There’s a couple of things to it. You either dig deep (10 to 12 inches down) and use REALLY good mix of soil and compost. I chose to go up, i.e. I did a 10” high raised bed. Some of them actually came up the first year.
I have one raised bed that I put in for lettuces and other salad fixins. It is around 8'x16', so I could devote 1/3 of it to asparagus. Is that too much??