use whatever mix you want, its your grow, the defficiency isnt boron, or the stems would look all fucked up, re-potting, (i think) will do the trick, i remember my plants slowed growth a little, (they were in the same pot) and i re-potted them and left them in a specially made greenhouse type box in my garden while i went on holiday, then when i got back they were nearly twice the size, bigger pots means better results, unless you do it too early - good luck