Asparagus needs a lot of real estate so plan accordingly - it is generally recommended to plant 5 crowns per family member, planted 18" apart in rows spaced 4-6'. Full, unobstructed sun and a deep planting bed with lots of organic matter. Takes 3 years before you'll realize a harvest - leave stalks to mature in the interim. If you intend to plant this spring (NOW is the time for planting asparagus in my area), I'd go with the compost and as much good garden soil as you can manage. Otherwise, prepare the bed this season and wait until next to plant so the other material has sufficient time to break down adequately.

Why raised beds?
Raised beds are good ideas if the underlying soil drains very poorly and cannot be easily corrected, but since the major reason for making raised beds is to improve drainage you may be creating a bigger problem by making beds that drain too well.

kimmsr, because I like the looks of it. And for me that is enough reason to do it. I didn't ask if a raised bed is the best idea for these plants, I asked what should I fill it with. Now if you can't answer the question then don't post.
Sandy

There are many reasons for using raised beds other than just improved drainage. For vegetables in particular, you can grow larger crops in a smaller space, you avoid any soil compaction, as the beds tend never to get walked on. You control soil conditions to far greater degree than you can in the ground - in areas where natural soil pH is incompatible with the plants intended, you can adjust the soil to your plants liking. Raised beds extend the growing season as the soils are faster to warm up in spring and slower to cool down in fall. There are design aspects of raised bed gardening that cannot be achieved to the same degree in in-ground plantings - think potagers or lifting smaller plants to a height where viewing is enhanced. And finally but certainly not least, raised beds offer a huge benefit to those who are physically challenged and can no longer bend or kneel or even use regular fullsized garden tools - most raised beds can be easily tended with hand tools.

I asked the question to help clarify in my mind why someone would want a raised bed and if trying to clarify the whys is offensive I am truely sorry.
You do not need to buy any soil to fill those raised beds just use what you already have and amend that soil with organic matter.

Wow, I'm new to Garden Web and excited to find a thread so exactly on topic for me, I was looking for help in figuring out how deep raised beds will have to be for rhubarb and aspargus as well as other insights to growing them by this method.
It seems like an old thread though so I'll see if anyone answers. Also seems to be a bit of attitude going on here, the first I've seen in the garden web-hopefully the last.
Anyway I am finally buying a house and swore that I would plant these two crops, rhubarb and aspargues, as soon as I was a home owner, because they take so long to establish.
The home I am buying has a concrete RV pad in the side yard with full sun. So my first contact with the garden web was results on a query to whether I could build raise beds right on the pad.
The answer was yes and there were great pictures of other folks beds made of used concrete blocks-satisfying all my criteria including going with surplus materials.
So I am very happy to have found you all and this terrific resource. I need to figure out how to get in a more current thread though. I'm looking forward to dishing dirt (garden soil) with y'all.
Hoping someone can help me out with the question of how deep the beds need to be. Thanks-Hatzi

Hi Hatzigirl! Welcome!
You might get lots more info & details by posting (1) new thread, (2) in veggie forum - and you might get different info depending on your zone, etc. - although keep hanging around Soil Forum b/c it has Great info!
All the Best,
Tree