Hey pullsit...a few questions
1. Are you using a flowering fert during veg....if so that would be a likely reason for yellowing.
2. If your soil(mg or whatever) is pre-ammended with nutrients then your going to want to be very careful what your feeding them for the first few weeks. They already are getting food from the soil, so water feeding maybe be a lil overkill. The soil nutrients should be used up after a few weeks and you should be able to start feeding normally.
I personally like to use base soil with no ammendments...this way i have total control of what the plant eats and when. Promix hp and bx are pretty good as is sunshine mix #4. hope this helps