You can grow a tree in mostly any soil and it will do fine, but that doesn’t mean it is the best bonsai soil for the tree. Growing trees in a small amount of soil increases the need to make sure you have the best soil conditions possible. Some trees enjoy sitting in large amounts of water and are not affected by their roots always being wet. Other species are known to experience root rot if the trees are not allowed to dry out a little between watering. Another aspect of biology that is commonly ignored is oxygen. Trees absorb oxygen from their roots, so roots sitting in water block this ingredient to success. Every type of tree prefers a different level of moisture in the soil, but I will discuss ingredients commonly used in bonsai soil in this post.
Compost – Compost is great to add to the soil mixture because it is high in nutrients. Using such a small amount of soil makes the need for it to be nutrient rich much more important.
Peat and bark – These are both used to add grit to the soil and increase drainage. These add some nutritional value to the soil while allowing the water to flow through the soil faster.
Akadama – This is one of the most used ingredients in bonsai soil mixtures. These fire-hardened clay pebbles allow the water to flow through the soil mixture very easily. Akadama can be easily obtained online for a relatively low price.
Charcoal – Charcoal is traditionally used as a soil additive because it increases the drainage of the soil and helps fight bad bacteria.
Sand – Sand is another commonly used ingredient to making good bonsai soil. You can find sand everywhere, but you want large grit well-draining sand for bonsai applications. This helps increase the drainage of the soil.
Moss is also very helpful in retaining moisture in bonsai pots. If you notice that your bonsai soil is drying out faster than desired, growing moss on the surface can help trap the moisture in. You can find “bonsai moss” online, or collect it from outdoors in sunny areas. Moss can also make a bonsai tree look older and more mature.