That's true, I use the soil once and put it in my veggie garden (trying to build the soil in that area as it's kinda week and will take some time to) so the hulls are not there for the long run......Pumice is go to otherwise.
Dolo seems to be Mag heavy sometimes,,,,,that is why I tend to use as little as I can......I have to use more Oshell as I feel it looses it's effectiveness in longer term Sativa runs if I don't......I like another short term solution too,,,,,dried ground egg shells. Living on and running a farm leaves me with a large ready supply......sooo.....Be aware that DG egg shell is a short term soil Calcification solution as it can become ineffective in as little as 3 months if you use smaller amounts.......2Tbl per gallon of soil mix or 1/2C per 5 gallons works longer term for me. Egg Shell does have an NPK value in the 1.1 - 0.5 - 0.2 area, keep that in mind if your mixing a "hot" batch!
Leaf "type" plants love this Ca, as seedlings do too. I really like DC egg shell for seedling mix's.