White widow first grow

Hi, this is my first attempt at a serious grow. I tried when I was in college on my windowsil but it kinda failed, so a couple of weeks ago I invested in a 250w metal halide lamp. They seem to be growing ok, from what I can tell.

However, the plants are starting to show a little yellowing on the leaves. I looked about and diagnosed this my self as a N deficiancy, so a few days ago I added some to the soil. It hasn't made much of a dif. They're not that yellow yet, I would just like to catch the problem early.

Here is some pics of my room and the plants.