Yellow dry spots on the tips and edges

Hi everybody, I've been learning a lot from the forum, it's my main resource for growing. Currently I'm growing 4 little plants in a filing cabinet. Everything is great, except for some weird yellow spots on some of the plant's tips and edges.

What could it be? Light to close? (4 x 26 w CFL's 2 inches above 'em) Nutrients? (I've been using Miracle Grow nutrient sticks)

I hope some one could help me

Hey there I have the same problem did you ever find out what it was