[harrymac]

I start off new worm bins with old compost, shredded newspaper and cardboard.

[SG6]

Could mix the old grow bag with some sand, put into a pot or trough and grow carrots in it.

They like a fine soil, compost and sand at say 5:1, and they are best in ground that hasn't been manured or enriched. So the old grow bag and some sand should be pretty good.