Yellowing seedlings

couple mouths ago I planted some seeds and they gave grown good but just of recent they have started to go a bit yellow and grow slow I put a white widow in three days ago and it is a beautiful green how it should be the only difference is that the others and in a brunnings seedling mix and the WW is in normal organic soil could it be because of a lack in nutrition I've gave them a tiny bit of nutrients to see how they will go just wanted to know what other people have to say! Thanks

hard to say without pics, but sounds like they've used up the nutes in the soil and need feeding.

Yellowing can be so many things, but I have never seen a plant die from lack of nutes. even in crap soil. I have seen many less than optimal situations where the plant needed a boost to reach it;s potential. if it is nutes remember less is more, until you see her reaction.