Can someone tell me my problem?

[cabbaged]

Greetings from Ireland,

I'm a long term browser of THCTalk, today I decided to create an account on here. I believe there is plenty of people on here that can help me with a problem I'm currently having.

The problem - I can't figure out what is causing the leaves turning yellow / brownish

Firstly, ill give you details about the growroom.

The setup is located in a makeshift insulated room within a confined attic/loft space.
The room is about 1.5m - 2 metres cubed. The inside of the room is wrapped in a highly reflective material.
There is a 600 watt HPS light including a cool-tube mechanism and a 12 inch cooling fan in the grow-room.
There is also an 5 inch extraction fan.

Here are the strains currently growing

Bluedream (Humbolt)
Budda Kush (Humbolt)

The plants are auto-flowering and are getting 18/6 hour light/dark cycles. The plants are currently in flowering. They were re-potted from 4 inch coco coins into 9 litre pots.
The soil im using is Plagron royalty mix.
I have only given the plants nutrients twice since flowering, I used the Biobizz grow and bloom.
The air temps are 17C - 25C, and humidity ranges from about 25 to 75%.
I gauge my watering by how heavy / light the pots are.
I haven't taken any p.H. reading from the water recently, I recall it being around 6.5. Plagron states that p.H testing is unnecessary in this soil mix due the soil /additives being able to buffer the p.H in the water.

Last week they got there 1st dose of molasses, but I started noticing the problem before dosing.

Can somebody please advise me what the problem could be. Or if you need any more info about the grow.

Looks like I can't post links or images. And it only allows me to add 5 pictures in attachments. PM me if you need more pics.

Thanks

[Skunky n funky]

Hey you never said if you are getting run off from the plant when you are watering. The yellowing looks pretty normal for the early stage of flowering, and the majority of the growth looks really good. Often times plants will shed those big lower fan leafs later in life, as they dont need anything from them and lighting is a lot less. Too me it looks like it very well may be a simple salt build up. If you havent been getting the 20% run off each watering then give them a decent flush, and then a low feeding. Its hard to see what the tops of the plants look like in the photos but you are real close and it should just be a small adjustment in what your doing. Im not sure on that soil either.....

[happyjack]

temps are a bit low lights out bit hot lights on need to close the gap, never done auto but i feed coco every day, humidity is to high.

[mrnice586]

Drop the feed, just read that that royalty mix is hevey fertalised. Iv only used there light mix and batmix.

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[SmobyToker#1]

HGaving "no idea what you've fed her, id plump for hungry mate, 2 feeds isnt much in a fair size floweering plant. temps touch cool at 17c but not awful. I would get some good all round bloom ferts into her asap. I cant see that shes locked up in the pot, usualy that comes from putting acid thru the pot or really cold temps or over nuted medium. TRry a feed of 3ml bloom (with a dosage range of 2-5ml on bottle.) if you donr see improvement then flush it out with 3 x pot volume. No acid in the pots ok lad, its kills the bene's in the soil which buffer the ph. The old yellow leafs wont repair, but keep an eye up top for new growth andd shoots, atb mate