What all products are you using for your nutrient? Are you using all canna? The local hydro store recently picked up the canna line, so I can go there to get the needed info to develop a profile. I'm pretty darn sure that one of their base nutes will have Calcium Nitrate in it. You may be able to change around what you currently have (mabye use a bit of mono) and increase your Ca too; though I'd want to sniff around with profiles first.
I think I'm stressing so much over this as my last grow had a very bad Cal- in week 4 of flower.
Chances are that the deficiency began in week 2-3 but became visible around week 4. Have you ever seen that .PDF that's floating around called the phosphorus myth? One of the nute companies had done tissue samples with various strains, each week through flower. The .pdf is usually brought up in relation to plants not increasing P by all that much in flower. However, there is a lot of other good info that you can get from that paper.
For instance with Ca (since it was also tested). You can see that, pretty much, with all strains they tested; Ca approximately doubled (in tissues) from veg, compared to 2 weeks in flower. As well, this higher level carried through week 3. After week 4 and onward, all strains lowered the Ca in tissue until (at the end of flower) the Ca in tissues was about the same as in veg.
This same effect is also apparent for Mg as well, I suppose. From the testing I've been able to do in my environment; it would appear that my results also correspond to the .pdf.
Having said that; to me personally. The most important time for Ca is from week 1 - 4 of flower. Before and after these points; I don't worry as much with Ca. For instance, I "stretch" with around 100+ ppm of Ca , whereas in weeks 6-9 I only run 50-60 ppm. Suppose similarly and as an aside I cut out any Si after week 3-4 for similar reasons.
You can adjust your profile to help prevent future Ca- issues and I would always encourage that. Though, since you did see Ca- and are worried for the moment; it may be a good idea to foliar apply Ca during the stretch. At the very least and with no other changes, that alone should help the problem.
I'm somewhat hesitant to point this out but; One other thing to consider too is a small bit of NH4 during stretch. (not sure if canna has much NH4) There is an interaction in soil between Ca and NH4 in that; for every ppm of NH4 goes in solution, 1 ppm of Ca will be precipitated out. Sounds bad right? Well, let's look at the flip side. For every ppm of NH4 that is removed from a solution; 1 ppm of Ca will be added to the solution / availability. I.e. removal of NH4 from solution via absorption can actually increase "instantaneous" availability of Ca.
