Dry crunchy leaves next to healthy ones, in veg, please help!

My plants are about 4 weeks into veg, i have 12 plants in there, 3 different strains.

On one strain (infinite euphoria), there are alot of dry , crispy, brownish leaves that are breaking off on their own. On the same plant, there are healthy leaves right next to it.

I was about to put these girls into flower but i wanted to find out what could be causing this first. The two other strains i have are fine, looking healthy. Its only 2 plants of the infinite euphoria strain.

Could it be pest, nute problems, ??? I have no clue, please help!

phosphorus deficiency id say, unless those are just from the bottom of the plant and in that case its okay that they fall off.

Or potassium, maybe a ph problem? Good luck

Howdy...yeah that's a real cunt of a problem right there mate...I get the same thing the odd time, and have narrowed it down to pH fluctuation. A LOT of deficiencies and toxicities manifest themselves in leaf necrosis, it's just about going over everything you've done! Check and recheck the water pH, before and after you've added nutes; check the NPK of your nutes to make sure there is enough phosphorus and potassium. Also cal/mag has to watched and equated too! Plants man! They're a deadly stoned challenge and I fucking love it.
A simple flush normally works for me, and then I start again slowly with the nutes and stuff. So I'd advise that! A small flush and just reassess if the nutes and pH are adequate...