Canesisters, potting soil is much lighter than top soil and far less likely to 'turn hard as a rock". Other light additions that should work would be shredded paper and shipping popcorn. These should be mixed well into the soil. This will keep the bags light, moist, and allow the tubers to grow in the dark. Of course straw, shredded leaves, or hay on top of the straw should work, too, but I rather like the idea of keeping some soil mixed throughout the growing area.