Use as much compost as you can afford. 100% is ideal.

Organic fertilizers will give you much better results. Bat guano or
rabbit droppings are about the best, but they are expensive. Cow
manure will do a fine job.

Additionally (or instead of if you have a thing about manure), dry
molasses and horticultural corn meal are good. As a bonus, the corn
meal will keep away fungus infections.

I always spray everything at least once a month with Garrett Juice.
Once a week is not too often.

Mark wrote:
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