Extreme curling + yellowing - WTH is wrong with my plant??

[silloroy]

hi people,
As my grow continues and reached day 20 of flowering,
The problems continue...
In the last 3-4 days 2 of my 4 plants started to yellow and curl a bit at the tips.
didnt seem that serious, thought its probably heat stress.
but its getting worse and today it really become visible and accute looking in 1 of the plants. :\
allso - the infected 2 seem to stop growing, while they were the tallest during veg,
now the other 2 are like 20-25cm taller. and way behind on bud develop - could this help understand the problem?

Im growing in a 0.6*0.6*1.6m tent
strains - the 2 infected are skunke#1, the other 2 are Northern lights and afgan kush
250 HPS + 65W florecent
soil coco mix medium
nutes - Sensi bloom a+b 0.2 per litter, B52 0.2 per litter and reive as needed.
dont have a PH meter.

 

[silloroy]

Important detail - The problem is at the top of the plant and on new growth.
most of the bottom seem fine

 

[PeytonManning]

Looks like lockout, severe pH problem. You really need to have a pH tester, probably shouldn't grow without one. Might be a salt buildup

 

[PeytonManning]

How far are your lights? The curling up could be light burn

 

[silloroy]

would you say that a fair way to go here is to flush right now? or can it harm or severe the problem?

 

[silloroy]

How far are your lights? The curling up could be light burn

the HPS is about 20cm from the top, i dont think that its light burn because the middle is also infected
not just the closest ones to the light, and i keep my temps down with an AC

 

[PeytonManning]

Can't harm it, but it needs to be flushed with pH corrected water. Coco is known for toxic salt buildup

 

[Kola320]

Root rot ??? did this happen over like a weeks time are the leaves really dry and crispy ??

 

[silloroy]

kola it hapenned pretty fast, leaves are indeed dry...
ok guys - i flushed them and i 2 days after i think they`r gettin better.
not sure thogh...now the leaves are curling down instead of curling all over the place,
and they showed some groath in the last 2 days, after about a week of halt.
the infected leaves are still dry and crispy - is it even suppose to get well?? new grow looks better.
WHAT DO YOU GUYS THINK? buy the way the 2 in the back are so beutifull, freakin models! they are Afgan kush and northern lights. will post good photo of them.

 

[Frea]

Got a similiar problem but mine are only three weeks old and there northern lights auto flowers my ph is on 6.1 temps are 25-27c humidity is 45%-50%

Can it be my lights to close ?

Or my soil ?