Potassium deficiency can definitely cause whole leaf yellowing. The elongated tip burning that we see here is also a classic sign of potassium deficiency. At this stage of the grow, nitrogen is not needed nearly as much as during veg.
If you google cannabis potassium deficiency, and look at various photos, from reputable websites, you'll see that OP's symptoms more closely resemble N deficiency. There could
also be some K deficiency as well, depending on which leaves we're looking at. BUT... Doc has been addressing the deficiencies, so what we are talking about here may likely be what was
previously deficient, which caused lower leaves to go yellow.
It would be hard to have a nitrogen deficiency while in flower.
Statistically speaking, I agree. But a percent of grows, when the plants reach flowering, may not be sufficiently fertilized with N and other nutes. I think this is probably more common than you think. There's multiple things factoring into this, not the least of which are growers relying on liquid ferts and getting confused as to what they should be using. Another factor is liquid fert products that are poorly labelled, contain unknown amounts of nutes, or are simply lacking completely in one or more nutes.
Sulfur is a trace element in our grows and almost never shows as a deficiency... it can happen, but it would be extremely rare.
Adding into the diagnosis that Iron, zinc and molybdenum are also deficient and adding to this problem, complicates this diagnosis to such a point that surely all of these elements could not be lacking and still have the plants look this good. The KISS rule says to simplify this to one thing, and that one thing is one of the macro nutrients needed at this stage of the grow, potassium.
If these trace elements have been used up, or were never there in sufficient amounts, then there will be negative effects – more or less. It all depends on what's in the growing medium, and what's being given for ferts. The key here is... depending on pot size and what were the inputs, trace elements could be depleted at this stage. Again, I would say this is probably a common occurrence. And then, a great looking plant starts to look a little shabby. Granted, that outcome is more likely to be due to macro-nutrient deficiencies, but not exclusively so.
I view micro-nutrients in a holistic way in terms of overall plant health, and not just in a simplified way regarding the appearance of symptoms, or the absence of symptoms. For example, if there's an N problem in the plant, the symptom of lower-leaf yellowing may simply be due to overall lack of N as an input, or it could be the result of a more complex interplay where available N is not being assimilated properly by the plant. Either situation could result in stealing N from lower leaves.
Another clue is that this yellowing is progressing from bottom to top. This shows that the plant is able to go first to the storehouses in the lower fan leaves, and then move upward as more and more is needed. This shows that the element that is lacking is a "mobile" element in the plant. There are only 3 mobile elements... NP and K. It has to be one of them.
Thanks for that reminder. Apparently also Mg and Mo are mobile. The upper leaves were robbing N, and possibly P & K, from the lower leaves. To my knowledge, the current yellow leaves will not become green again.
Also, its not too late to save this plant. Given the nutrient that she needs, this plant would easily recover within 3 watering cycles. You could immediately see the yellow stop progressing and the leaves stop dropping. Boiling banana peels would give you enough extra potassium to see a difference, fermenting them would give you even more. I suggest Terpinator, as it is a great source of extra potassium.
It's not clear from Doc if the yellowing is still progressing at this point. He's been keeping up on the ferts. I don't think this plant is in danger of not being saved.
