Leaves Dying...Part II

I have 9 plants under a 600watt sodium light, 4 of which are Pablo Cheese and 5 are Duchess. The Pablo Cheese plants have a couple of leaves which are turning yellow. Though it does also seem as if the tips of the leaves are going brown and curling slightly.

I have attached the pics of both the set up and the effected plants/leaves. I would appreciate it if someone could get to the bottom of this little problem. The plants have now been in the vegetative cycle for almost 4 weeks. Thank you.

if they have just started to turn yellow could be a nitrogen problem if you are not useing any nutes try adding a small amount next time you water