Philip, this "unavailable element" is magnesium - because Epsom salts is magnesium sulfate, a salt. I'll quote "Magnesium sulfate is an inorganic salt with the formula MgSO4(H2O)x where 0โคxโค7. It is often encountered as the heptahydrate sulfate mineral epsomite (MgSO4ยท7H2O), commonly called Epsom salt. ". From:
https://en.wikipedia.org/wiki/...
It can be appropriate to use Epsom salts. It can also be inappropriate to use Epsom salts. There is no one size fits all. Just because it works for you does not mean it won't cause a problem for someone else. The symptoms on the hydrangea do not suggest magnesium deficiency.
Yes I did read that far but I don't think you've read any of the references I've given you or you would understand why it's wrong to suggest applying magnesium unless it is actually needed. It occurs naturally in soil, you only need it if there isn't enough in the soil.