Redworms are heat-tolerant litter dwellers that find little to eat
in soil. Mixing large quantities of soil into worm bedding makes a
very heavy box. However, the digestive system of worms grinds food
using soil particles as the abrasive grit in the same way birds
"chew" in their crop. A big handful of added soil will improve a
worm box. A couple of tablespoonfuls of powdered agricultural lime
does the same thing while adding additional calcium to nourish the
worms.