I keep my chickens in an old garden area that is no longer used. The chickens are digging up the entire thing and bringing up old perlite from potting soil, bits of charcoal and other old debris from the plants of long ago. There is TONS of this stuff and there is no way to clean it out of the ground. Is this perlite stuff safe for them to eat? They dine on it like they are starving to death!