I've seen this study before, but since I just came across it again and because it's a good one, I thought I'd share it with you guys. Only keep reading if you are interested in optimal potassium levels in your grow, otherwise this will be boring!!.... Here is the link:
https://www.frontiersin.org/articles/10.3389/fpls.2021.764103/full
To begin with, I've had the feeling for a long time that we are overfeeding out plants with Potassium (K) in general....but then, "potassium deficiency" questions are pretty common, if you keep up with the new posts here. Of all the "deficiency" type questions, "cal/mag" is the most common, but I think potassium comes next in popularity. Well, it's just a feeling I've had for a long time. I think most of those potassium "deficiencies" are actually due to having too
much potassium, or in some cases, too much calcium.
What got me thinking about this again the other day is because someone mentioned to me that they "read" that canna coco has about 100 ppm of potassium available by the time the flowering phase comes around. I heard something similar years ago, but I can't seem to find any official mention of it, but I'll send canna an email because I'm sure they have that data. Anyway, if that is true, then it means a lot of folks feeding a moderate to high EC of pretty much any 2 part nutrient system is right on the edge of K toxicity. In that scenario, all that excess K also reduces the amount of available calcium and magnesium-perhaps explaining a lot of the "calmag" deficiencies we see (and also the K "deficiencies").
This article found no increase in flower yield when the K ppm went from 60 to 340 ppm, also, if found no increase/decrease in cannabinoid content in that range, which surprises me because usually in cannabis as yield goes up, cannabinoid levels go down, also, as nutrient EC levels go up, cannabinoid levels also decrease. So, according to this study, feeding high ppms of K doesn't increase yield or THC, though it does potentially lock out cal/mag as the levels increase.
There is one other recent study that looked at optimal K levels-
https://www.frontiersin.org/articles/10.3389/fpls.2019.01369/full
This study came to some of the same conclusions. First of all, optimal K supply is genotype specific, like everything with this plant. As growers, we need to experiment to find the optimal K supply to fit our specific cultivar. Next, it found 240 ppm to be the max concentration of K before overfeeding damage was noticed. If you use a nutrient calculator you'll see 240 ppm K is not uncommon....add into that 100 ppm extra K from coco (for those of us using that media) and even more people are probably overfeeding K with absolutely no benefit. It isn't just people who use "bloom boosters" who are overfeeding K.