You're right. I didn't even answer any of your questions. Oops. Everything here should help you better understand what is going on, even if it doesn't directly answer your questions.
With so many veg ferts pushing such high levels of potassium is it the cal/mag or nitrogen in the calmag thats making the difference?
I'll be honest, I'm not sure how to answer this question, mostly because I'm not sure what you're really asking. Running high levels of potassium is mutually exclusive to calcium, magnesium, or nitrogen. So, some clarification might be necessary.
Could it be true to say that a modern veg formula or npk 3-1-3 needs more liquid calmag which contains nitrogen than an older formula like 3-1-2 or lower even 3-1-1/4-1-2?
I would say this is false. If you're NPK is 3-1-3, you don't need to be adding much more nitrogen in that the plants don't really need it. Do not confuse this with "adding more nitrogen will suddenly cause serious problems" because it won't immediately cause problems. While NPK are the 3 nutrients that require blatant front of the label listing on a product, it is important to always look at your calcium and magnesium quantities in a fertilizer blend before considering the addition of anything else.
Arent elevated potassium levels the shortest way to a calmag deficiency and surely cannabis dosent need an n-p-k-ca such as 2-1-2-2 which seems common for many brands?
Calcium deficiencies can come from a number of things.
If you're growing in soil, a high phosphorus concentration in the soil will cause the calcium to bond with phosphates and form an insoluble calcium phosphate. pH and CEC are other factors that effect calcium in the soil. Low pH soils are more likely to contain low calcium. Low CEC soils have low calcium holding capacity.
Calcium is a positive ion, like potassium and magnesium. It competes with potassium and magnesium in the water and soil. So, you are right that elevated potassium levels can reduce calcium uptake by the plant. However, high potassium levels usually mean potassium levels that are out of control as opposed to just being high relative to calcium. Just stating the ratio of each nutrient to each other isn't going to immediately give you the answer you're looking for either. You've got to look at the concentration in the water or soil.
Now, before I get into this next section, I need to talk about NPK labeling for a second. N as total nitrogen. P as P2O5. K as K2O. When we look at ppm concentrations of different nutrients, we aren't looking at P as P2O5 or K as K2O. We look at P as total phosphorus and K and total potassium. This is important because fertilizers show P and K a little different than what a laboratory analysis will show. Below, you'll see what I mean, I think.
So, consider a feed solution that is the following:
Total N - 200 ppm
Total P - 56.7 ppm (130 ppm P2O5)
Total K - 166 ppm (200 ppm K2O)
Ca - 130 ppm
Mg - 65 ppm
The ratios are about 3-2-3-2-1
Those concentrations of each are acceptable and will give you a strong fertilizer blend. If you were to double all of those, the ratios stay the same, and you may have problems, you might not. If you were to greatly increase your potassium, to say, 600 ppm, then you'd start to have calcium and magnesium deficiency problems due to the high concentration of potassium.
Most ferts carry enough calmag hence my reference to the 2-1-2-2 npk which youve quoted at 2-1-2-1.5-.75 including the mag in a 2:1 ratio which i ommited to leave out but none of that goes to answer the 3 questions more based around potassium levels i posed and why the use of calmag is so prevelant in most grows that use npk formulas that are already 2-1-2-2-.75 which as said earlier is what most standard bottles of modern mj specific nutes already have.
You could go deeper but a simple 3 part answer will suffice.
First off, I may have not been clear about something. The ratios I mentioned are ratios I use in my own blends and do not necessarily represent the ratios used in any consumer liquid fertilizer blend.
Now, if you're asking why growers use so much CalMag even though a lot of other liquid blends are reasonably balanced, I don't know if I have an answer. While I'd like to be able to say that everyone has done soil quality tests and so on, I can't. I know a lot of growers grow in decent soils and many good soils don't need a lot of fertilizers. However, there's plenty of soil blends that are deficient in some nutrients, or just become deficient over time, so you might supplement calcium nitrate and magnesium nitrate during veg stages. It's hard to give a definitive answer here. Unfortunately, I think like a scientist and don't pass strong judgement on questions unless I have strong data to back it up. This is one of those cases.
