We use backyard dirt. It is of the finest soil you can find an Easy County SD.
Honestly though we water and tend to the track for optimum conditions. Before hitting the track, the jumps and obstacles require a make over. And after wards the porch requires a good sweeping (Major roost!)
The cars really hook up and the track last for longer than the batteries.
My father-in-law has a crusher thats set upo for concrete material so can obtain anything. When I was at elsinore this weekend, Team AE had a course set up with what looked like silt but packed and watered.