(Submitted By An Anonymous But Thoughtful Donor)
Placing 2" x 10" boards along the bottom of the posts creates a 10 inch wall for adding footing to any round pen, be sure to leave a space with chicken wire between boards if you live in a particularly wet environment so as not to trap too much water in your round pen.
Composting stall materials with sawdust for 3-6 months in either a barrel composter or in a pile 3 at least 3 feet deep and 4 feet wide at the base turning it at least once during that time and keeping it slightly moist will create a nice dirt like mixture that can either be used back in stalls, over your garden or in the base of your round pen for footing, and bonus it grows fantastic grass so you might even be able to use your round pen as short term grazing. :)