Bonsai soils are similar in many ways. By using a number of components, bonsai soils are designed to drain quickly and offer good oxygen contact with tree roots. Different soils are really just different combinations of these components. By understanding the components of the soil, you can either mix your own soils or be better prepared to select a commercial or nursery-blended soil that will work well in your local growing environment. Different species of trees can also benefit from changes in soil composition.
In the past, people growing bonsai in the West tended to use organic soils that drained well. Peat, bark and leaf litter were used as primary soil components. One problem with these components is that, although they drain well, they retain too much water for bonsai root health. In some climates, peat can become difficult to water because it dries out too quickly. Another problem with organic soils is that they break down. A soil with properly sized particles for bonsai health will eventually break down into fine particles that will interfere with drainage. However, a small percentage of these organic components can help with a number of trees that need a little additional water retention. In general, 10 to 20 percent organic material should be sufficient in a bonsai soil.
Most Asian bonsai masters use a soil made primarily of inorganic components. Inorganic soils are made from baked clay. These soils retain their size and shape over a longer period, and will maintain their ability to both drain properly and retain adequate water for your bonsai trees. The best-known Japanese baked clay soil is Akadama. However, Seramis, Turface and Oil-Dri are three baked clay soils commonly available in the United States and the United Kingdom. Unscented, pure baked clay cat litters can also work well as a bonsai soil.
Certain baked clay soils, such as Akadama, will eventually break down over a year or two and need to be washed from the roots. Therefore, Akadama type soils are not recommended for trees, like pines, that will not tolerate bare rooting.
Sand and Rock
Sand and fine pebbles, like aquarium gravel, can help improve drainage for trees that either need a good portion of organic material or require very fast drainage. In general, 10 to 20 percent of sand and rock should offset an equivalent amount of organic material, if you are using organics.