Soil does the following: it anchors plants, it feeds them, it waters them,
and it allows roots to breathe. Each of these things is important. In
bonsai, the nutritive properties of the soil are not very important, since
we fertilize the trees frequently. So what's really important in bonsai
soil is textu the soil must retain water but also allow lots of air to
reach the roots. To do this, you need large particles, so that lots of
air spaces are left in the soil, but some of the particles should be made
of a water-retentive material, usually some kinds of porous rock or
organic material. Generally, bonsai people mix gravel, high-fired clay,
and fine bark particles. I make my bonsai soil out of pine bark,
grani-grit, and turface, but I could use completely different ingredients
and get the same combination of air-space and water retention.

As Kitsune said, you have to adjust the proportions according to your
climate. In Arizona, you'd need more water retention than you'd need in a
cooler, wetter climate, so you might decrease the size of the air-pores
and increase the amount of water-retaining ingredients. You learn this by
trial and error. It will also vary somewhat by tree species; some plants
need better drainage than others.

Nina.

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