Basic points about soil and pots for a tree in a container. This is not a mix recipe for larch or yew, and it does not replace the grit, sand, peat, and sphagnum recorded in the raft case study.
North Carolina State University’s Extension Gardener Handbook, chapter 18, is about container plants in general, not bonsai. It is used here because it states a few limits that apply to any pot. Field soil, it says, often holds too much water, has too little pore space for oxygen, is heavy, and may carry diseases, insects, and weeds, which is why container growing moved to soilless mixes. Ingredients it names as common are perlite, pine bark, sand, sphagnum peat moss, and vermiculite, in combinations, not as one mandatory formula. It says drainage holes are necessary so roots do not stand in water and rot. It says a layer of gravel or rocks in the bottom does not improve drainage: water sits in the mix above the gravel until that mix is saturated. Porous pots such as unglazed clay dry faster than plastic or glazed pots and need more frequent watering. It says to repot in early spring, before plants start actively growing, when the reason is a healthy plant that has filled the pot. Those are container-gardening statements. They are not a bonsai show calendar, and they are not a hardiness zone.
The Piedmont Bonsai Club in North Carolina, a society rather than an extension service, answers whether special bonsai soil is required by saying a pre-made bonsai mix is optional. Many of their members use sifted pine bark soil conditioner, fine gravel, and a little peat moss or coconut coir. That is their everyday mix, described for their group. It is not the tray mix in the raft case study. On the raft larch he prepared a tray of grit and sand and peat, dusted damaged bark with hormone rooting powder, and top-dressed with sphagnum moss for eighteen months. Later he moved through ceramic pots and wrote that a stone slab was his preferred option once he had prepared it. The yew went into its current pot after a spring bare-root. Those details stay on his pages. This page does not correct them into pine bark.
The same club says that in their area, trees should be repotted in early to mid-spring, and that once the first 80-degree days arrive in May their potting season is over. Some members repot in mid-fall, which they call riskier because of early cold. That is Piedmont North Carolina, not Wales, not Scotland, and not a date for the larch the raft notes say are unforgiving once the buds have burst. Season, as far as it can be said without inventing dates, is on seasonal timing.
His beginner note still stands beside all of this: do not be in a hurry to put a tree in a bonsai pot, and do not wire and repot on the same day. A pot with no drainage, or a root ball left standing in a saucer of water, fights the “moist but not soaking” line in his myth replies.