The clay/dirt topic is an interesting one.

My son and I went down to North Carolina (Carolina Speedway) in early December to get some track time in. The clay/dirt down there is red and very soft. It was an extremely smooth track. I bet my son turned 150 laps that day and the tires looked the same when we finished as they did when we started.

Whatever it is they have down there is very easy on tires. The only down-side was getting the car cleaned up.....that stuff was everywhere.