The topic of soil is one of the most complex in the world of bonsai and often causes disagreement, even amongst bonsai experts.
The health of your bonsai is largely determined by the health of its roots. The soil your bonsai tree is planted in provides the foundations for those roots and thus the soil is the key to its growth, health, and survival. You can imagine how important it is to make sure that you provide the correct foundations. Normal garden soil is of no use to your bonsai as it will harden when it dries. This is extremely detrimental to the longevity of your bonsai. It is often the case that bonsai which do not thrive, grow or mature well are a result of being planted in poor soil.
It is possible to buy pre-mixed soil for your bonsai. If this is what you opt to do it is important to choose carefully as the quality can vary greatly. We recommend TinyRoots if you do choose this route. With this in mind, many bonsai owners choose to mix their own soil.
Key Factors in Bonsai Soil
The first step in choosing the best soil for your bonsai tree is understanding the process required for the roots to grow. This will allow you to understand better the choice of components used to make the substrate for your bonsai. The roots of your bonsai tree need access to water, so the substrate they are planted in must have good water retention capabilities. They must be able to breathe air, so the substrate must allow aeration. To ensure a balance of water retention and aeration, there must also be good drainage. Let’s take a closer look at these three requirements:
Aeration
Aeration is the process of allowing air to circulate through something, in this case, the soil which you are planting your bonsai tree in. Air is just as crucial to your bonsais roots as water. Without air, the roots will suffocate and rot. The key is to try to plant your bonsai in a substrate that will soak in enough moisture to water your bonsai roots, whilst draining well enough to allow aeration.
Water Retention
The soil of the bonsai tree needs to be moist so that the roots can have access to plenty of water which is crucial to their growth. Of course, our bonsai roots need access to water to grow. Trees planted in the ground will adapt to their surroundings. If there is not enough moisture readily available then they will spread their roots out widening their access to moisture. This is obviously not possible for your bonsai tree since it’s confined to a small pot. This means that you are responsible for making sure it has the correct balance of moisture and air. Very few bonsai trees can withstand completely drying out.
Drainage
This is the removal of excess water, which will leave through the bottom of the pot. As crucial as it is for the soil to soak up moisture, it is also vital that it drains well. The perfect amount of drainage will allow moisture to soak into the soil but will also allow the excess to drain thus preventing the roots from suffocating.
Now that we understand the basic requirements of our Bonsais roots we can look at the most common components that are used to make Bonsai soil and the differences between them.
Organic Bonsai Soil
Dead plant matter is what we consider organic. This includes peat and pine bark. In previous decades, a lot of Bonsai growers used organic soil mixes, assuming them to be the healthiest option for their Bonsai trees. Now, after much research into the topic, most enthusiasts opt for inorganic soil, recognizing that organic substrates do not fulfill all the needs of the Bonsai roots. Organic soil can retain too much water during rainy seasons and yet it can be difficult to water when there are high temperatures. Organic soil also continuously breaks down and can become compacted. As we mentioned above aeration and drainage are extremely important to the growth and health of the Bonsai. When the soil becomes compacted aeration and drainage become difficult. There are still some who use organic matter in their soil mixes and have success, it should, however, be the smallest component in the mix.
Peat Moss
This is used for water retention. Peat moss holds a lot of water, for this reason, it should be used in small amounts, if used at all. It has a slightly acidic PH so it can be helpful if you are growing a species that prefers slightly acidic soil, azaleas and camellias are two examples.
There’s plenty of peat moss on the market. We have used and had great results with this “Peat Moss In A Bucket“.
Pine Bark
This is possibly the most popular of the organic materials. When sifted into very small particles, it will not breakdown easily but will breakdown slowly. This will provide nutrients for the Bonsai. Also, it holds moisture well and is readily available.
You can find pine back in many different sizes. Avoid using the larger chunks for bonsai, and go with pine bark about 1/4″ for most bonsai. This package of Bonsai Jack 1/4″ pine bark is perfect for most bonsai trees.
Inorganic Bonsai Soil
Inorganic soil has many more advantages, importantly it doesn’t break down easily and for this reason, it provides good aeration and drainage. Once it has soaked up a certain amount of water it will drain the rest through the bottom of the pot. Some popular inorganic substrates are akadama, pumice and lava rock.
Akadama
This is a fine clay which occurs naturally, mainly in Japan. Translated akadama means red ball, which is what it looks like in its natural state. Akadama are particles of volcanic matter. They are collected and sifted before being packaged. It is touted by many Bonsai masters as a crucial component in the soil mixture. Some who find it too expensive or difficult to obtain, suggest that alternatives can be used. Akadama is good for moisture retention but also drains well. Uniquely roots can grow through as well as between akadama. It does slowly break down as time passes. This is actually a good thing, as you want the growth of the roots to slow down over time. However, over a very long period, this breakdown will cause slow drainage and soil will need to be changed.
Pumice
This is a type of volcanic rock. It is excellent for ventilation. It supports the growth of the Bonsai roots well and does a good job at absorbing nutrients and water. In general, it should not make up more than 40% of the soil mixture unless it is being used to treat fungal infections. It is best sifted. Ideally, use smaller particles for smaller bonsai trees and larger particles for larger ones. Pumice is frost proof which makes it among the best substrates particularly if you live in a climate where temperatures are likely to fall into the minuses.
Lava Rock
This substrate retains water and is able to add structure to your Bonsai pot soil mix. It has natural nutrients which can be very beneficial to your Bonsai tree. Lava rock is great at preventing soil compaction, it won’t break down over time. It is sold in various particle sizes, however, for most bonsai, it is best to use smaller particles. It is heavier and more porous than pumice but has a similar function in the soil. The PH is neutral and therefore it can be used with any Bonsai. With its rich mineral content lava rock is thought to be good for encouraging bonsai roots to form well. Lava rock is also one of the cheaper inorganic materials, given the benefits its definitely one of my preferred choices.
Mixing the Soil
As with most aspects of caring for a Bonsai, there is no hard and fast rule. Therefore checking the requirements of your Bonsai species is always recommended. Also, adapting the mix according to local climate is essential. If you live in a hot, dry climate and you find the soil is drying out quickly, you can add more akadama. Alternatly, if you find that the opposite is true and your Bonsai to be too moist, you will want to enhance the drainage capabilities of your soil. This can be done by adding some lava rock or grit.
We can break the Bonsai Trees into three categories, deciduous, coniferous and Tropical. Each one will need a different type of soil and substrate:
Deciduous Bonsai Soil
Deciduous trees will lose their leaves annually. They need to be kept moist and should not be allowed to dry out. They should generally be repotted every two years. During Spring, Summer and Autumn you should make sure the soil is fertilized regularly.
Coniferous Bonsai Soil
Coniferous trees have cones. They are dormant in the winter months, so fertilization is unnecessary. These plants are usually able to tolerate drier soil than other Bonsai Trees. They typically don’t like a lot of moisture, so make sure their soil has good drainage.
Tropical Bonsai Soil
Tropical trees need a steady warm environment with fresh, humid air. They can dry out quickly, making the correct balance of water retention and drainage imperative to their health. Tropical Bonsai should be given fertilizer during spring and summer. This is when they do most of their growing.
Bonsai Soil Basic Recipes
| Tree Types | Species | Akdama | Pumice | Lava Rock | Organic | pH Level |
| Deciduous Bonsai | Elms, Maples, Ginkos | 50% | 25% | 25% | None | 6.5 |
| Coniferous Bonsai | Cedar, Junipers, Pines | 33.3% | 33.3% | 33.3% | None | 6.5 |
| Tropical Bonsai | Ficus, Fukien | 40% | 30% | None | 30% | 6 |
Deciduous Bonsai (e.g. Elms, Maples, Ginkos)
- 50% Akadama
- 25% Pumice
- 25% Lava Rock
Coniferous Bonsai (e.g. Cedar, Junipers, Pines)
- 33.3% Akadama
- 33.3% Pumice
- 33.4% Lava Rock
Tropical Bonsai (e.g. Ficus, Fukien)
- 40% Akadama
- 30% Lava Rocks
- 30% Organic component
Some people put a fine layer of gravel or grit in the bottom to help with drainage. However, most enthusiasts find this to be unnecessary. It is important to remember that there are no hard and fast rules, only guidelines based on what has worked well for others. Monitoring your Bonsai tree and adding certain components based on the trees needs is a crucial part of loving your Bonsai and ensuring its longevity.
How Often Should You Repot Your Bonsai Tree?
So as we have established every Bonsai tree needs air, water, and sunlight. Since your Bonsai will be growing in a small pot and therefore a small amount of soil, its roots can over time fill the voids in the soil. Thus the airflow in the soil will reduce, this hinders the growth of the roots which in turn affects the health at the top of the Bonsai tree. This will cause your bonsai to stop growing, if not dealt with over a long period of time.
How often you will need to repot varies greatly depending on the type and size of the Bonsai tree you are growing.
Watch for Indicators
One of the biggest indicators that it is time to repot your Bonsai and change the soil is when the soil becomes very hard and compact. When this happens you will notice that the soil becomes difficult to wet. Other signs that it is time to repot and change the soil might include a reduction in the leaf size and a reduction in the glossiness of the leaf.
Your Bonsai may also experience leaf discoloration, early shedding of the leaves, and slime forming over the soil’s surface. One of the key skills to mastering the art of Bonsai is getting to know your tree, this is how you will get the best out of the experience, however, if you’re looking for a quick answer, most Bonsai literature will suggest that you repot every two years. Do keep in mind that there are many exceptions to this rule. Chinese Elms tend to need repotting more often due to their fast growth. I would recommend doing this yearly particularly if it is in a small pot. Some more mature trees can go as long as 10 years. After repotting your bonsai tree, make sure to wait at least 2 months, or until you see signs of growth, before adding fertilizer.
When repotting your Bonsai, you should ensure that the soil fills the pot to the point where the small oval at the top of root is just visible. Make sure that your tree has plenty of soil to give your bonsai structure and that it is evenly distributed over the pot. After that, you’re done.
If you’re new to bonsai and you’ve read this article. Congratulations! You now know more about soil than most bonsai growers. This is sure to give you a leg up! Check out our Beginner’s Growing Guide for more info.
Happy Growing!