The great information and link on gypsum provided by clay all over below reminds me that I wanted to thank NAT and the poster who helped on my mushroom in gravel shed floor over old woodland base question last fall.

I did not get to concrete the shed floor this summer as I was leaning towards....but the response to my post about mushrooms not liking salt was extremely helpful. Two bags of water softener salt raked in to the gravel floor worked beautifully! A LOT less expensive than 40 zillion sq ft of concrete! No mushrooms where I used the salt, a new crop coming where I did not - how's that for scientific evidence! The whole floor now has a bit of rock salt raked in.

Thanks again to originator DKO, NAT, and the people here that make it work!

The best all night coffee shop in the Ag business!

Jim at Dawn