Does setting posts in foam give them some protection from dry rot?

[George Makra]

When i was a kid i dug thousands of fence post holes. Back filling with gravel was the best way to secure the post since once packed the gravel locks together and allows for drInage.
Concrete will retain moisture and will slowly deteriorate the wood even if it is treated.

[Tom Bender]

So much depends on your local soil moisture and on the species of wood.