One question first:
Is it common in your area to see Acer palmatum's 30 feet wide and tall?

Let's start there.

Thx,

Dax

Also, tell me about your clay. Does it rain enough where you are that you can consider your soil type to be "workable" or is it like concrete... requiring serious tools, to excavate it? Also, if you know, what is the ph?

This will help (myself)/others, immensely.

Kindly,

Dax

I live in coastal SoCA; its quite cool with a lot of morning cloudiness. Rarely above 75 here except occasional hot ultra-dry Santa Ana winds in winter. This tree would get direct afternoon sun, but it's had full sun at the nursery with no ill effect. I've never seen a 30x30 ft wide A. Palmatum, even when I lived in Seattle. Around here, most Japanese maples are seen around commercial buildings.

My clay soil is reasonably workable unless really wet. Other trees and shrubs do quite well. Almost anything will grow well here once it gets going. My concerns with this tree are starting it properly. I only have hand tools and a Mantis tiller. No means for serious excavation.

In your case I wouldn't be all that concerned, then. I would use that mantis and amend only with perlite to help fluff it up a bit and provide a bit better drainage. Other than perlite don't use any organic materials except mulch to top-dress.

You'll want to amend at least 4 times the width of the rootball. 3 is the norm but the more the better.

With a lot experience...

Regards,

Dax

The soil in the box is about 20" deep. To what depth should I add perlite and what ratio?

I would skip the perlite and back fill the planting hole with native soil only, to an immediately post-planting height of about 2"-3 above grade (to allow for settling), then add your topsoil mix up to the basal flare ht and taper outward. Perlite won't/can't improve aeration or drainage in clay soils unless it's by far the most significant fraction of the whole.

To illustrate: How much perlite do you need to add to a bowl of pudding to increase aeration and improve drainage? I can't be done. The perlite would need to be more than 50% (more like 60-75%) of the whole before it begins to have an impact, then you would technically be adding pudding to perlite. The clay particles will simply settle in around the perlite after a few irrigations or a rain or two & the o/a affect on aeration/drainage will be exactly the same as if it wasn't even there.

Even if the perlite was able to fluff up the soil or improve drainage, it would have to be as a result of the creation of air spaces in the soil, which we can see it will not/can not create. Taking it a step further, if it WAS able to create a fluffier soil with more air and better drainage, it would have created a bathtub effect, with the air spaces in the 'tub' filling with water whenever it rained or when you irrigated heavily - something that, if it was possible, you undoubtedly would not want.

Al