Bonsai trees require a very well-draining soil with special characteristics and frequent watering to ensure the correct mix of air and water in the soil. Planting bonsai trees in garden-variety potting soil can quickly result in root rot and damage to the trees. Most bonsai soil is made up of three components. The exact mix of the components vary slightly, depending on your climate.
Many bonsai soils use an inorganic component like baked clay as a main component. In some cases, you can keep a bonsai in only baked clay as a growing medium with no adverse effects. Akadama is a Japanese baked clay. Akadama is available in a number of grades, based on its durability. Higher grades break down more slowly and are best for trees that do not deal well with root baring, like pines. Although Akadama is very cheap in Japan, it is often expensive and hard to find in the U.S. Any medium to medium-fine baked clay potting soil works as a good substitute.
Pumice is a volcanic rock that acts as a grit, which reduces moisture retention in your bonsai soil. Although pumice often appears whitish, some bonsai pumice is made from black pumice. The exact mix of pumice with your clay depends on how much water your trees require. A good starting point for people who prefer to water their bonsais every day or two is often 15 to 20 percent pumice. Pumice also helps to increase the air content of your bonsai soil.
Sifted pine bark retains water and adds organic components to the inorganic pumice and baked clay. Add a larger percentage of sifted pine bark to your bonsai soil if you live in an arid or semi-arid climate. If you live in a humid region, add less bark to limit water retention.