacteria populations in compost. Thermophilic bacteria thrive in a range of 130 to 160 degrees F. A two-week period where temperatures remain within this range is optimal for bacterial colonization, but is too warm for many weed seeds and plant pathogens to survive. Bacteria can more effectively carry out decomposition if the proper temperature is maintained by aeration. Turning the compost periodically prevents the pile from getting too hot and killing off the bacteria.