foliar feed will provide quick response
1/4 tsp per gallon 2x 3 days apart for a mild def
1/2 tsp per gallon 2x 3 days apart for a more severe def
theres a balance between your potassium and calcium and magnesium, if one is high it can possibly reduce availability of the others
foliar feed is a quick remedy but you still need to address why your getting magnesium def issues
is it from lock out or basic def
but to answer that wed need to look at more info
medium type, nutes used, how you mix the nutes