MICROORGANISM IN A COMPOST PILE

• Microorganisms such as bacteria, fungi, and actinomycetes account for most of the decomposition that takes place in a pile. They are considered chemical decomposers, because they change the chemistry of organic wastes. 

• The larger decomposers, or macroorganisms, in a compost pile include mites, centipedes, sow bugs, snails, millipedes, springtails, spiders, slugs, beetles, ants, flies, nematodes, flatworms, rotifers, and earthworms. They are considered to be physical decomposers because they grind, bite, suck, tear, and chew materials into smaller pieces.

•  Of all these organisms, aerobic bacteria are the most important decomposers. They are very abundant; there may be millions in a gram of soil or decaying organic matter. You would need 25,000 of them laid end to end on a ruler to make an inch. They are the most nutritionally diverse of all organisms and can eat nearly anything.

• Bacteria utilize carbon as a source of energy (to keep on eating) and nitrogen to build protein in their bodies (so they can grow and reproduce). They obtain energy by oxidizing organic material, especially the carbon fraction. This oxidation process heats up the compost pile from ambient air temperature. If proper conditions are present, the pile will heat up fairly rapidly (within days) due to bacteria consuming readily decomposable materials.

•  While bacteria can eat a wide variety of organic compounds, they have difficulty escaping unfavorable environments due to their size and lack of complexity. Changes in oxygen, moisture, temperature, and acidity can make bacteria die or become inactive.

•  Aerobic bacteria need oxygen levels greater than five percent. They are the preferred organisms, because they provide the most rapid and effective composting. They also excrete plant nutrients such as nitrogen, phosphorus, and magnesium. When oxygen levels fall below five percent, the aerobes die and decomposition slows by as much as 90 percent.

•  Anaerobic microorganisms take over and, in the process, produce a lot of useless organic acids and amines (ammonia-like substances) which are smelly, contain unavailable nitrogen and, in some cases, are toxic to plants. In addition, anaerobes produce hydrogen sulfide (aroma-like rotten eggs), cadaverine, and putrescine (other sources of offensive odors).

• There are different types of aerobic bacteria that work in composting piles. Their populations will vary according to the pile temperature. Psychrophilic bacteria work in the lowest temperature range. They are most active at 55° F and will work in the pile if the initial pile temperature is less than 70º F. They give off a small amount of heat in comparison to other types of bacteria. The heat they produce is enough however, to help build the pile temperature to the point where another set of bacteria,mesophilic bacteria, start to take over.

•  Mesophilic bacteria rapidly decompose organic matter, producing acids, carbon dioxide and heat. Their working temperature range is generally between 70º to 100º F. When the pile temperature rises above 100º F, the mesophilic bacteria begin to die off or move to the outer part of the heap. They are replaced by heat-lovingthermophilic bacteria.

• Thermophilic bacteria thrive at temperatures ranging from 113º to 160º F. Thermophilic bacteria continue the decomposition process, raising the pile temperature 130º to 160º F, where it usually stabilizes. Unless a pile is constantly fed new materials and turned at strategic times, the high range temperatures typically last no more than three to five days.

• Thermophilic bacteria use up too much of the degradable materials to sustain their population for any length of time. As the thermophilic bacteria decline and the temperature of the pile gradually cools off, the mesophilic bacteria again become dominant. The mesophilic bacteria consume remaining organic material with the help of other organisms.

• The drop in compost pile temperature is not a sign that composting is complete, but rather an indication that the compost pile is entering another phase of the composting process. While high temperatures (above 140º F) have the advantage of killing pathogenic organisms and weed seeds, it is unnecessary to achieve those temperatures unless there is a specific concern about killing disease organisms and seeds. 

• Many decomposers are killed or become inactive when pile temperatures rise above 140º F. If the pile temperature exceeds 160º F, you may want to take action and cool the pile by turning it. A number of research projects have shown that soil amended with compost can help fight fungal infestations. If the compost pile temperature goes above 160º F, the composting material may become sterile and lose its disease fighting properties.

• While the various types of bacteria are at work, other microorganisms are also contributing to the degradation process. Actinomycetes, a higher-form bacteria similar to fungi and molds, are responsible for the pleasant earthy smell of compost. Grayish in appearance, actinomycetes work in the moderate heat zones of a compost pile.

•  They decompose some of the more resistant materials in the pile such as lignin, cellulose, starches, and proteins. As they reduce materials, they liberate carbon, nitrogen, and ammonia, making nutrients available for higher plants. Actinomycetes occur in large clusters and become most evident during the later stages of decomposition.

• Like bacteria and actinomycetes, fungi are also responsible for organic matter decay in a compost pile. Fungi are primitive plants that can be either single celled or many celled and filamentous. They lack a photosynthetic pigment. Their main contribution to a compost pile is to break down cellulose and lignin, after faster acting bacteria make inroads on them. They prefer cooler temperatures (70 to 75º F) and easily digested food sources. As a result, they also tend to take over during the final stage of composting.