As organic matter degrades, it lets off heat. Too much heat from tightly packed organic matter, possibly dried out, with heat from the outside can lead to spontaneous ignition.
The process sparked the fire that burned four units at the Parkway Plaza Apartments on Saturday morning.
Fire investigators found a single ball of earthen coal at the site, still hot hours after the fire had burned. That earthen coal was the remnants of a plastic potted plant container; the potting soil had spontaneously ignited.
Some types of potting soil are filled with bits of organic matter. As that matter decomposes and dries out, it creates its own heat, which can add to the speed of decomposition and more heat. Enough ambient heat, combined with heat from the sun and the heat from the decomposition, was enough for spontaneous ignition.
“It’s pretty rare that this happens,” Carson City Fire Chief Stacey Giomi said.
The investigators found the point of ignition but were “scratching (their) heads,” he said. “Then they picked up this ball of coal that was still glowing.”