Two-year old plants are nice and green, but now some of the leaves have started to turn yellowish. High pH?
Blueberry Leaves Turn Yellow
Certified GKH Gardening Expert
There are many things that could be causing the yellow leaves. I would try a fertilizer first and also make sure that they are getting enough water and that there is not any damage to the roots that would be preventing them from taking up water.
This article will have some other possibilities: