More field research. This time I had my friend Eddie play with burying things in the dirt / place cardboard.
He didn't seem to like the idea of touching dirt with his bare hands -- his hands will get dirty! -- but once he started and embraced the idea, he readily experimented with dirt. At this point I'm glad I didn't choose perlite, because even with the potting soil Eddie was accidentally flinging some out of the box in his digging frenzy. Perlite, being exceedingly dusty, would've been a disaster.
I'm against providing gloves for players though. Living in a snow-filled frozen cesspool of a city, I haven't seen dry dirt in a while, so playing with dirt -- touching it with bare hands -- is a relatively novel sensation. It's kind of like some sort of communion with nature, I suppose?
Before I was thinking the cardboard would serve as some sort of impervious surface to stop the other player from digging, but in-practice the digging player will always use so much force that they rip the cardboard out of the ground. Now I'm thinking that it can instead serve as some sort of trap -- when the other player tries to dig forcefully, they end up caving in all the surrounding dirt around them, which compromises their mining operation and they have to dig more to excavate the minerals.
The interaction is getting more interesting now, I think...