more info needed and more pics...from what I see its over fertilizing..frying the older fan leaves starting at the tips....BUT more info on the plant Is needed ...the run off ph the ec the food brand ..lighting temps everything you know ....will only help us help you... good luck and good grows keep us posted Sent from my SM-G600FY using Tapatalk
true the leaves look like pottasium problems ...but assuming he is using a good fertilizer with plenty of pottasium...something else is happening....excess of other ferts can lock out potassium...and or the root zone ph fluctuated which will also lock out potassium....it also looks like it could be heat damage....really more info is needed to be sure. good luck and good grows keep us posted Sent from my SM-G600FY using Tapatalk
Serious nute burn flush and leave to drain overnight or better potup no nutes for 2-3 weeks water well good luck
It looks like it's not very wide spread. Usually nute burn will be on most of the leaves. Many leaf conditions can mimic others. Looking at all those healthy leaves in the picture with no burn and the few that are displaying the browning/yellowing it does look a little like a potassium deficiency. Potassium deficiency can mimic nute burn. The difference is K deficiency displays yellowing between the burn and the green part of the leaf. It kinda looks like yours. It seems like I also see hints of calcium deficiency. Potassium Deficiency | Grow Weed Easy This is typical nutrient burn.
Having a deficiency doesn't always mean that you have to add more like others mentioned. You can get lockout from too much salt buildup in the medium. I'm not sure what is the cause of your condition from one photo. We could use input on your feeding schedule and lighting, ect.
Looks like nutrient burn. Flush it out with water until it runs clear coming out of the bottom. Will stress it out but it'll get better.
to correct a nute problem use 1/4 strength nutes NOT STRAIGHT WATER....doing so will force the plant back "down" until it levels off ...and then you'll see the new growth come out with no problems ...THEN gradually within 7 days return to the normal ec for your stage.....plenty of info on the net why not to flush with straight water when you suspect a deficiency or excess of any of the elements.. good luck and good grows keep us posted Sent from my SM-G600FY using Tapatalk