On magnesium deficient plants
older leaves yellow from the center outward
while veins remain green.
which can lead to dead patches in the affected areas
leaf tips and edges may discolor and curl upward.
Growing tips turn lime green if the deficiency
progresses to the top of the plant.
Damage is similar in appearance to zinc deficiency
but far more likely to have.
Overdoses of potassium can block magnesium uptake.
Hope that helps.