Sorry for the delay in posting here, but I finally got some time to pop in again.
@Quantrill Yea I'm aware of Trichomes and their connection with Ca; that was why I posted and underlined the sentence
Within the leaf, Ca follows the apoplastic route of the transpiration stream and accumulates in either the mesophyll cells, trichomes or epidermal cells adjacent to guard cells, depending on the plant species. Both the [Ca2+]cyt in guard cells and the closing of stomata in detached epidermal strips are sensitive to apoplastic Ca2+ concentrations within the range of [Ca2+]xylem."
About being a Calcifuge, I wonder here, if you may possibly have the wrong perspective on what the label Calcifuge / Calcicole means. That categorization is in no way connected to Calcium "Use" or "Requirement" for plant species. If it were, then ecologists would not have been able to group all plants into just 2 categories. Rather, it was originally brought forth from noticing species diversification in differing soil conditions; which eventually led to this "bulk" classification. As well, the connections with these 2 plant types and their corresponding sensitivity / insensitivity to (Al,Fe,P,Ca and Mn) was also realized early on. However, the exact reasoning and function behind this differentiation took quite a bit longer for scientists to deduce.
So, then to cover what the difference is with these plant types and why Calcifuges are more sensitive to P and Fe; I'll just use one of the papers that is fairly straight, to the point and has good information.
From the abstract:
"Until now, mechanisms regulating this differing ability of plants to colonize limestone sites have not been elucidated. We propose that contrasting exudation of low-molecular organic acids (LOA) is a major mechanism involved and show that germinating seeds and young seedlings of limestone plants exude considerably more di- and tricarboxylic acids than calcifuges, which mainly exude monocarboxylic acids. The tricarboxylic citric acid is a powerful extractor of Fe, and the dicarboxylic oxalic acid a very effective extractor of phosphate from limestone soils. Monocarboxylic acids are very weak in these respects."
From the Discussion:
"The tricarboxylic citric acid was, by far the most powerful extractant of Fe, followed by the dicarboxylic tartaric, malic, succinic and oxalic acids, whereas the Fe solubilizing power of the monocarboxylic (formic, acetic, lactic) acids was very low. Oxalic acid proved to be the most potent solubilizer of limestone soil phosphate (Table 1)."
...
"The exudation of tricarboxylic anions, including citrate, was on average one order of magnitude greater for limestone species than for acid soil species. Exudation of dicarboxylic acids by limestone species was four to five times that from acid soil species. Exudation of the monocarboxylic formate and acetate did not differ between the two categories but approximately three times more lactate was exuded from the acid soil species than from the limestone species."
.....
"Comparing the relative proportions of LOAs exuded thus reveals clearcut differences in the exudation with no overlap between the patterns of the two plant categories (Fig. 2)"
--This paper covers only the germination and seedling phase, however it is brought forth as the easiest to cover this aspect. You can easily find more papers on this subject with various keywords in the paper above, though there is no variation, that I've found; to these ends. I.e. they all agree.
--The differing composition of root exudates, then, is what ultimately makes the difference between Calcicole and Calcifuge plant types. It is worth noting that these powerful acids exudated from Calcicoles to solubilize Fe and P; are extracting these elements (most typically) from Sulfate, Calcium or Carbonates.
Finally with the Calcicole and Calcifuge relation, there is a few more points that might be good to cover. These are taken from "calcium in plants" linked on the last page of the thread here.
"Nevertheless, calcifuges generally grow well at low Ca2+ concentrations in the rhizosphere ([Ca2+]ext) and respond little to increased [Ca2+]ext, which may even inhibit growth (Fig. 2). Conversely, the mechanisms that enable calcicole plants to maintain low [Ca2+]cyt in their natural habitat are believed to restrict their growth at low [Ca2+]ext by inducing Ca‐deficiency."
"One characteristic of calcicole plants,is a high soluble Ca concentration. In these plants, which are also termed ‘calciotrophs’ (Kinzel, 1982), Ca accumulation is stimulated greatly by increasing [Ca2+]ext. By contrast, calcifuges generally have a low soluble Ca concentration. They include the ‘potassium plants’, .....which are characterized by high shoot K/Ca quotients, and the ‘oxalate plants’, which have high tissue oxalate concentrations.
"Plants that accumulate oxalate can be subdivided into species that contain soluble oxalate and those in which Ca‐oxalate is precipitated. Interestingly, the uptake of Ca does not appear to increase with increasing [Ca2+]ext in plants containing soluble oxalate, such as the Oxalidaceae. In plants that precipitate Ca‐oxalate ......, there is a proportional increase in both Ca and oxalate concentration with increasing [Ca2+]ext."
The largest note with the last section is a relation that Calcifuges respond rather poorly to increasing Ca in the rhizome. This along with the exudates explains why certain growers have never been able to supply enough Ca via the medium with certain strains.
If we then put together all of this information it would make sense why Cannabis and Calcium issues seem to be so common. Not only considering calcifuge / root exudate differentiation but also from the previous page in which; even when proper Ca is available it may not be able to to be supplied to new tissue fast enough.
I believe I've mentioned before that often times when looking into history or understanding; there is no large change or epiphany that comes with it. None of this is intending to say that there is some great shift needed at all, but rather a possible understanding to why these things occur.
So, finally then; where does it leave us with all of this? If we add in the beginning aspects of no set Ca:Mg ratio specifically and add to this that Calcicfuges do not tend to respond well to varying Ca levels in the medium; it would be logical to conclude that the more important point here, is to maintain / achieve proper Ca Saturation levels in the medium (in relation to other elements) for your specific environment. Also, that irrespective of availability, exudates and transport rates; Calcium may very well not be able to be supplied in large enough quantity during times of rapid growth. Combining this further, it would seem logical that the best option of alleviating / averting a possible Ca- would be to focus on Foliar application to bypass nearly all issues covered. It should be noted too though, that since Ca is not transported to any degree after deposition ; that it does tend to build up in older leaves and that any fully matured leaf would not have a calcium demand in excess of delivery rates. If we further consider the Cystolic Calcium signaling, it may well also possibly be inhibitive to raise Ca levels in mature leaves beyond a certain point; as they would no longer contribute to the signals / perturbations. As such, it may serve well to only focus the foliar application on newer growth and most importantly during times of high growth.
I think that about covers things and I apologize for all of this to be so long. Hopefully this can provide help or understanding to some.
