Help please!! Leaves are dieing

Ok, I've been lurking on here for awhile, doing my research and getting all the knowledge I can from here. I've decided to post up cause I need some help with my current grow.

Heres my setup:

FFOF soil
Not feeding nutes right now
400W HPS with 4x 26w CFL's on the side 22/2 Lighting schedule
6" pots

My girls are Seedsman Feminized WW. They are about 3 1/2 weeks old right now. About 7-8" tall, does that seem short for how old they are?

My problem is the lower sets of leaves are starting to turn yellow and getting very brittle to the point where they just crack and fall off when I touch them. Are they dead? Should I cut them off? If so, do I just cut off the dead part or the whole leaf set?

This is my first grow, everything has been going great until these past couple days. I'm learning as I go and trying to make the best out of it. I dont have a camera so no pics sorry guys. Thanks

PS- HELP PLEASE

quality pics would help. my first guess, if it is the first seed leaves, it is fine for them to yellow and fall off. If this has already happened and it is some of the mature leaves that are yellowing it could be a nutrient defficiency or a watering problem. BUT, like I said pics would help

I use fox farm soils and 3 1/2 weeks is usually when I have to start feeding. I'm gonna guess its N deficiency. Just give them about a 1/2 strength veg feeding for there next 2 or 3 waterings. Then use a little stronger solution every other or every third watering. Just keep a close eye on the plants they'll let you know what they want.