Our 4 wk. poults starting bathing in the dirt last week too. They are the funniest things to watch, I agree. Today one of them stood up to shake off, and as it walked away a rock half the size of it's head fell out of it's back feathers. Who knows what else they have hiding in there!
Ash will help with pest control, It's an old poultry thing to add Ash to the litter and in and around the nest. The small ash particles with it tiny sharp edges will kill some small pest you can't even see.
It's best to mix it with your dust bath dirt, and not full strength.
We have several small feed tubs we fill with dirt and ash. The tube aren't really that big about 10 to 20 inches in diameter, and about 2 to 3 inches deep. The dirt around here has a lot of clay in it so when dry it really fine.