| | Re: How-To: Mineralized Soil Substrate, by Aaron Talbot
Dolomite is used as a source of magnesium in the substrate - the calcium is just icing on that cake. You don't necessarily need it, because you can get magnesium other ways, in the water. Your hard water may or may not have lots of magnesium. Your water quality report doesn't say how much of either calcium or magnesium the water has, but the combination seem to range from 10-15 dGH.