I grew on a farm. We would burn stuff we didn't want (like hay that went bad, etc) in piles that would burn for many, many hours. The soil underneath is intact,

I grew on a farm. We would burn stuff we didn't want (like hay that went bad, etc) in piles that would burn for many, many hours. The soil underneath is intact, there is so much mass behind it that it may get hot but never so much that it would decompose.