Spring greetings!

I am looking for suggestions about how to amend the soil in raised bed
boxes. These are tall ones - 15 inches high - and I've been using them for
four years. What I essentially have is huge, unmovable window boxes with
ten cubic yards of soil in them. Things have grown less and less vigorously
over the four years, although anything that stays in the ground long enough
does great once it reaches the native soil below (longest parsnips you've
ever seen!).

I use initial fertilizer at planting/sowing time, and feed throughout the
season, but what's the most effective thing to do now for the coming season?

I'm in Portland, Oregon - wet winters and cool springs. Last average frost
date is May 10, but we've hardly had a frost this year.

Many thanks for any and all suggestions.

Marcia