A combination of things, the first being weed and grass killer. The second, he’s got this rig with his riding lawnmower where he has a 4ft section of chain link fence. He weights the section of fence down with bricks, and then just keeps dragging it around the track using the lawnmower. That actually served multiple purposes - it turns over the dirt a little, pulls up the smaller grass blades that escaped the killer, flattens out most of the bumps, and kicks up any rocks so they are easy to find and toss aside.