Bonsai soils are more about the physics of the soil than they are about what material the particulates are mad from. In most cases, we avoid peat because it is small enough that it collects in macro-pores, turning them into micro-pores that hold water instead of air. In my estimation, compost is worse than peat because it holds water tighter and its water release rate is slower. Plus, peat and compost break down too quickly to be of value in most soils for bonsai.

Particles of a uniform size and shape will yield best drainage. Experiment: In a clear bottle, add a hundred or so large marbles - the boulder type, twice as large as a regular marble. These fill a 2 qt clear container and represents a soil with uniform size particulates and plenty of air spaces, a beautiful thing, this soil. Then, take a quart of BBs, dump it on top of the marbles and give it a little shake. The BBs find their way into the large pore spaces, but still fit handily into the "full" bottle. Then, pour a quart of sand (you can substitute the word peat or compost for sand here, too) into the "full" bottle. The sand filters down, clogging the remaining air pores and making the soil a terrible choice for plants. Sand is death to aeration and how did I get 4 qts of soil in a 2 qt bottle? By jamming small particulates into my air-holding macro-pores.

You can build a wide variety of soils if you have a few things on hand. A porous mineral product that holds air, moisture, and nutrients - like Turface, Haydite, akadama, pumice, others. An irregularly shaped mineral product, preferably with rough or jagged edges that doesn't hold air or water - crushed granite or large silica (almost BB size) is good here, and an organic component in a suitable size that will resist rapid breakdown by soil organisms. We often use conifer bark to fill this requirement because the combination of a high % of lignin and suberin (in the bark) makes it difficult for micro-organisms to cleave hydrocarbon chains and destroy the structure of the organic part of your soil.

I can't think of anything I grow in bonsai soil (houseplants too) that I use more than a 1/3 organic component on. I usually keep a large amount of soil that is 1/3 each of Turface, crushed granite (grower grit), and screened fir bark. I amend this basic mix as needed. Your mix may vary & use other ingredients, but this one won't be too far from what will work well for you.

I grow most deciduous plants in the equal parts mix. Pines & junipers I use up to 90% inorganics in the soil. I even have a few plants that are perfectly happy in 100% Turface. Experiment, but don't be tempted to compromise your soil mix too much to simply extend intervals between watering - your plants will pay.

Al