Soil question

Ok, everything was going along fine for a while. Now, I feed and nothing. I water and nothing. Especially the first box I built. It's 3 or 4 seasons old. I added some soil this season before I planted, but I guess that didn't really help enough.

I took a soil test and although the PH is fine, there is NOTHING in the soil for nutrients. What is the best and most inexpensive way to put everything back in. I would like to avoid taking the plants out, but will if I have to. (carefully)

The only thing to use in any type of an EB is container mix at a cost of $7 per 2cu ft. Never use soil such as top soil or anything stating container soil.