Yellow spots on leaves. Is it the different soil?

[Ghostface]

Hi. I have 7 plants each around a month old under flouresent lights, I'm getting them ready to be taken outside, so every day for the last 3 days I have taken them outside. Yesterday I noticed some yellow spots on the second true set of leaves on the the biggest plant, today they have increased and the second biggest has also started to get them. It's a dull yellow, and looks like a rash. The two infected are both in a different compost from the others. The compost they are in is significantly lighter than the other type, which is for seedlings and young plants. The compost from the 2 'rash' plants is also alot thiner with know lumps. Could you please help me to find out why these spots are coming up, and if it has anything to do with the compost/soil they are in. Maybee they need some nutriants? Also why should they be bigger in the lighter compost, but have spots?
Thanks for any help. Cheers, -Ghost.

ps. unfortunatly the bags do not tell me what's in the soil/compost.