payote cookies

Not completely safe sailing, seem to have simile issues as I normally do around week 3/4. My soil seems to do ok buffering the ph of my very hard water but come weeks 3/4 it gives up. From what I can see it seems to be zinc deficiency but any ideas are welcome? My run off was coming out around 7.4 and my water (with nutrients added ) about 7.3 Strickly speaking I don't like to adjust the ph of my water when using soil but in this case don't feel I have much choice. I'll be adding a few drops of PH down for the next couple of feeds along with a mini flush to wash away any build up of salts.