Naoki,
we used information, from two sources, our Forestry folk and Bonsai Today Magazine.
Our local pine tree forests are grown on sand soils, with little organic matter. As one officer put it, the organic matter is in the trees.
If grown in clay soils, they are usually short and will often die.
So we use builder's gravel [ silica] 3 mm for seedlings and 5 mm for growing trees , an organic [ compost / coco peat / peat moss ]
and earthenware pots.
The training of pines requires trunk thickening, which the Japanese can do in clay pots.
Ask Sifu [ Adair ] for help when you past the health stage.
After germination they are left as single seeds in single pod germination trays. You know the common black plastic
trays used for seeds [ tomatoes etc ] Sun - morning sun and until around 1.00 p.m
Then 6 months to year [ depends on if you give a gentle tug and squeeze the tray cubes gentle and the plant comes
out with all of the soil ]
Simple transfer to 8 cm diameter x 6 deep cm clay containers.
Water and fertilise when needed. Placement full sun or sun until 2.00 p.m.
Next is a year later or by observation, into a clay pot that is 15 cm diameter but still only 6 cm deep
This is just because the potter's make that size, not any special reason.
However, we have noted that the clay container can get wider, but no deeper than 12.5 cm to 15 cm deep.
They seem to prefer a broader pot, not deeper.
The clay containers should be porous.
We just keep getting wider as containers go, but not deeper.
The Idea here is health and just getting the pine to grow and trunk thicken. Our sunlight encourages adventitous buds.
I will stop here as you need to just grow for three to four years.
I read over at AUSbonsai, that the J.B.pine, when training, in zone 10 and milder climates, might be treated more as
a simple tropical tree. We are exploring the idea, too soon to discuss.
Left images over in the Conifer section of BNut, will locate and add new images here, hopefully by Saturday.
Plate is a bit full presently.
Thank you for your patience.
Good Day
Anthony