what is wrong?

Hi,
I have had over 15 successful grows with 0 problems. Now my last 2 grows have been suffering from unknown issues. The only change has been a new grow space within same building but with same conditions (temp etc). some room stats:
3 x nirvanna lithiun og kush in soil scrog
1.5m x 1.5m
1x600W + 1x400W hps
cool tubes with own fan (pulling air in from out and blowing air out of the room)
carbon filter with own fan blowing out of room
GHF 3 part formula @ 1/2 recommended strength
Day temp 26C Night 20-22C and humidity @ 30-40% (day and night)
no light leaks
PH is good.

both last 2 grows beginning of grow has been good all the way till week 4 of flower. Then larger leaves start to get yellow/brown spots in middle of leaves, edges and tips become dry and brittle, grow become slow and almost stops. Last grow i tried to flush but no help. now i have the same starting again. attached is a pic of the leaf from one of the plants.
can anyone help me with this issue?

Hmmm I'd say ph problem,probably problems with lock out if feeding full strength nutes but if you've flushed to no avail then It's ph imo. Should be 6.5 in soilless mix.

Whatcha got there is toxic salts buildup my friend. TSB

Your humidity is good now for budding but has it been like that all thru veg and early flower too? Was the RH the same at the previous location or was it maybe higher there?

I kept having the same problem and I thought it was too much N but it kept happening then a year or so ago I found out about VPD. Vapour Pressure Deficit.

When your RH is low the plants transpire a lot more water and drag a lot more nutes along with it. The extra nutes get stored in the leaves all over your plants and the older fan leaves are the first to go bad. Looks like some kind of pH or deficiency problem but adding stuff to correct the problem only makes it worse. It doesn't present like ordinary nute burn but that's basically what's happening. The leaves get so much salt built up that they start burning up from the insides all over. The leaves get all thick and crispy dry instead of thin and soft like when they start yellowing at the end of the grow.

If you've just fed recently it might be a good idea to flush a bit out with water. RO water would be best as the minerals in tap water also add to the problem. I only use RO water and still got TSB. Now I feed a lot less and am trying to come up with a plan to keep my RH up. Really low in the winter and not much higher the rest of the year. It's 76F and 34% in my grow room right now and only 22% here in the mancave with a temp of 81.

There's not a lot you can do about it now but cut way back on nutes and give RO or distilled water.



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thanks. i did not realize it could be humidity. it was even dryer during veg (about 18-30%RH) but it has been the same before in previous room. last time, i fed them the same as i always did before and clearly the problem was worse. i cut back to half of what i was feeding them before and now the problem is a bit slower to progress. also, since i used ready soil mix i fed them first time only in week one of flower. it was just water (spring water @ PH 6.) till then. i did mix about 15% crushed expanded clay(washed) into the soil. can this be part of the cause? the are in 60 liter tubs... i have a humidifier i can try....