Bonsai soil requirements
What is good bonsai soil?
Before mixing your own bonsai soil, you should know what type of soil for what purpose to mix. Good bonsai soil must meet the following requirements.
- Good water retention,
- Good drainage,
- Good aeration,
- Adequate soil pH (slightly acidic), and
- Low in nutrients.
Good water retention
During photosynthesis, plants use sunlight as well as carbon dioxide from air and hydrogen from water absorbed through the roots to create their energy source, glucose (sugar). This means they need a constant supply of water to generate energy.
Soil with good water retention enables bonsai trees to absorb water necessary in the process of photosynthesis.
Bonsai soil needs good water/air entry and exchange for the roots. Plants need water to create energy but the roots need oxygen to burn carbohydrates for the roots’ energy to grow and repair. During this respiration, plant roots use glucose made by photosynthesis for their various functions, like nutrient uptake and maintaining and growing their tissues.
If the soil is always moist and wet, the roots cannot respirate appropriately and will drown in water.
Adequate soil pH (slightly acidic)
What is soil pH?
Soil pH (“potential of Hydrogen”) is a measure of how acidic or alkalic a soil is. The pH scale varies between 0 and 14, in which
- a pH value of less than 7.0 is acid
- a pH value greater than 7.0 is alkaline, and
- a pH of 7.0 is neutral.
When the scale is closer to zero, the soil is highly acidic, and when the scale is closer to 14, the soil is highly alkalic.
pH and bonsai tree growth
Soil pH influences nutrient uptake and tree growth. Many nutrients change their form in reactions to other chemicals in the soil, which are largely controlled by soil pH. Trees may or may not be able to use nutrients when they change form.
Most plant nutrients are optimally available to plants in general within 6.5 to 7.5 pH range, which is generally compatible with root growth as well. For the majority of bonsai tree species, soils with a pH between 5.5 to 6.5, meaning slightly acidic soil, generally provide the best growing conditions.
That said, the vast majority of bonsai tree species can live in a broad soil pH range if the proper balance of required nutrients is maintained.
Ideal pH for each of the bonsai species
The majority of bonsai tree species prefer slightly acidic soil but optimum soil pH level differs slightly from species to species. Here are some of the pH levels they prefer.
Bonsai soil must contain little or no organic matter but should be able to hold the nutrients well when we fertilize it.
If the bonsai soil is rich in nutrients, controling the strength of tree growth becomes harder since the trees grow vigorously using abundant energy in the soil. Branches and internodes tend to elongate when trees grow fast with rich nutrient soil.
It is very difficult (or almost impossible, I should say) to make a bonsai tree grow new branches in between elongated internodes in a natural way once they are grown. You have to cut off the whole branch or major part of it hoping new buds will come out where you want. It may or may not happen depending on the species as well as the tree itself.
Bonsai trees aim to mimic a natural landscape created with plants in a bonsai pot, and elongated branches and internodes prohibit that. So, managing the growth by cutting the nutrients in the soil should be a priority rather than making nutrients always available in the soil for the trees to take in.
How to make bonsai soil at home
What you need for making bonsai soil
To make bonsai soil mix at home, you need;
- Soil of various types for bonsai soil mix,
- Soil scoop,
- Soil sieve with 3 screen mesh sizes (large, medium and small), and
- A few containers to mix and store soil.
Types of bonsai soil
Main types of soil used in bonsai soil mix are;
- Akadama/lava rock,
- River sand, and
- Organic potting compost.
You can also use pumice and vermiculite in a homemade bonsai soil mix.
Akadama/volcanic rocks should be the main ingredients for bonsai soil mix because they have all the characteristics necessary for bonsai soil; good aeration, drainage, water-holding capacity as well as nutrient-holding capacity. Also, Akadama is mildly acidic with pH of 5.1-5.5.
Akadama is a granular clay-like volcanic pumice found in the region 90-100 miles north/northeast of Tokyo. Since lava rock is formed from lava erupted from a volcano, both have similar qualities.
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If you want to know more about Akadama, please read the following post.
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River sand is used to improve soil drainage and aeration, which is especially necessary for coniferous trees.
River sand is the type of sand collected from the river bed that is made from weathering or erosion processes of rocks. River sand for bonsai soil should be the one collected upstream because the sand from upstream drains water well but can hold water to some extent as well.
You cannot use sea sand because it has almost no water-holding ability.
For more detail about which type of river sand you should use for bonsai, please read the following post.
Organic potting compost
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Organic compost is used to make nutrients available to the trees which require relatively abundant nutrients such as flower and fruit bonsai trees. Organic potting compost is never used alone or in high proportion in bonsai soil mix.
You can also use pumice and vermiculite in a homemade bonsai soil mix. For more information on these types of soil you can use for bonsai, please read the following post.
Here is how you should make your own homemade bonsai soil mix.
- Before mixing the soil, use soil sieves to separate out the different particles by their relative size and put each size in different containers.
- Check the ratio of soil you need to mix for your bonsai trees.
- Use the soil scoop to measure the mixing ratio and put the soil in a container.
- Mix the soil well using hands.
Why and how to use soil sieve
Why use soil sieve
It is important to get rid of silt (particles less than 0.04 inchs/1mm in diameter) from the soil before you prepare your bonsai soil mix. If the soil contains fine particles like silt, the soil becomes muddy when water is given, and hardens when dried. This kind of soil has very little room to contain air and does not allow water to pass through. The roots cannot breathe and drown in water.
The size of the particles is important as well. In general, the larger the size of the particles, the better the drainage and aeration, and the smaller the size of the particles, the better the water and fertilizer retention. As a broad rule, bigger particle sizes are for big bonsai trees and smaller ones are for mini bonsai trees. The soil with small particles can be used as a top dressing as well.
How to get rid of silt using soil sieve
To get rid of silt,
- Place the soil into sieve #1 (the largest mesh). Shake soil over sieve #2 (medium mesh) for a few minutes so that sieve #2 collects smaller soil particles.
- Put the soil collected from sieve #1 (the largest mesh) in a container.
- Shake the soil collected in sieve #2 (medium mesh) into sieve #3 (the smallest mesh) for a few minutes.
- Put the soil collected from sieve #2 (medium mesh) in a container.
- Shake the soil collected in sieve #3 (the smallest mesh) for a few minutes to get rid of silt. You can throw away the silt.
- Put the soil collected from sieve #3 (the smallest mesh) in a container.
- Repeat the process for each type of soil you will use in the bonsai soil mix and put them into different containers.
Alter the soil mix
The soil mix should be altered according to the climate you live in and the environment where you place your mini bonsai trees. Basic bonsai soil mix ratio shown in the next section is a guideline. Tailor your own bonsai soil mix that is suitable to your environment, and your bonsai tree will grow healthy.
The things you should consider are the climate you live in, if you live in the city or countryside, where you place your bonsai trees and the age of the tree.
If you live in California whose climate is warm and dry year-round, you might want to use less sand and the kind of soil to retain more water. But if you live in New Jersey whose climate is hot and humid in summer but very cold and snowy in winter, you might want to use more sand for drainage and protection from freezing weather.
City or countryside
If you live in the city where most of the ground is covered with houses and concrete, the humidity rate is likely to be much lower than in the countryside even though they are in the same region.
I now live in a countryside house surrounded by rice fields and small mountains, after leaving the big city. I found out that humidity is so much higher here than in the city that the basic soil mix holds more water than the bonsai trees need, especially when it stopped raining and the sky is still cloudy. Once before summer, the bonsai soil did not dry up enough and the roots of my bonsai trees started to rot.
This would not have happened if you live in the city.
Bonsai tree placement
You might also have to consider where you place your bonsai trees. The soil mix has to be altered depending on whether you place your trees outdoors under the sun or indoors with less sunlight. Think about how much sunlight your bonsai trees get during the day and how dry the soil would be. The more sunlight the trees get, the more dry the soil will be.
Age of the bonsai trees
Younger trees need more nutrients and water to grow than older ones and they are less resistant to water shortages. On the other hand, older trees need more drainage in the soil because they absorb less water to create energy.
How to mix the soil
- Akadama/lava rock
- River sand
- Organic potting compost
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- Prepare a clean bucket or container box. If it is used before, wash it thoroughly to get rid of any mold or bacteria that may cause infestation.
- Pour each type of soil of the same particle size you prepared into the container, according to the soil mix ratio. Make sure to use an appropriate particle size. Preparing a bit more than needed may save you time. Remember, you can put more soil in the bonsai pot when you water it.
- Mix the soil well using hands or a shovel.
Basic soil mix ratio for bonsai trees
Here is the basic soil mix ratio for bonsai trees by species. Flower and fruit bonsai trees need nutrients to bloom and bear fruits.
Basic soil mix by species
Why no organic matter for coniferous trees
Coniferous trees like pines grow in rather barren lands. They do not require abundant water and are resistant to water shortage. Coniferous trees planted in a high drainage soil are able to grow thick roots which leads to thicker branches. Lesser water also results in rough, coarse branches and trunks, a good bonsai aesthetic for coniferous trees.
Dr. Thomas L. Jensen, SOIL pH AND THE AVAILABILITY OF PLANT NUTRIENTS, IPNI Canada, 2010.