you already have topsoil, so I would use the compost. At least thats my plan for my fall garden.

If you want to add 8-12 inches you want a mix. Compost has lots of nutrients, but good soil adds mineral content and structure you will want in your garden.

If you are making supported raised beds you could consider "mels mix" which is 1 third of each, soil, compost, and peat

I just went through this. The County Extension Agent advised me to include some clayish soil in the mix as it helps retain moisture and nutrients and increased the Cation Exchange Capacity (I thinks that's the ability of plants in use nutrients in the mix.

Mike

I use 100% compost in my gardens - all 4X8, or 4X12, and about 12 inches deep.

I've only got a couple years under my belt so far, but I can't imagine better results than what I've had.

My vote is 100% compost, as that's what I've used, and have been very successful with.

What type of soil you'll be putting this extra 8" to 12" is quite important.

If clay, don't use 100% compost.
If sandy well drained soil, 100% could be fine.
You want the top 8-12" layer to hold water at least as well as the soil beneath it.

Below is a copy of a post I made earlier:

Just got done REMOVING huge amounts of compost from a garden bed and shoveling in CLAY.

Just a fyi warning. Last year I got a few truckloads of free city compost and just dumped it on top of the ground (clay soil) to make raised beds. And I dug some 2feet diameter 2 feet deep holes and filled them with the compost to plant tomatoes in.

It was the worst garden results in a long time. The compost was a heavy compost, but even so, when I watered it, it would drain relatively fast down to the clay. The result was the compost was usually dryish, while the clay beneath was totally saturated. My plants were only 1/2 their normal size. Did get an ok harvest, but it was actually like growing in container because the plant roots couldn't grow into the saturated clay. If I didn't water a lot, the compost would completely dry out.

So out when most of the compost and I double dug down adding some/not much compost and leaves.
The plan is to not double dig again, and go with no-till. Just have overwintering cover crops to help break up the soil.

In a previous house, I double dug for the first 2 years, added a couple inches of compost on top and had very productive garden for years.

Clay is great.

All the farms in California grow great vegetables in pure clay, no amendments.

Some things to consider when weighing your options.

Like others mentioned, if you already have soil, adding compost is about personal preference and your requirement for drainage or moisture retention.

Last fall, I added 100% compost to my raised beds that already have some soil and mixed it in. After compost addition, soil volume increased at about 4 inch per bed. What started out as 4 additional inch compacted down to 1-2 inch after one season.

For my new raised beds with no soil, I use planting mix for consideration that 100% compost may be draining too fast for our summer drought.

It really does depend upon your conditions, but I had quite good results with tomatoes by spreading leaves (mostly maple and oak, overwintered and not completely deteriorated) over an area I was converting from hardpan to a 6 inch raised bed, and then using almost all compost on top of that...I might have thrown in a little sand and topsoil, but not much, and I didn't mix anything.

What I did do, though, was plant my tomato plants deep enough that their roots were at least 2 inches into the actual soil below. This wouldn't wok as well with plantings fom seed or some othe plants, but possibly planting in deep furrows would wok.

We had a fairly rainy summer when I did that, but even during dry spells the plants were fine. The leaves seemed to keep moisture in the compost, but the roots were also deep enough to go into the actual soil. By the fall, the raised bed was full of enriched soil, with no trace of the leaf layer.

My soil is heavy clay, and after years of struggling with it, my approach is to now let the earthworms sort it out. I try to put down whatever I think they will like and hope that they will come and do their job. I never use poisons, which helps. One of the reasons for my dislike of digging too much into heavy clay is that, if it is too wet, it will turn to brick when it dries, and if it is already dry, it is like digging into brick to begin with. There are of course more nutrients than in sand, but you really have to watch how you work with it. Unless you work with it under optimal conditions (faintly damp and crumbly) you are better off not disturbing it.

I also think that smaller ants sometimes help to aerate the soil. They like to dig under upside-down plastic plant containers full of soil. (I don't think I would encourage that where thee are fire ants, though). Larger ants seem to cause things to dry out too much directly next to or in their nests, but in large plantings they are probably generally beneficial to most of the plants, if they are not into aphid farming.

Man, do I love to let bugs do my work for me. I am willing to wait a season for optimal results if they will just pitch in and help. I can't do heavy digging anymore, anyway, so I really have no choice, but I have never been a fan of heavy digging. My DH put in an asparagus bed last summer, and it almost killed him.

I want to use 100% compost but I feel like it's a risk!

100% compost isn't generally recommended for several reasons - mineral deficiencies as already mentioned, the absence of a good soil micro-herd, root stability, drainage issues, to name just a few. And 100% really offers no benefits over using a 60:40 or even a 50:50 mix.

Don't get me wrong, compost is garden gold but that doesn't mean it is perfect. :)

You can find any number of discussions about this issue over on the Soil & Compost forum here as this is the focus of that forum.

Dave

I would want to know more about the topsoil and more about the compost. Topsoil is very generic. Some venders sell good topsoil and others sell a mixture of sand and composted horse manure and call it topsoil.

Compost is also very generic. Composted sewerage sludge is commonly sold as compost and you probably shouldn't be using that in a vegetable garden. Or it might be green waste which can also be suspect.

Ask the sand silt clay and organic content of the topsoil. Ask about the ingredients in the compost.

Zeuspaul

I always use just compost in my raised beds.