Having similar probs in soil, have 600hps in tent, well ventilated, various strain, at varying stages, I usually add 4 when I harvest 4... Anyway the problem seems to affect all plants in tent even though they are not all having same water/feed mix or volume. It is since the outside temp. Has dropped in last few weeks. I suspect it may be due to variation in humidity, the grow is in a small portacabin. As cold air enters and is heated by an electric wall heater which is thermostated- (and HPS bulb 12hrs a day) the relative humidity falls dramatically as temp. rises from say 5c to 23c, the air leaving tent feels humid enough but the intake will likely be low rH..... I have considered nutrients and lock out and flushed with water, ( the tap water here is ph6.8 spring water) I have had healthy results with the same nutrients and watering regime at same location in warmer weather. I can't just decrease ventilation because the Walls get wet with condensation as they stay cold these days so I don't have a solution for problem yet- changing to another location not possible........ If yr in a more stable environment you could perhaps control humidity accurately with ventilation, same for temperature......
This may or not be relevant to yr problem, I will watch this thread to see what others advise. I found very good photo diagnosis for multiple deficiencies just by googling relevant words...... Some posted on this site I think