What is the best fertilizer to use on a Juniper Nana in early spring into summer?

Any fertilizer will do as long as you use it as it is on the label. Im using organic pellets and liquid fertilizer as well on my junipers and they respond very well to this mix. You need to experiment with different products and different specimens. You can also try to shorten or expand the time between feeding depening on the growing medium it is potted in. Try to look for signs from the plant itself. It will tell you what to do, and you will learn a lot.
If you dilute as per instruction, you won't make big mistakes