What deficiency is this?!

so I’m at the start of week 4 off flower, I have got 3 plants all the same strain (bag seeds) and I’m running in to a few problems... on all 3 plants I’m having problems with foliage going a tan colour, then brown and crispy. Right now it is only on 2 or 3 leaves per plant, but just can’t figure out what the problem might be! There environment is as follows...
RH 35-50%
TEMPS 16C (night) up to 30 (day)
Constant air movement with clip fans and in-line fan.

Any ideas as to what the problem could be?

Hmm, judging by that pic alone and general lack of info on medium and feed I would be looking at the root zone pH.

Renfro said:

Hmm, judging by that pic alone and general lack of info on medium and feed I would be looking at the root zone pH.

Click to expand...

I’m using soil as my medium, the only problem is that it has slow release nutrients in it... I have since changed to an organic soil for further grows (the girls in my vege tent.) I do not have anything to measure the soil PH with unfortunately... I know that there last run off was 470ppm, but that’s all I know as far as what’s going on down there.

Renfro said:

Hmm, judging by that pic alone and general lack of info on medium and feed I would be looking at the root zone pH.

Click to expand...

And here is what I’m feeding, it recommends 5ml per week, but I was having nitrogen toxicity ’claw’... so put it down to 3ml per week, and they picked up for a bit... now this

I'd need to see more damage to say with any confidence but it could be phosphorous related, the feed you have is really low in P. Soil nutrients may have become somewhat exhausted leaving them hungry for P.

Renfro said:

I'd need to see more damage to say with any confidence but it could be phosphorous related, the feed you have is really low in P. Soil nutrients may have become somewhat exhausted leaving them hungry for P.

Click to expand...

Here’s some more photos, this is the same plant but another leaf, it seems to be getting worse quit quickly!! The end off the entire leaf is a tan colour and shrivelled downward. Ph of water has been 6.1 for there entire life, I’m really wanting this sorted because the flowers are starting to look great! After looking at pictures I could see how it would be phosphorus related, but then again looks a little different, as this is my first grow it’s still all a little confusing... but I’m getting there (somehow) lol. Appreciate all your help bro

Yeah honestly I can't say for sure what it is. Keep an eye on it I suppose.