exactlyDepends on the situation, if its a mag def due to lockout because of too much calcium for example, theres already more than enough magnesium in the medium and adding more will not help. So a foliar spray which is immediate will allow the magnesium into the plant. But it wont resolve the lockout. Which has to be tackled too.
Foliar spray off Epsom salts is good, Foliar spray of cal/mag solution is never a good idea.
Mulders chart, the antagonism and synergism of one nutrient in relation to another.corey, where do you get such info?