I have a room in my house that is above a crawl space and soil. It's the only room like that.

In the winter and most of the time, everything is fine but sporadically, I have this horrible odor in that room.

Brand new carpet (odor resistant in fact) -- bleached the floors before putting carpet down -- this has to be from the crawl space.

This odor is always toward the late afternoon, when it happens, and it's not related to moisture -- if anything, I've noticed it more often after dry spells.

I do notice a similar smell in "some" of my soil on my land -- any ideas?

Not a methane type smell but a strong putrid "unusual" odor -- more chemical like.