I have bought an electronic soil tester. Measures PH level and Nutrient
level (fertility).

The PH it generally shows OK around 7.
But fertility is low.

1.What should I do now with plants potted in containers? Is adding
standard 15-15-15 fertilizer enough for that?

2. When next time I prepair soil for containers. I usually mix half a
soil from the ground and half the compost I got from the dump. Should I
mix 2 parts compost to 1 part soil?, So I do not have to add fertilizer
right away after planting?

Thanks