There is no difference in what you buy in a bottle. The periodic table is what it is. Potassium is potassium no matter where it comes from. We don't make potassium in a lab. Only marketers want you to belive that something called "natural" is better than some other product. It is just hype - and in some cases the "natural" product for your own health could be worse since many of these types of products (herbal remedies . . .) are not FDA regulated. If I wanted, I could start selling my own "Ginseng" tablets today - and you'd have no way to know what I was actually selling you.
In the case of a planted tank, I guess you could call fish the "natural" fertilizer. Anything else you would buy most likely went through some sort of purification process before getting to you anyway.
72g bowfront planted, CO2, 2x - T5HO, Eheim 2213 and 2013, red tiger lotus, vals, java fern and moss, crypts, swords, mbuna tank: Yellow labs, Ps. demasoni, clown plecos, SAE's.
10g mini-reef. 16lbs LR, green mushroom, star polyps, wagon wheel polyps, purple mushroom, candycane, hammer, other zoas, CUC.