Honestly, Lava Sand, Mushroom Compost 1:1 with a dash of bat guano and earth worm castings.. sometimes some Volcanite.

If it's a tree or shrub that's not going in a bed, I'll take the above as part one and mix that into an equal amount of the soil it'll be going in later to get it used to that soil.

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Shepherd of the Trees
It is not our part to master all the tides of the world, but to do what is in us for the succor of those years wherein we are set, uprooting the evil in the fields we know so that those who live after may have clean earth to till.

What fertilizers were used with the topsoil?

Sounds like you're over watering. With an organic program you don't have to water as much. You want to stimulate the microbes and feed the soil, not the plant. Might want to find other containers because you might not have proper drainage.

Mixing the organic program with the synthetics is a waste of time and money.

Sorry I missed the boat. Just trying to help.

In my experience what you want to aim for is a mixture that not only drains well but is light and fluffy - easy for the seedling roots to grow through. That's where our native soil (even amended) fails. You're right - Metro Maples uses a mixture of forest humus and pine bark in their mix. If you look at Japanese Maple or Rhododendron seedlings/small grafts/rooted cuttings from the most reputable growers in the Pac NW (Iseli, Van Veen, etc.) you'll find they use pretty much 100% peat.

For my young Maples I use a mix of about 25% acid-mix soil from Soil Building Systems (substitute quality organic soil enriched with forest humus and a little sand - acid mix not necessary for non acid-loving plants obviously), 25% peat, 25% pine bark (I mash it up into smaller pieces than come in the bag), 25% Perlite, and this time of year I add a healthy dose of ground up oak leaves. Once I have the basic mix I adjust until I have a good fluffy result. (Note, a lot of Maple hobbyists I know don't use any soil, just some combination of the other ingredients.)

If my ultimate goal for the seedling is to plant it in the ground or even in a raised bed I'll introduce percentages of that soil into the mix as the seedlings grow and I repot.

I saw the comment about using native soil for oaks too - I took some red oak acorns from my yard and potted half in a native soil/compost mix and the other half in the mix I outlined above. We'll see what happens.

Lou-
The goodly amount of Lava Sand I use in my potting mix keeps a good balance of moisture, which is why I use it. Holds a bit of water in it to release later when the soil dries too much, but in the mean time keeps the overall soil light enough to shed excess water and keep a reasonable gas exchange going. End result- no soup.

I use organic fertilizer in this environment and it's been fine. There's probably a good chance that the decomposed granite Howard likes will have much the same effect.

_________________
Shepherd of the Trees
It is not our part to master all the tides of the world, but to do what is in us for the succor of those years wherein we are set, uprooting the evil in the fields we know so that those who live after may have clean earth to till.