bottum leaves turning yellow

i got some 5 week old babies, im veggin them wth flouro tubes, their in organic soil, the bottom leaves (1st, 2nd set are turining white and yellow, any clues as to whats goin on? ph of the soil maybe?

it could be a number of things, but usually it means it's nutrient deficient... on lower leaves it's usually nitrogen, phosporus, potassium, or magnesium...

check the ph of your soil and your runoff, if you are using fert there may be salt build up, and you may want to flush it sounds like nitrogen deficiency, but who knows? no pics and limited info.
if your not using ferts they maybe telling you they want nutes!!
http://www.icmag.com/ic/showthread.php?t=11688 there is some very good information in this link