Bait shop worm dirt is not the same soil that they were raised in. When the worms are moved into the US from Canada, the dirt has to be changed before they are allowed to come to the US. This is no shit, I can't make this up. The worms are welcome from Canada, but we want the worms in US dirt, not that Canadian shit.
I really don't know the reason behind it, but I do know it for a fact. That is just plain old dirt in those flats of worms, not much at all for any casings. At least not in the night crawler flats. Red wigglers may be a different story as they are farmed in the US. Crawlers are not able to be farmed and are picked up off the ground after rains, mostly from Canada, but there are some here too. Once again, not in the soil that they were rased in but plain old dirt.
Good luck with your grows.