I'm in the process of learning about this since I'm setting up my first 29 gallon planted tank as we speak, so feel free anyone to correct me if I'm wrong.
The biggest things plants need to conduct photosynthesis and grow are ,besides light and co2:
Nitrogen - Comes from fish waste
Phosphorous - Comes from fish food/waste/biomatter
Potassium - must be supplemented
Ferrous Iron - which should be readily available in your substrate (assuming you purchased a high quality plant substrate)
You would also want a source of micronutrients, like Seachem's Flourish or the one you are currently using.
So really at the end of the day, every tank is different, with different bioloads, different fish, different plants with different needs. I don't think there is a cut and dry solution on how you should fertilize based on factors like your tank size for instance.
Algae will be the main indicator of whether or not you are lacking nutrients/light/co2, and apparently (I don't know what they are at this point) there are tale-tale signs on both the type of Algae you get, and how your plants look in determining which nutrients you lack exactly.
It's a big balancing act. If you provide enough light and co2 to spur growth, a heavily planted tank might dry up the available source of nitrates (fish waste/nitrogen) and force you to supplement nitrogen for your plants, but not everyone is going to need an outside source of nitrogen/nitrate. The same can be said for any and all of the nutrients -- though Light and co2 vs plant-load will always be the driving forces for how much outside nutrients you should be inputting into your tank.
Potassium is one of the keys I think, since it is the only one that would not come free from fish.
Again, I started in this hobby about 2 weeks ago, so what I'm saying could be totally off base