Lower leaf yellowing...

I've grown a few times when I was younger, w/ relatively good success. I never cared about problems w/ the plants until my current attempt at growing, which is becoming a somewhat more involved project. My plants are about a month old, and about a week ago I started LST to get the full advantage as early as possible. Before I started training I had a problem w/ some of the lower leaves yellowing, then dying off. I decided to give a very weak solution of food to see if it was possibly N deficiency. It cleared up for a while, but since training, the lowest leaves on the lower branches are starting to do the same thing. Yellowing, but not yet necrotic (if thats a word, lol) I know its hard to diagnose w/o pics so I'll give as much info as possible before posting pics tonight. I tried searching first, but haven't found my exact problem...

They're in a 20 something gallon rubbermaid, w/ a 120mm, 85 cfm exhaust fan, four 2" passive intake holes, and a decent size desktop fan inside for circulation.

Lights- I'm using 4 26w 6500k CFLS, and 1 23 watt 2700K CFL. Not sure about temps or humidity yet, even though I know the importance.

They are in pots that are roughly 1/2 gallon, w/ some off brand potting mix and 50% perlite. I am using MG nutes designed for acidic plants (I know not optimal) at 1/4 strength every other watering. If any other info is needed, or anyone has any ideas, just let me know. Sorry about the long winded post, and all help is appreciated. Thanks guys

Pics definitely help, but so do knowing more about your grow, as you've already pointed out. Knowing the pH of your water is a biggie, too. It helps to fill out the troubleshooting form as best as you can.