Day 15 of Flower, Is it a phosphorus deficiency?

Hey guys,
First off thanks to everyone as I have read a lot on this forum for advice, tips, and growing techniques.
This is my first closet grow and I have OG kush. I am using Fox Farm soil and basic nutes. Currently my ventilation is great and I have a 135watt UFO LED light and some 300watt CFL's to help along the way with flowering. My grow has been going great, especially being my first! I am 15 days into flower and recently had some good bud action, with hairs starting to stretch out and look like actual buds.
Although I have recently seen some dark spotting occur in the larger fan leaves about midways on the plant. I want to think it may be a normal phosphorus deficiency but not completely sure.
I have been watering about every 3 days with PH at about 6.6-6.8 and with a teaspoon of cal-mag per gal. Also i apply FFBB and FFTG at very very small amounts (not even 1/4 tsp per gln) as I do not want to burn them. I have been letting the plant tell me what she wants, and so far I have gotten away pretty well with research, thanks to you guys.
Can someone help me with diagnosing this problem? I don't want to mess this up as I have gotten this far into flower and have been pretty successful so far.

Thanks for your opinions and advice, I appreciate your time spent.
Pics are below!

I'm having a very similar problem i just posted a thread, hope we figure it out, good luck to you

i believe you are correct to assume phosphorus
http://gr0wing.blogspot.com/2012/05/marijuana-problems-symptoms-how-to.html

I'm gona go with tek^^ but I might add your leaves are longer and skinnier than an Indica OG....I'm growin' OG #18 right now and the leaves are short and fat..
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RIKNSTEIN said:

I'm gona go with tek^^ but I might add your leaves are longer and skinnier than an Indica OG....I'm growin' OG #18 right now and the leaves are short and fat..
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Thanks for the indication RIK. I am curious about the strain and genetics of this OG after reading about other strains from different people. Hopefully in the next 6 weeks or so I can finish it out and start my second grow with even better results.

Thanks for your valued input.

That looks like calcium deficiency to me.

It looks like a GHOST "cut" OG.....They are early sativa pheno's of the breed.....Dr. Greenthumb has beans for that breed if your interested.....That's the "Ghost" in "ghost train OG"
http://www.drgreenthumb.com/cannabis_seeds_GreenthumbSeedsEntrance.htm

It does look like a P problem....I'd like to see pic's of the whole plant,,,like where exactly did this start on the plant? You said "midway" lower or higher then center and how fast did it get to this point? How quickly is it spreading???? and in what direction???

Looking at my notes, I find I've had that problem in FF soil's before (co-winky-dink). I don't care for FF soil's. Thrips and fungus knats have come in it before.....To woody and short on perlite also....Make your own sub's super-soil, or use something like Potter's gold or Dr. earth's POTting soil.....
https://www.rollitup.org/organics/539844-subs-supersoil.html

Dr. Who said:

It looks like a GHOST "cut" OG.....They are early sativa pheno's of the breed.....Dr. Greenthumb has beans for that breed if your interested.....That's the "Ghost" in "ghost train OG"
http://www.drgreenthumb.com/cannabis_seeds_GreenthumbSeedsEntrance.htm

It does look like a P problem....I'd like to see pic's of the whole plant,,,like where exactly did this start on the plant? You said "midway" lower or higher then center and how fast did it get to this point? How quickly is it spreading???? and in what direction???

Looking at my notes, I find I've had that problem in FF soil's before (co-winky-dink). I don't care for FF soil's. Thrips and fungus knats have come in it before.....To woody and short on perlite also....Make your own sub's super-soil, or use something like Potter's gold or Dr. earth's POTting soil.....
https://www.rollitup.org/organics/539844-subs-supersoil.html

Click to expand...

Good info bud...you need some rep

Sorry, I can't upload a pic at the moment but will tonight.
It seemed to start from the bottom on the older and bigger fan leaves towards the bottom center and dissipated towards the top pretty rapidly. although the array of my led light masked it well until it it had some leaves with sure signs. I did flush it pretty well and feed a little extra tiger bloom (1tsp per gal) and cal-mag. Also picked up some unsulphured molasses and added a tsp in there along with it. I fed this back to back waterings with a couple days in between (second feeding was lighter).
So far it hasn't gotten any worse and buds are forming rapidly. About 3 or 4 center fan leaves dried and died but so far it's been over a week and seems to be doing fine. When I get home I will load a photo of my first grow!

I appreciate the strain information, this is very interesting. The bud and leaf structures are very similar! A comparison in the future would make for an interesting project. Thanks!!

It takes 5-7 days for an adjustment to show results. So then, six days later with your statement of "hasn't gotten any worse". I would say you have a handle on it!

Good luck