What do they do with dirty baseballs in MLB?

These days these days any baseball that touches a dirt surface is pretty much immediately thrown out of play. Some of those balls are then used for batting practice and some are shipped to minor league teams. Of course the actual number of baseballs used each game will vary throughout each game of a season.

Do they reuse baseballs that hit the dirt?

Even a conservative 100 balls means each one would survive only an average of three pitches — roughly a quarter million balls used in a season. Hmm, maybe it’s time to buy some Rawlings stock. So, yes, umpires will almost always toss out a ball after a pitch hits the dirt.