For many bonsai enthusiasts, their first tree came from a big box store, mall, or roadside stand. Typically, these trees come planted in garden or potting soil, which is not a suitable substrate for vigorous bonsai growth. In this article, we’ll explore why bonsai trees need special soil considerations, and discuss the proper type of bonsai soil to provide for your tree.
It’s All About the Roots
So, why do we care so much about bonsai soil? After all, we can plant a tree in our yard and it grows fine. Why does the miniature version of that tree need special soil?
The answer lies in the roots of your tree.
Most first-time bonsai owners don’t think much about what’s inside their bonsai pot, as they’re more interested in the beauty of the tree itself. But that beauty can only sustain itself if the tree has a strong foundation. And that foundation are the tree’s roots.
When it comes to bonsai, we’re interested in developing fine, fibrous “feeder roots.” These roots consume the nutrients in the soil and feed the rest of the tree. Without a strong network of feeder roots, your tree will become weak.
Normal potting soil doesn’t provide an environment which encourages the growth of feeder roots. So we must consider a different type of soil for bonsai trees.
Is Bonsai Soil Really “Soil”?
The irony in the term “bonsai soil” is that there isn’t really any “soil” in most bonsai soil mixes. Instead, a mix of substrates like crushed lava rock are used which more closely resembles gravel than soil.
This type of mix provides an environment where the tree can more easily grow feeder roots that it needs to thrive.
How Can Bonsai Soil Encourage Root Growth?
Before we go into specific soil mixes for bonsai soil, let’s take a quick moment to understand the type of environment needed to grow strong, vigorous roots.
The roots of your tree require:
To provide those requirements, we need bonsai soil that has these properties:
- Water Retention
Bonsai Soil Aeration
The roots of a bonsai need oxygen. If they are tightly packed in garden soil, it’s difficult for them to receive the oxygen they need. A loose bonsai soil that has room for fresh air to permeate between the particles provides proper aeration for the root system.
Bonsai Soil Drainage
Think of a bonsai tree planted in garden soil in a small container. When you water it, that water will remain in the soil for several days, or even a week. During that time, the roots won’t access to the oxygen they need. Starving your tree of oxygen will weaken the tree.
Additionally, many bonsai trees don’t like “wet feet.” That is, they don’t like to soak in water for long periods of time. Over-watering a bonsai planted in garden soil can actually rot the tree’s roots. This “root rot” is detrimental to the tree and may eventually kill it.
A proper bonsai soil allows for excess water to escape through the pot’s drainage holes after watering bonsai.
Bonsai Soil Water Retention
As much as we need good drainage, we also need some water to remain. If we used actual gravel as the substrate, water would flow directly through the pot and out the drainage holes leaving no water behind for the roots to utilize.
Because of this, we need to include some type of material in our bonsai soil mix that retains some water.
A clay-like material called Akadama, which originates in Japan, is commonly used to achieve the proper water retention in bonsai soil. Akadama is available as granules that have been baked to create a gravel-like substrate.
Organic vs. Inorganic Bonsai Soil
Bonsai soil is commonly classified into two categories: organic and inorganic.
Just like it sounds, this type of soil has no organic compounds. This means that any nutrients that the tree needs will be added via fertilization, because there are no inherent nutrients in the soil itself.
Three types of inorganic substrate are commonly mixed together to form bonsai soil.
- Lava Rock
This volcanic rock is light-weight and porous. It retains some amount of water, but is typically considered a dry part of a bonsai soil mix. It is used as a component in well-draining soil mixes. Pumice is very hard and should not break down.
Also a volcanic rock, lava rock provides bonsai soil mixes with water retention using its porous properties. Its rough, jagged surfaces, also help develop fine feeder roots.
This clay from Japan is the primary component in soil mixes used to hold water. It is mined, then fired, creating a hardish material that slowly breaks down over the life of the soil.
For conifers, like pine and junipers, the mixture is typical 1:1:1 ratio. Meaning, 1 part akadama, 1 part lava rock, and 1 part pumice.
For deciduous trees, like elms and maples, that mixture is slightly different, with a 2:1:1 ratio. Meaning, 2 parts akadama, 1 part lava rock, and 1 part pumice.
Some bonsai soil mixes include organic compounds, like peat or bark. These elements add nutrients to the soil, but can also break down and inhibit drainage. For that reason, the popularity or organic bonsai soil is decreasing.
Where to Find Bonsai Soil
If you’re lucky enough to live close to a bonsai nursery, you may already have access to premixed bonsai soil. For those who don’t, other options are available.
If you purchased a bonsai planted in garden or potting soil, you can set it up for success by repotting it at the proper time into a well-draining soil mix more that is more adequate for strong root growth. This root growth will provide vigor in your bonsai tree, helping you produce a healthy plant to enjoy for years to come.