This is one of those issues that makes you wish some vet school would develop a niche in the systematic study of chameleons. My personal hunch is that its got something to do with humic acid in the soil. I cover the dirt in the pots with big riverstones just to be sure the chams won't get into anything I don't know is in there, but now I also leave a couple tablespoons of humus (leaf compost mostly) in a tupperware lid on the bottom of the cage (out of the drip stream!). Once a week or so, the chams will decend and, with gusto, take a few large mouthfuls. Perhaps in nature they get bits of decaying leaf litter from the bugs they zap off the forest floor, and maybe that provides trace elements/minerals that don't work as well in powdered form. Or, maybe they need the fiber -- we worry about chitin but my guess is that their natural diet includes a lot more bettles/shells than I am able to provide. In any event, I wouldn't worry about it as long as, as others have said, you've got safe soil in there and no little stones/twiglets/debris that could get stuck in their digestive systems.