I see a lot of people posting in Facebook group that their plants are dying, they don't look healthy and they want to know which nutrient is missing. So of course someone will post a chart that looks like this.
After some deliberation, the group will come to a consensus that the person needs more Potassium in their tank. There is asking the hobbyist about their fertilization regimen, they just go out, by a bottle of Seachem Potassium and start dumping that into the water. Most of the time we won't see a change in the plants at all. Or sometimes they start looking a little better for a couple of days, but then something else starts looking off about the... and they got back and start the process all over again.
The reason why nutrient deficiency charts don't work is because of Liebig's Law of the Minimum. The law states "that growth is dictated not by total resources available, but by the scarcest resource", so as soon as you up a single nutrient like Potassium, you're most likely going to run out of another nutrient like Phosphates. This is why the Estimative Index (EI) was created. The EI method floods the tank with all the nutrients a plant could need within a given week. Furthermore, this is why we also inject CO2. Carbon is one of the scarce resources in a planted aquarium. Very low amounts of CO2 are naturally occurring, but by increasing the available carbon really jumpstart the growth process of plants underwater. That coupled with the EI method, there is never a time that a plant needs a nutrient and isn't getting it lending itself to optimal growth.
So the next time you see one of these charts, link them to this post and educate them on Liebig's Law of the Minimum. Also, look into the EI method for fertilization, or at a minimum, a holistic fertilizer like Thrive All in One... stay away from Seachem Flourish.