anonlooker wrote:So dirt is a foreign substance? On a softball field?

Well, not if you're playing ball in the US of A and using good old fashioned American dirt!
But seriously, you're missing the intent and nuance of the written rule...
The rule addresses a substance "foreign to the ball", not "foreign to the playing field". Is dirt (or, for that matter, grass, sweat, pine tar, chalk, vaseline or spit) normally found on new balls right out of the package? No. These substances are not normally present on fresh game balls, thus they are "foreign to the ball".
More importantly, the rule specifically addresses substances being
purposely placed on the ball by a defensive player. Yeah, sure dirt gets on the ball during the course of normal game play. But that isn't illegal until the point that it is being purposely placed there by a defensive player.