I had an interesting situation at work. One of the Faculty has a very large plastic planter filled with store bought soil. It has appearantly been sitting outdoors unused for quite some time and has become incredibly dried out with some dry dead plant material still mixed in. Yesterday there was smoke wafting up from the soil. Something was definitely burning in there but there were no flames and there wasn't any material directly on top that appeared to be burning. I poured a bottle of water I had near by on it, approximately a liter, and then thought that it may not have been the best idea. The water seemed to boil as it went into the soil and there was a loud hissing noise. After wetting the soil smoke was still coming up from under it. I was wondering if it is possible for soil to spontaneously combust if dried out and left to sit in the sun. I understand that there are a number of additives in store bought soil but have no idea what they are or how they would react.