First leafs turning yellow (pics)

Greetings;

Woke up to find that the first set of leaf on one of my plant had turn yellow. After checking around I am guessing it's Nitrogen related.
Found this really well made FAQ Grow Marijuana FAQ, Cannabis cultivation - marijuana growing tips & photos

Before I feed the plants I wanted some advice, tips...

Plants are 15 days old in soil; using a 200w Ecolight. Soil ph is a 6.5 and using water 6.5. The soil is organic with no ferts. temps are around 78/80 fahrenheit.

its abit hot for the seedling, and i think that its abit low in the pot.
and and she's gonna get rootbound pretty quickly in that little pot!