Since you are in the small cups you should look around the site for ideas for soil mixes. That way, you can transplant your seedlings into richer soil. I tend to keep my soil mix very simple because I'm lazy and I don't care about super huge plants and yields, I only grow for myself. So basically my mix is 1/3 peat moss, 1/3 compost mix (several different types of compost), 1/6 perlite, 1/6 vermiculite, and whatever amount of nutes the back of the bag recommends per volume
, or close to it. Oh, and 1 tbsp lime per gallon of soil. So for 3 gallons (my typical pot size) I toss in 3-4 teaspoons of 4-4-4 granulated organic nutes (using Jobe's right now) and 3-4 tsps of 0-10-10 organic granules. Neither of these is fast release so I mix it w few weeks ahead of transplant and moisten the soil to "activate" everything. The 4-4-4 typically carries most sativas into bloom, with help from the various composts, and the 0-10-10 already in there seems to last the whole bloom cycle. If I see something the plant needs, I tend to top dress with various guanos because they will release their nutes fairly quickly. Again, mine is a simple set-up, but I am certain others have much better mix ideas.