This is what I read, a cut n paste of what Gee64 posted on CalciumWell that is interesting.
Its more than a nutrient. In living soil its the main ingredient as it sets the colloidal charge to fill the colloidal plates with the proper ratio of all nutrients (aka organic chelation, but as nature designed it, not General Hydroponics, FoxFarms, etc..) so if its low, then your soil kitchen isn't able to properly load those plates and what does get loaded can and will get locked out as the charge isn't correct so nutrients stick to each other.
Different foods get locked out depending on how low it is.
PH is what allows cec to release the nutes from the plates. Different nutes swap out with hydrogen at different ph's and fall off the plate (become available, no longer locked out).
Thats why Cal-Mag is such a good rescue tool. It corrects the chelation and it also neutralizes magnesium back to its needed electrical state.
When calcium gets low in the soil the cal to mag ratio gets out of whack.
Magnesium starts to get electrically sticky and ties up other nutrients. It likes nitrogen the most so low cal in living soil usually shows as a nitro def 1st.
Calcium also, by using its double positive charge, which makes it your strongest electrolyte, sets the floculation of your soil.
Think of your soil particles as dinner plates stacked in the cupboard and held together by magnetism. Changing the charge with calcium changes that magnetism.
When you hold 2 magnets together they stick. Turn one around and they repel but spin around and stick by their own means.
When Calcium corrects the electricity in the soil structure it changes the polarities in the particles and every 2nd plate in the stack stands on edge forming I-beam structures in the soil particles that become hallways for air,water,microbes fungi... you get the picture.
That particular grow was 1/2 cup to a 30L pot and it worked a treat.So just to clarify that was half a cup as a standard measurement to approx 50L?
On my current grow, I recently gave 1/8 cup to a plant in a 30L pot because its yellowing was fairly minor. But the other 2 which were showing yellowing a bit worse are both in 50L pots, for them I gave 3 tablespoons to one, and 4 to the other (1/4 cup).
I am totally with you on the concern of trying to correct it and creating a risk of making it worse.Then I put a veg mix in with a higher N ratio at about 1/3 strength.
Neither fixed it.
Im hesitant to really give it a liquid Blood and Bone, or dry ammend as it might solve one problem but create another. I say that as each is adding P, K, and trace on top of whats available.
My next try is a Ca + N mix at 1/3 strength. Which goes back to your Calcium observation. Could it be that simple?
I would have started with that had I had it on hand.
The hardest part is the recommended application of the top dress. Its pretty vague. It could be entirely be just an underfed issue. Been hesitant since day 1 not to give too much as its better to be underfed and correct than crash them, just like my BB. You can see the difference within a day or 2 after application, its relatively fast.