I think I can do this
Mg is preferred as a mobile cation and is important as an enzyme cofactor besides being a major component of chlorophyll. It must be in balance with K and Ca. In toxic levels, it can interfere with the uptake of other cations like Ca, K and NH4. Reduced growth rate is a symptom of deficiency and toxicity of Mg.
No one in this hobby has ever once seen toxicity, that much I assure you.
Regarding Ca and K, these references are from terrestrial soils where salts can increase due to irrigation 10-2000X more than we might EVER see.
In otherword, all the mumbo myths on aquatic plants sites are grossly misapplied. There is no factual support over the concentration levels we see in aquariums.
I tested this a lot, and have not found any evidence to support it with a wide range of suspected sensitive plants.
Edward made a bunch of claims as to Mg being toxic above 10 ppm, see the pic of the pantanal? 25ppm with nothing but hard to grow weeds looking might nice. This clearly falsifies such this myth, ignorance is no excuse.
You no do, you no say.
It's that simple.
I am not certain at what upper range we will see toxicity, but it's a lot more than 20-25ppm. I have not dosed that high and few folks will ever experience Mg in that range. So there's little point to go farther.
See what the ppm's level are for hydroponic solutions for N, K, and P:
210ppm, 235 ppm, and 31ppm for most modified hoagland's solutions.
About 5x more than EI would ever get close to.
Mg levels are 52ppm............pretty juicy.
What does Tropica and Oriental use for their nutrient solutions for growth?
I'll let you guess.
Deficiency is seen first in older leaves as chlorosis, necrosis in severe cases, and new leaves are severely puckered.
So, Mg is merely a cofactor in carotenoid production but only when it is mobile, so the Mg in chlorophyll is locked away so instead when we see a Mg deficient plant we are looking at the affects of low mobile Mg+ which would not effect existing chlorophyl... ...so we would expect a Mg deficient red plant to still be red but with ugly twisted little new leaves because what Mg is in chlorophyll is fixed.
Why would it be twisted? There's little reason for stunted tip growth, mostly just a lack of pigment color from Chl a.
New growth is where the N limitation occurs, likewise, since the old Mg is locked in Chl a, and the new growth is limited........not much will translocate to new growth which would result in redder leaves.
Recovery from Mg deficiency is rapid also.
The older leaves can easily supply the reduced carbon to the region of new growth, so carotenoid production is not affected much here.
Therefore, excess Mg might facilitate chlorophyll production and enzymatic actions but would have a negligible at best affect on carotenoid production though it would cause other problems by inhibiting the efficient uptake of Ca, K and ammonium, which would lead to other problems.
And N limitation does not do this or have much effect on enzymes.....??
Comparatively speaking, Mg would offer a far better candiate if you go with the pigment method of nutrient limitation.
For Ca, K and NH4, the transportors can simply use H+ and ATPase's, Mg is required for some things, but a limited range should be similar to a N limitation.
Is that where I should be thinking? Am I even on the map
? C-/D+ for effort at least
Naw, I'd give you an A- hehe
I'm hard teach, but grade easy. If you survive the first few weeks
Got to put the fear of poor grades(the great motivator) in new students. They are there to learn and make mistakes, not suffer grade wise if they work hard.
The co factor aspect and mobility are good ideas and relevant, all the Mg is not in the Chl a, but the same can be said for N. I'm not aware of anyone that has writtren abiout Mg limitation or tried this critically.
That is interestinjg in and of itself.
I managed to induce Mg limitation but it was long ago, I have been interested in toxic and inhibiting levels at the other end mostly.
NPK, Fe etc.........I've done those nice and low..........but not Ca++ and Mg++ at the low end limiting.
These tend to be tough without some RO and serious modified dosing and decent controls prior. Even my DI/RO like tap has too much to run strong limitations. We did some in SF in the 1990's. But not individual nutrients.
So it's a knowledge gap.
No one really knows if the red can be popped out using Mg limitation.
Might be easy.