Check the roots, I bet you have rot causing no intake of nutrients

Thanx for the reply!
Since it is in soil, am not sure how to do that.
a quick google search turned up a lot of info related to hydro grows.

What soil and please give a little more details.

My bad on the details. I entered all that in my profile and thought it would show up like a sig. anyway, upon further searching I believe it to be light burn. notice it is primarily the center leaves that are affected. This is my first experience with a hps lamp, i started it under a 250W floro, then moved it to a 60ow mh and now flowering under 600w hps and have been keeping the light about 10" from the tops but moved it up another 6-8" when the lights came on earlier.

OG Kush
600W HPS for flower
approx. 4 sq. ft but not 4'x4' dimensional will get more accurate measurements later. (i know 600w is overkill)
190CFM inline fan w/carbon filter and lamp fixture has inline cool tube.
190CFM forced air input pulled thru portable a/c
She is in soil and have been using foxfarms soil trio increasing dosage small amounts at a time last feeding was:
Big Bloom = Full strength
Grow Big = 50% strength
Tiger Bloom = 75% strength based on foxfarms feeding schedule. i feed, water, feed.

Just received Advanced Nutrients Sensi Bloom, that pH Perfect stuff and am planning to switch to that on next feeding at 75% strength as i read someplace someone having no problems with it at that dosage for most strains.

But, i digress... i do believe it is light burn. can anyone confirm?

Thanks!
*EDIT* If it is light burn, how will it affect my harvest?

I keep my 600 at 18" from the canopy, but I run open reflectors.
As you said it's you're first grow so I would worry about getting a harvest period and not worry about yield at this moment.

Sounds like good advice. was just wondering how it will affect harvest (hopeful harvest... a lot can still go wrong).
18" is about where i raised it to yesterday.

Thanks!

Best of Luck to you.