Oh wow, I'm not sure I'm ready to take on that method. Maybe if I get to having multiple planted tanks, but for just my first one and just starting, I think I will stick to the generic/commercialized options. So I'd hope that a Flourish liquid or tab would be comparable to this solution?
I did say it was a slippery slope... I wasn't kidding. It is a steep muddy slope with a very deep pool at the bottom...
Let's look at Liquid fertilizers, which fertilize the water column and mostly feeds plants that feed through their leaves or live as floaters:
You can get the same stuff out of the seachem liquid fertilizers as mixing your own from dry.. It is 100% doable. It just gets expensive because you need to buy 4-5 different bottles, and they all go fairly fast at non-limiting dosage rates. The Seachem liquid fertilizers are intended to be used as a set, not singly.
Let's compare nitrogen as a simple example:
In a 75 gallon tank, EI low-light would call for 43ml of Flourish nitrogen every week. A 2L bottle costs $30 and would last a bit over 46 weeks (just under 1 year).
In the same 75 gallon tank using KNO3 you'd need 4.629 grams/week. At this rate 1 pound (453 grams) would last 97 weeks (nearly 2 years), and costs $3 (+s/h as you won't find it locally).
Repeat this kind of expense difference for phosphorus, potassium, micronutrients and iron.
Now, substrate fertilizing (tabs) is a different ball of wax. Some plants root feed, and do great on tabs. Other plants leaf feed or float and tabs do little or nothing for them.
Again, there's the Seachem option, or the DIY option. DIY'ers take Osmocote+ and fill empty gelatin capsules with it, then cram them all the way to the bottom of their substrate....