I planted eight arborvitae trees three years ago in a river bed. The soil there is very rocky. Seven are doing very well, very green. This season in late spring the one in the direct middle of them started to turn yellow. It's not dead, but it has more yellow than green. I have a service that deep root fertilizes them every fall. I can't figure out what's going on with this one tree. There is no black on the bark. What can I do to save this tree?