Start with untreated mulch, pine bark nuggets or tree trimming mulch as a base. Then as you get it, add some straw, leaves, small twigs/branches broken up and yard waste(old flowers, weeds, ect...). I have pine cones and corn stalks in mine also. A good mix of bigger and smaller items. Bigger stuff helps the water drain down into the ground. After a while you will have to add more stuff as it will all break down to a nice rich soil, just keep a balanced variety. If you have to use more of anything, use one of these, untreated mulch, pine bark nuggets or tree trimming mulch. They will not compact down as much, like leaves or straw. Oh, you can also add your pine shavings when you clean your coop out, as long as it's not going to be mostly them.