Chances are that the leaves at the bottom are yellowing because they are not getting a lot of light. In two weeks you should stop using ferts alltogether so that 1. the soil gets cleaned and 2. they start using the nitrogen built into the leaves. Once it starts to use energy from the leaves they all start to yellow out. Yellowing on the leaves at this far into the plants' lives is no problem at all. Don't freak out about them. The plants are beautiful and I have a feeling the bud they give is going to be superb. You've done a great job. Not only have you done a great job, you did it all on your first grow. You should be proud. Goodluck.