Soil is one of the main ingredients used in the art of bonsai. To state the obvious: your bonsai needs soil. It needs soil to entangle its roots in and, most importantly, to get its nutrients from. Because the soil is responsible for all the tree its nutrients, the mixture of the soil is essential for its success. Different trees have different needs and therefore require different soil mixtures.
A soil mixture can vary in number of components, texture and purpose. Especially the latter is important when deciding on what soil mixture to use. Some bonsai trees may need a soil mixture with extra drainage capabilities, others may need a more moist holding mixture. There are also species that require a more acidic soil mixture.
Soil mixtures can consist of a lot of different components. Below you will find a description of some of the most common soil components used in bonsai culture. This is not a conclusive list. Bonsai trainers in the global community have come up with some really interesting and inventive ways to create a specific soil mixture. As a bonsai trainer you alone are in control of the soil mixtures you trees grow in. So, you can decide for yourself what components fit your tree’s needs best. It all starts with an understanding of what the individual components are used for.
Some of the most common soil components and their characteristics:
- Akadama: Akadama is a natural volcanic Japanese clay. It’s a (commercially produced) hard-baked clay that is widely used for bonsai purposes. Akadama is used to improve the drainage of the soil mixture and comes in different grades (from small, to medium and ever larger particle sizes). Because it’s a hard-baked clay it produces fine grit or dust, which should be sifted out before adding it to any soil mixture. And even after carefully sifting, some fine dust will get mixed into the soil. You should counter this by simply watering any newly planted bonsai tree until clear water floats from the container. Why? Well, the fine akadama grit can counter the exact purpose that Akadama is used for; drainage.
For more information about Akadama, check out the Daily Bonsai Akadama Wiki page.
- Pumice: Pumice is solidified lava foam. It is used to retain water, give off nutrients and provides a fantastic base for root growth. A lot of different sorts of pumice are available, but the light weight, soft pumice is the one most commonly used in bonsai cultivation. This form of pumice has an open and brittle structure that works specifically well for bonsai trees and their roots.
For more information about pumice, check out the Daily Bonsai Pumice Wiki page.
- Lava rock: As the name would suggest, this type of rock originates from volcanic lava. Lava rock is commonly used for the same reasons as Akadama and pumice; water drainage & aeration. Lava rock is harder than Pumice and roots cannot grow on it. It will retain water quite well and provide a good structure that improves aeration within the mixture.
For more information about lava rock, check out the Daily Bonsai Lava Rock Wiki page.
- Grit: Grit, or fine gravel, is a garden-variety alternative for Akadama, pumice and lava rock. It is used for water drainage and aeration of the bonsai soil mixture. Since the above mentioned alternatives became popular, grit is hardly used anymore. But it is still an adequate option if you don’t have any Akadama, pumice or lava rock lying around.
For more information about grit, check out the Daily Bonsai Grit Wiki page.
- Organic potting ground: organic potting ground, or potting compost, is actually already a mix of several components. Usually it consists of sand, peat moss (humus), perlite and sometimes conifer bark and other composted organic materials. Organic potting ground is highly water retentive. Be cautious when adding it to your soil mixture because it could very well become a threat for the entire tree. The main reason to still add it to your mixture is because of its retentive and bounding characteristics.
For more information about organic potting ground, check out the Daily Bonsai Organic Potting Ground Wiki page.