I let our chickens out for a few hours each day to free range. When I do, they often will go to a spot under a specific tree and roll in the dirt. They do not take these dirt /sand baths anywhere but in this one specific spot. I dont want to just dig up the dirt in that spot and put it in a small swimming pool as a sand box for them. Has anyone here made a sand box, and what type of dirt/sand did you use?