I guess what I was trying to say is that I'm mystified by statements here that can only be called guesses. Maybe this, maybe that, maybe calcium, maybe magnesium. I don't understand why people respond to threads with guesses when the OP hasn't put up any significant info to speak of. Without close up pics of those leaves, I don't know if it's mag def or not, and I've seen this problem close up before so I know what it looks like. The yellowing seen could be any one of a variety of problems. Guessing doesn't do the OP any good. You have to diagnose the problem properly from all the info you can get before trying to solve it.
Years ago, the most common cause given on the web for every problem was pH. Everyone was saying "check your pH." Checking pH and flushing is lame. The thing to do is get the grow medium right from the beginning.
Ideally that's how it should go, but many people either are new or simply don't know and they run into issues along the grow..it can't be undone and can only be corrected in the now. As for giving guesses..if the OP chooses to leave out critical info, i give my opinion on what the OP delivers in their statement, and based on that, yes i firmly believe it is magnesium. The location of the yellowing, the time of flowering he is at, absolutely i dont think it's anything else. Now, the main question is, why is magnesium missing..that's the real question and that cant be known without more info..like ph, and feeding habits and source of water. it may be there just not accesible..so adding more wouldn't help. once the OP gives detailed info, then a more precise answer can be given..for now, all i can persoanlly say..is the plant is not getting enough magnesium, as to why...that's another story that needs more data to be put in
I wasn't guessing, no other deficiency or toxicity would cause that kind of yellowing in that location and especially at this time of the grow. I firmly believe it is a mag deficiency..and no..not cal-mag..just magnesium! Add to it the fact that he's got heavier purpling petioles, which suggests phosphorus is having a harder time getting through, because it needs magnesium to get transported.