I have a few 3 x 4 pens set up to see what works best. One has sand and one with clean pea gravel. So far, I am leaning towards the pea gravel. I put an empty molasses tub, filled with used compost in a big pen and they tore that up. So, I plan on setting one of those 3 x 4’s with used compost, just to see how that works. I am guessing it would be similar to the way Kenny uses horse dung. Then there is regular dirt, but that gets messed up when it rains.