Advice on outdoor yellowing leaf tips

I tried posting this in the problems forum with no replies. Hi everyone, first time posting here and this is my first grow. Basically I have 2 plants in one pot (my wife accidently threw one in without my knowledge) and a third in another pot. The 2 in the same pot are now 52 inches and 32 inches and the smaller one is much further along in flowering. All 3 have been doing well with little problems to speak of. The little one started exhibiting this discoloring in the leaves about a week ago. PH is around 7-7.5 and I am having a hard time getting it down. I have been using organic nutes like earthworm castings, kelp, guano, molasses etc. and lime for PH. Seems like a K issue to me but I don't know why. The bigger plant recently started flowering pretty heavy a few weeks ago, do you think that she is sucking up all the K from her little sister? Thanks for checking out my grow!

here are 3 pics:
PS: this is in a greenhouse

How far are you into flowering?

that looks like a fairly severe zinc deficiency. zinc is not very available to plants at higher ph levels. you really do need to get that down a bit. one of the organic growers would probably be more help than me in getting the ph down unless you want to use chemicals.

Thanks guys! I'll look into the zinc thing. The little one with the problem is about 5-6 weeks into flowering and the other one is about 3 weeks. I would prefer to use something organic but I'm more concerned with fixing the deficiency quickly.

Thanks and if anyone else would like to weigh in I'm all ears

Ok so was just reading up on zinc deficiency and stumbled on "Chelates": an organic molecule that forms a claw-like bond with free electrically charged metal particles. Natural chelates include humic and citric acid. So if I dilute some lemon juice into water and add a little liquid seaweed for zinc (and other micro nutes) do you think this will help my ph problem as well as beef up the zinc? If this is actually a good idea, should I apply the 2 additives separately or together?

thanks