Probably not one you are expecting. I give my chicks dirt from the main run where my adults are starting around Day 2 or 3 in the brooder. This gets grit in their system, gives them probiotics the adults have, and gets them started on flock immunities. You probably don't have any adults so you won't get the probiotics benefits but there might be something in your soil they need to work on immunities against.
Instead of trying to raise mine in a sterile environment and then tossing them into the real world unprepared I start to strengthen their immune system as young as possible. The main thing I'm thinking of for you is Coccidiosis, that bug may be in your soil. If you keep your brooder dry the chances of it becoming a problem are pretty slight and after eating that dirt for two or three weeks they develop an immunity to that strain of Coccidiosis. When they hit the ground they don't get sick from Cocci, they already have immunity.