It can be tempting to think that “soil is just soil”, and you can plant bonsai trees in any old soil you purchase in a bag or find in your garden. But in fact, nothing could be further from the truth!
Planting a bonsai tree in the wrong soil type is one of the biggest mistakes new bonsai tree keepers make.
Soil doesn’t just provide your bonsai tree with the nutrients it needs to grow. It is crucial to give your bonsai tree just the right amount of water and oxygen.
Bonsai soil ensures that water is retained and drained away as it should be and that the roots of your bonsai tree receive enough oxygen.
In this article, we will get our hands dirty as we take a close look at bonsai soil.
What makes bonsai soil different? What is it made of? What types of bonsai soil exist? And how can you make custom soil for your bonsai tree?
How Is Bonsai Soil Different From Normal Soil?
The soil in your garden is made of eroded materials, such as rocks, and naturally-occurring organic materials, including lots of decaying plants and animals that have accumulated over time.
Bonsai soil is different. It uses a carefully formulated mixture of both organic and inorganic materials and is created with a precise balance of ingredients to suit the particular needs of different types of bonsai trees.
There isn’t just one type of bonsai soil, but many variations.
Each type of bonsai soil contains different ingredients, carefully chosen by gardeners depending on the needs of their bonsai trees.
Bonsai Soil Or Bonsai Substrate?
You’ll often hear experts talk about “bonsai substrate” rather than bonsai soil.
Substrate means “an underlying substance or layer”, so bonsai substrate is the material surrounding your bonsai tree’s base and roots.
The term substrate is often used instead of soil because “bonsai soil” usually doesn’t include much actual soil!
When you look closely at the material surrounding a bonsai tree, you’ll often find that the bonsai substrate looks more like gravel than soil.
What Soil Is Best For Bonsai Trees?
The best soil for bonsai trees is a type of soil that allows the tree’s roots to get the oxygen and nutrients they need and just the right amount of water.
The amount of oxygen that reaches the roots is determined by the aeration of the soil. And the amount of water than reaches the roots is determined by water retention and drainage.
To understand how aeration, water retention, and drainage combine to make the perfect bonsai tree soil, let’s look at these three factors in more detail.
Bonsai Soil Aeration
People often think of tree roots absorbing water or nutrients from the soil, but they forget that roots need to absorb oxygen too.
A bonsai tree’s roots need an oxygen supply in the form of air pockets within their substrate to grow.
If a bonsai tree is planted in a very dense material with very few pockets of air, the roots won’t be able to get the oxygen they need.
This is why bonsai substrate usually resembles loose gravel rather than tightly packed soil. The larger particles of a gravel substrate allow lots of air pockets to form, which gives the bonsai tree roots all the oxygen they need.
Bonsai Soil Drainage
If your bonsai soil does not have proper drainage, excess water will remain in the soil rather than draining away.
Your bonsai tree needs water, but too much water can be harmful.
If water accumulates around your bonsai tree’s roots, rather than draining away, it will prevent oxygen from getting to the roots and will cause root rot.
There are two ways to provide sufficient drainage for your bonsai tree.
Firstly, use a bonsai pot with drainage holes in the bottom. And secondly, use a bonsai tree soil that allows water to pass through easily, rather than a material that becomes waterlogged and lets too much water accumulate around your tree’s roots.
Bonsai Soil Water Retention
As important as it is for water to drain away from your bonsai tree’s roots, we do still need some water to be retained within the soil. After all, that’s the point of watering your tree!
This is why creating a bonsai tree soil is such a delicate balance.
The soil needs to provide enough space for air to accumulate and enough space for water to drain away, but not so much space that the water drains away too quickly, leaving your bonsai tree thirsty.
What Is Bonsai Soil Made Of? 8 Common Ingredients
Popular bonsai soil ingredients can be divided into two types: organic material and inorganic material.
Organic soil is made up of material that originated in living creatures. This organic matter is made by plants and animals dying and decomposing.
Inorganic soil is made from materials such as rocks and clay, which do not contain significant amounts of organic matter.
Your bonsai soil will likely contain both organic material and inorganic components.
Below, we list eight standard bonsai soil components, including three organic and five inorganic ingredients often found in bonsai soil.
Organic Ingredients
- Potting Compost: A loam, peat, and sand mixture packed with nutrients. Use sparingly to ensure your soil drains well.
- Peat Moss: Moss and other organic material decomposed in peat bogs. Good for retaining moisture and binding your soil together.
- Pine Bark: Aged chips of bark from a pine tree. Pine bark can retain some moisture while also draining well.
Inorganic Ingredients
- Akadama: A type of hard-baked clay that retains a lot of water.
- Turface: A ceramic soil conditioner that facilitates good aeration in the soil.
- Pumice: A porous volcanic rock that is good for drainage and aeration.
- Lava Rock: Another volcanic rock, similar to pumice, and good at retaining heat in the soil.
- Gravel – Small loose stones that allow air and water to flow easily around bonsai roots.
These ingredients and others are mixed in various combinations to make types of bonsai soil with different properties.
Adding more of one ingredient might make the soil drain more quickly, while another component could make the soil retain more water.
The key is to get the balance right for your bonsai tree.
How to Make Bonsai Soil
The best mixture of bonsai soil ingredients depends on the species of bonsai tree, but the following is a good rule of thumb:
Deciduous trees (trees with leaves that fall off annually): 50% akadama, 25% pumice, and 25% lava rock
Evergreen trees (trees with leaves/needles all year round): 33% akadama, 33% pumice, and 33% lava rock
To ensure enough water absorption, you may also want to add an organic ingredient, such as potting soil. However, this isn’t always necessary.
We recommend starting without potting soil or only adding a small amount initially – no more than 20% of the total mixture – then monitoring how well your soil drains and retains moisture.
If your soil is draining too quickly, add more potting soil or another material that retains moisture, such as pine bark.
If your soil is retaining too much water, or there are signs that your tree is not getting enough oxygen, then you need to add more porous material, such as lava rock.
Buying Bonsai Soil Mixes
Of course, if you don’t want to dive right in and create your own bespoke bonsai soil mixtures, you can buy ready-made bonsai soil instead.
Our favorite commercial bonsai soil brand is Tinyroots, and their soils are available in deciduous and conifer varieties.
Tinyroots Deciduous Blend
Containing akadama, turface, river sand, and pine bark, Tinyroots Deciduous Blend is perfect for deciduous trees such as maples, hornbeams, elms, and beeches.
Tinyroots Conifer Blend
Tinyroots Conifer Blend is designed for junipers, pines, cedars, cypress trees, and other coniferous trees. It contains the same materials as the deciduous blend but in different proportions to suit coniferous trees.
The advantage of buying Tinyroots blends is that they are incredibly well-respected, proven to promote vigorous root growth, and it saves you the hassle of mixing your bonsai soil.
However, there is a downside as the products are not cheap and may be unaffordable for some bonsai tree keepers.
Can Bonsai Trees Grow In Normal Soil?
If you don’t want to spend money on premium bonsai soil and are unsure about making a bespoke soil mix, you may be tempted to plant a bonsai tree in ordinary garden soil.
While it is technically possible for a bonsai tree to grow in potting soil, there is a high risk of excess water being retained in the soil, which will cause root rot.
To prevent this, you will need lots of drainage holes and will almost certainly need to add other materials, such as gravel, to the potting soil to increase drainage.
However, if you’re going to start adding materials to garden soil, why not just make your bonsai soil from scratch?
In our opinion, the best way to get soil for your bonsai tree is to buy some akadama, pumice, and lava rock and make your own bonsai soil blend.
Making your own soil will also help you learn more about your tree’s needs and get the most enjoyment out of the bonsai tree hobby.
Read Next…
We’ve learned that bonsai soil needs to be well aerated to get oxygen to your bonsai tree’s roots and that you need the right balance of water retention and drainage. But there’s much more to looking after a bonsai tree properly.
To keep your bonsai tree healthy and growing well, you’ll need to water your bonsai tree and add fertilizer to your bonsai soil.
Browse more Bonsai Care articles from Keep Bonsai and you’ll be well on your way to becoming an expert bonsai tree keeper!