I live in NH and I use All Purpose Sand in their run and coop. As long as it or part of it is dry(ish), they'll dig a whole for bathing. Chickens are remarkable as you know with tilling. Once in awhile I'll throw in some wood ash that I've presifted and sand as the weekly part of my maintenance. I screen the wood as before hand to remove the unburned chunks of all sizes. Once less thing they'll jump down on from the run roost and get bumble foot.