The combustion of potting soil fertilizer is so rare that manufacturers don't put warning labels on their products. But homeowners whose hobby is container-gardening on their backyard decks be warned: In very hot and dry soil, the problem can occur.

Nancy Osborne recently returned to her home in Aurora, Colo., a suburb of Denver, to find the deck on fire. A plastic pot holding a tomato plant, soil, and fertilizer, which Osborne had bought at a local Home Depot, had caught fire and melted into the deck. Firefighters told her she might not have kept the soil damp enough, and when temperatures climbed to near 100 degrees, fire broke out.

"[Firefighters] could find no other reason," she said. "They were pretty confident that this is what happened."

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