This sounds like more of a light issue than anything else... what kind of lighting are you using?
The lower leaves always die off as the plant ages..but it sounds like you do'nt have enough light penetrating the canopy

As for nutrients... it depends on the soil medium you're using and overall strength of the plant.
Fertilize to early and you'll do more harm than good

Nitrogen based nutrients for vegging, Phosphate based for flowering