Enclosed raised beds are a hybrid between container growing and inground conditions and often combine the best of both as well as combining soil requirements. Unenclosed raised beds are more like inground plantings and other than needing to import enough soil to achieve the bed height, you can pretty much follow all inground planting techniques and recommendations.

I generally avoid anything labeled 'topsoil' simply because it doesn't mean anything - topsoil is whatever the provider wants to call it and can be (but not always) pretty crummy stuff. I would look for a planting mix - this is typically some combination of loam, compost and a larger textured drainage enhancing amendment, often bark or bark fines. This will be easier to locate in bulk than bagged and will also be far cheaper for the quantity you will need.

If you cannot locate, you can go with the topsoil but I'd examine it closely first to make sure it is not full of rocks and other junk and to determine what other amendments might be required. Compost may very well be called for as well as fine bark (sames as bark 'fines' -often sold as a mulch or soil conditioner). Avoid very chunky bark nuggets.

Just keep in mind that Japanese maples prefer an organically enriched, well draining soil that is slightly acidic and aim to duplicate those conditions as closely as possible.

I used the pine bark nuggets from Lowes. I added in some perlite, some compost, and topsoil. All from Lowes and HD. It works well in a raised bed and the maples are VERY happy. I'd say it was 50% pine bark, 20% perlite, 20% topsoil, and 10% compost. I doubt you can go wrong as long as you have enough pine bark. Don't buy the premixed stuff. I've used it before and it just about killed the maples. I doubt you will get good enough drainage unless you are using a cactus mix.

Improved drainage is pretty much a given in ANY raised bed situation just by virtue of the elevation. And it is unfair to dismiss ALL premixes as unsatisfactory for this purpose when they can differ so widely. Buying individual bagged soil ingredients for a raised bed, especially one that is 18" deep, is generally going to be cost-prohibitive also. Just a 4'x4' raised bed 18" deep requires the better part of a cubic yard of soil to fill.....bagged soils/amendments are most often just 1-2 cubic feet of product and there's 27 cubic feet in a cubic yard. You do the math!!

Find a good local bulk soils products provider and check out their stuff. Look at what they may term "landscape" mix or a 3-way soil mix. It should have a rich dark color and good texture and just a good earthy aroma. You should be able to see some bark remains in the mix also. If this is unavailable, you might want to consider using the topsoil combined with compost or composted bark.

I've built a good many raised beds and berms in my time, both for myself and for landscape clients and this combination of products is what I use. But stay away from the individual bagged stuff........way too expensive and too much work to assemble and mix for a decent sized bed :-)

Most bark, east or west coast, is ground up at some point and time. From there, in may go to a bark "wholesaler" who may run it across screens to get certain size particles (like the small pine nuggets you found). The "fines" are normally the dust and debris that falls through the smallest screen. It may not be bagged or sold in your area...maybe closer to where the bark orginated and available in bulk (sometimes it can end up being the "waste product" of the bark industry.

Schmoo

Thanks for all the good info everyone. I got about six yards of a 50/50 mix of topsoil and leaf compost. The topsoil portion was a really nice dark brown color and very fine in texture with no rocks. It had that good earthy smell that was mentioned.

Home Depot has a bagged product called "Clay Breaker - Soil Conditioner". The product has gypsum added in. It is in an orange bag and appears to be very fine bark pieces. Would the gypsum help or hurt or have no effect. I'm thinking about mixing in a couple bags of this where the JMs would be.

Thanks

Gypsum will have pretty much no effect other than adding some calcium to the nutrient base. Gypsum as a soil conditioner is really only of benefit to sodic clay soils. But the bark fines will work with any soil type :-)