lower leaves turning yellow

i'm thinking nitrogen deficiency? maybe magnesium?



Click for larger pictures

Again the burn on the leaf in the 2nd pic is just from getting too close to the light.

Should i just up my dosage on my Pure Blend Pro Grow (3-1.5-4) up to a full dosage instead of half? or maybe look for some nutes with a higher N rating? I'm just afraid of giving it too much and causing problems.

oh shit, forgot to post my setup.....

5 weeks old
75* - 85*
30-40% humidity
water it every like 3 days.....when the moisture starts to drop off at 4-5"
half strength Pure Blend Pro Grow (3-1.5-4)
2x 6500 (23w?) 100W equivalent bulbs
2x 5000 (23w?) 100W equivalent bulbs

could this be the result of doing ALOT of training after starting the training kinda late?
at first i was just training the main stem but then I started doing the sets of leaves growing out too to make it grow out wider


Click for a larger pic

as you can see there is alot of training going on....it seems like 1 stem from each branch off location is doing fine and 1 is dying. could this be due to light? or it jsut cant support the 2nd set of leaves or like since the first set is really growing out the 2nd set like isnt worth growing so its just killing them?