If the problem is spreading then you have a different problem. Yellow lower leaves is usually a lack of nitrogen. If you have black spots on leaves, that is a disease like mold. I'd suggest taking clones from the top, get rid of the mother plants and try again. If you to use a chemical fert, use Shultz brand or go to a grow shop and get a bottle of Dynagro for veg cycle, NPK of about 7-9-5. It's a complete fert with micro nutes and will do what you want but for soil it's only about 1/4 to 1/2 teaspoon in a gallon of water, probably every time you water when soil looks dry. Use less fert if you water alot.