1 of my outside runs is only 12 foot square and it's bare dirt. Once a month I take a shovel, over turn the ground in 4 areas. Chickens love to scratch in it and take dirt baths. Only other thing I do is throw in treats for them to eat. Adding materials( straw, wood chips etc.) that will flatten and hold moisture is asking for trouble unless you can get them to scratch at all of it every few days.