I have no personal experience with it but lots of other folks recommend against it because compost may lack some of the micronutrients present in mineral soils. Now, whether your nursery's soil is good quality to begin with is an entirely different question, but if you or others have had good results with it, I'd try to make sure that the dirt is at least 25% of the mix.

If you layer your compost on top now to plant in spring your plants will grow through the compost into the bottom soil.

If you've built boxes you might want to use a blend of soil + compost like Mel's mix from Square Foot Gardening rather than pure compost.

Here is a link that might be useful: Making a raised bed with just compost ?

Agree with above. 100% compost is not usually recommended. It lacks both micronutrients, some of the needed mineral, and most importantly, the soil bacteria and are necessary to convert the nutrients in compost to form useable by plants. It has various other minor problems but those are the big concerns.

Of course how much of an issue these concerns are all depends on the quality of the compost - its components. If it already contains some soil as good compost would then they are less of an issue. It is it straight organic matter with no soil - no dirt - then you need some.

Common % recommendations range from 30% to 60% compost and the rest good old dirt.

Dave

I've been growing in partially finished compost for a couple of years. The two issues I have is maintaining moisture (the more unfinished the compost the faster it dries) and settling. Next year I am going to add more dirt to the top 6 inches. My beds are about 2 feet deep. The plants grow well if kept watered.

I tried this as an experiment this year when I thought my soil was harboring fusarium yellows. The broccoli grew pretty well there, but I had to add quite a bit of nitrogen and water. What happened is exactly what ljpother suggests - the compost settled over the season from 1 foot deep to about an inch.

Thanks everyone! I will NOT be adding just compost to my new raised beds. I think I'll continue to do what I have been doing, a mix of half soil, half compost.

Michele, I just went through this process three years ago. I loosened the soil in the bottom of the box so that plant roots could grow down into it and then filled two of my beds with compost alone (finished and homemade). I could see no appreciable difference in the performance of those two boxes with the others that were a native soil and compost or manure mix. I had to add nitrogen to ALL of them. A certain amount of settling is a yearly issue. I add compost and some manure each year for both soil quality and the settling. My advice to you is whatever you use, get a soil analysis this winter so that you really do know what to add next spring. Good luck!

This link may be of interest to you.

http://forums2.gardenweb.com/forums/load/soil/msg1008133312877.html?19

I tried that for a pumpkin plant and ended up having the shallowest root system because of "shrinkage" if the mound. However, the least amount of soil you can add would be good because my experience in Arizona has been the more compost you have, the lesser amount of destructive insects you will encounter as well.

Thanks again. Great link Joeworm, that was very helpful :). I can't wait to get started this weekend. My goal is to have all my beds ready for planting next spring.

I guess I'm the lazy gardener. We have thick sod at our place. Last weekend I started two new beds using the lasagna method. This is how I've started all my beds. First, a thick layer of newspapers and then layers of grass clippings, kitchen scraps, dead plant material (i.e. old bean plants but not tomato plants that may be diseased), chopped leaves from the lawnmower bag, and a heavy sprinkle of wood ashes, and some aged manure. You're supposed to alternate greens and browns. This pile will sink a lot by spring. In the spring, a layer of aged and composted horse manure (mixed with bedding, kitchen scraps, etc) is added to all the beds. Right now, the two beds are about 24 inches tall.

Sometimes the used soil/dead plants from pots gets added.

I built my first bed in 2007, planted it in 2008. I've been very happy with this system since we have plenty of the ingredients. I get the composted horse manure from a Craigslist find. Perhaps I add enough soil, wood ashes and manure to get a good mix. (We heat with wood.) I started the first bed with old grass clippings from a neighbor and kept adding different material as it became available.