Q: Can potting soil burn or spontaneously combust?
A: Potting soil is composed of combustible materials like peat and shredded wood fiber so I’m sure it can burn if exposed to something hot, like a cigar or a stick of incense. There are news reports of this happening.
I presume that the incidents happened with dry potting soil, not damp.
I don’t think a bag of potting soil is large enough to spontaneously combust. Sure, it might get hot in the center but the small insulating factor of the material surrounding the hot spot would probably prevent any development into flame.