A wide array of microorganisms live in a compost pile. Bacteria are especially abundant and are usually divided into several classes based upon the temperatures at which they best thrive. The low temperature bacteria are thepsychrophiles, which can grow at temperatures down to -100C, but whose optimum temperature is 150C (590F) or lower. The mesophiles live at medium temperatures, 20-450C (68-1130F), and include human pathogens. Thermophiles thrive above 450C (1130F), and some live at, or even above, the boiling point of water.
Strains of thermophilic bacteria have been identified with optimum temperatures ranging from 550C to an incredible 1050C (above the boiling point of water), and many temperatures in between.20 The strains that survive at extremely high temperatures are called, appropriately enough, extreme thermophiles, or hyper-thermophiles, and have a temperature optimum of 800C (1760F) or higher. Thermophilic bacteria occur naturally in hot springs, tropical soils, compost heaps, in your excrement, in hot water heaters (both domestic and industrial), and in your garbage, to name a few places.21
Thermophilic bacteria were first isolated in 1879 by Miquel, who found bacteria capable of developing at 720C (1620F). He found these bacteria in soil, dust, excrement, sewage, and river mud. It wasn't long afterward that a variety of thermophilic bacteria were discovered in soil — bacteria that readily thrived at high temperatures, but not at room temperature. These bacteria are said to be found in the sands of the Sahara Desert, but not in the soil of cool forests. Composted or manured garden soils may contain 1-10 percent ther-mophilic types of bacteria, while field soils may have only 0.25% or less. Uncultivated soils may be entirely free of thermophilic bacte-
Thermophiles are responsible for the spontaneous heating of hay stacks which can cause them to burst into flame. Compost itself can sometimes spontaneously combust. This occurs in larger piles (usually over 12 feet high) that become too dry (between 25% and 45% moisture) and then overheat.23 Spontaneous fires have started at two American composting plants — Schenectady and Cape May — due to excessively dry compost. According to the EPA, fires can start at surprisingly low temperatures (1940F) in too-dry compost, although this is not a problem for the backyard composter. When growing on bread, thermophiles can raise the temperature of the
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