what causes the leaves on my gardenia to turn yellow?

it is normal for one or two leaves to yellow every now and again and drop. it sheds leaves this way instead of the typical autumn leaf fall.
if lots are doing this it could be a magnesium deficiency, a dilute dose of epsom salts will help put it back in the pink, or should that be green ;o)
if it is in a pot it may need repotting or just a feed of a good liquid feed. [baby bio type thing.]

Thanks so much for the advice. I'm having similar problem with tropical hybiscus and sense that it may be a soil deficiency of some sort. I'll keep you posted. (The gardenia is in a large pot outdoors and the hybiscus is in the ground.I spend january to june in Naples florida. thats where the plants are located.) Summer to fall is spent in southampton New York with hydrangea, viburnum,azalea lilies of all types and many other cold weather plants.