Bonsai soil is gravelly. Usually different grades of gravel are used. Screens are used to grade the gravel. Bonsai need a free draining mix and for a small bonsai fine gravel is good. Larger trees in bigger pots use different grades of gravel. Watering may change because gravel will dry out faster than peat based mixes. You would fertilize the same. I use a slow release fertilizer in my pots.
I live in the tropics, my plants are outside all year round and I water nearly every day so I use pure gravel and slow release fertilizer. The junipers get a little bit of added peat moss. Succulents, desert rose don't like very wet soils. Ficus is pretty forgiving. If you don't water that much and your plant dries out fast between watering you can use a mix that contains some compost. Aeration and drainage is the most important thing in keeping any potted plant alive.
BTW wash the gravel before you use it to remove as much of the fine dust as possible. As the rocks erode and dust is collected from the air, it blocks air spaces, roots take up air space too. The fine dust and rock dust settles at the bottom of the pot. Most bonsai only have small drainage holes so you don't want mud at the bottom of the pot or anything to impede free drainage.. That is also one of the reasons why bonsai are so frequently repotted. I prefer unglazed bonsai pots and concrete or terra cotta training pots because they breathe.
Recommended soil mixtures
Different tree-species demand different soil-mixtures. Below are two main mixtures, one for deciduous trees and one for coniferous trees. Both mixtures consist of Akadama (to provide the mixture with aeration, drainage and medium water retention), organic soil (nutrient rich, water retaining) and split (drainage).
Note that both mixtures can, and should, be adapted to your local circumstances. When living in a relatively wet climate, use more grit/akadama for your bonsai plants.
Deciduous tree bonsai soil
25% Organic potting compost
Coniferous / Pine tree bonsai soil
10% Organic potting compost
Kaizen website has a good explanation of how, why, and what to use for bonsai.
https://www.kaizenbonsai.com/bonsai-tree ... ONSAI-SOIL
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