You're not going to be able to do anything about the color of the lower leaves but that's ok... What I do - and I've had the same issue with one of my grows - I measure the ph of the soil before watering/fertilizing and then try to balance the ph of what I'm giving them... i.e., if the soil is testing out at 7.0 and I want to drop it, I try to make my water/fertilizer come in at about 6.0 so as to reduce the soil ph afterwards. This has given me great results! I find I now test my soil before watering/fertilizing no matter what just to be on the safe side! Your plants are young enough, they'll bounce back with the right soil ph!