When I built my raised beds (4'x8'x1') I filled them with triple mix (equal portions topsoil, compost, and peat). I borrowed a friend's pickup and bought a bit more than 2 cubic yards of the stuff. Way cheaper than buying by the bag. Worked well, had good production. I did notice that it was pretty dense stuff. Additions of compost in the fall and spring have greatly improved the texture.

You are right that you will get "differing opinions" as there are many options for fill and most any of them can be made to work especially in small beds such as yours. So it often comes down to what you have available locally and what you can afford.

Like aniajs my raised beds were initially filled with a 3 part mix of quality compost that included aged manure, local soil, and peat. Regular amending with both peat and compost (I make my own) keeps them working well.

I would caution against a 50:50 mix of soil and compost as it can have water retention and rooting issues. And also be sure the compost you get is a high quality made from multiple ingredients as some have reported issues with commercial compost that included ingredients treated with weed killers.

Dave

I used potting MIX and compost and some perlite the first season. Then worked in shredded leaves and more compost in the following falls.

Get your wallet out and just pretend that the beds are large containers; I use a soilless mix like ProMix BX or Fafard's #2, combined with compost--at least a third since the soilless mix is almost nutrient-free, unless you use something like Miracle-Gro.

If you are square foot gardening, do a search for "MEL'S MIX" formula, which uses about 3 or 4 ingredients including granular fertilizer (not water soluble like M-G.)

Most of my raised beds were started with a 3-part mix including topsoil, and regret it; now, I try to refresh them with compost or a mix of peat moss and M-G Garden Soil, which looks exactly like the compost I used to truck in (the town's compost facility got bought by a commercial topsoil company and no longer sells a pickup truck bedful for under $20. They also sold a garden mix, but I really needed just the compost.)

Most of my raised beds are either 3 or 4ft by 7 or 8ft and everything settles over winter, so I add a bale of peat moss and a couple bags of the Garden Soil to each bed, all tilled or forked in.

Hope this helps!

My raised beds at a former home were about 12" above ground and a similar amount underground. Because of the quantity of rock in our weathered granite soil, we put the native soil through a sieve to remove rocks and then added composted manure and top dressed with composed or shredded leaves. It was a great garden!