The dry method I've used is because the trees I've planted had a root ball too large to easily fit in a 5 gal. bucket for the slurry method. I read about the dry method - mixing some dry crystals in with backfill soil - somewhere on this forum or the QDMA one, or another one a few years ago. It works for large root balls, if you run into that problem. I think once the crystals get wet in the soil, they swell and retain the water.
Good luck either way, gents !! I hope we all get our trees planted safe & sound this spring.