A General Guide To Bonsai Soil

Bonsai soil can be one of the most controversial topics in the hobby. Soil is one of the fundamental elements required to keep a tree alive and everyone believes their way of doing things is the right way. The reality is there are a number ways to approach the topic and they can all be as equally good as each other.

So long as you follow a few key guidelines, which I will discuss throughout this article, you can’t really go wrong with your soil.

Why Do We Use Bonsai Soil?

Bonsai soil is used for one main reason, it’s better for the trees health. Pretty much everything in bonsai is centred around the health of the tree and the soil is no exception.

You need to take into account that a lot of bonsai trees are grown in very small pots with quite small amounts of soil. In order to keep them alive it is important that this small amount of soil is as efficient as possible.

How Does Bonsai Soil Work?

Broadly speaking the soil needs to be able to give the tree enough water and oxygen to survive. It needs to be able to hold enough water to sustain the tree, but not so much it causes the roots to drown and rot. 

The soil needs to be –

1. Quick draining
2. Still be able to hold some moisture
3. Allow plenty of air flow

So long as you can achieve these 3 key requirements, your tree will be very happy.

Bonsai soil is basically a mix of different components that should allow you to achieve all 3 of these basic soil requirements.

Changing the recipe of these components and their ratios within the mix will change how the soil works and will change the effectiveness of these 3 key requirements.    

Bonsai Soil Is Loose On Purpose

When you look at bonsai soil you will see it is quite loose, it pretty much looks like a handful of stones, especially when you compare it to more denser soil, such as garden soil.

This loose consistency and the larger particle size is not by chance, it makes sure the 3 basic requirements are met.

When the larger particles are side by side, they create little air gaps in between each particle. Which, you know… contains air.  These little air gaps are literally essential for the tree and allow plenty of air flow.

These gaps also allow for water to basically drain straight through the pot.  When you water a tree the majority of the water will drain straight through. This sounds like it’s a bad thing, but it’s not. The water flushes through from the top to the bottom. This sort of acts like a sucking action and drags air down with it. So this is how the air gets replenished in the gaps of the soil.

But what about the water? Doesn’t the tree need water? Yes, of course. Not all the water gets flushed through. Some does remain in the soil and this can be used by the tree. (You can more about – How To Water A Bonsai)

This is also where the ratio and composition of the soil comes into play. If you have components that retain water, they will absorb water when you water the tree. This can then be slowly released and used by the tree throughout the day.

This is how the soil can be fast draining; yet hold enough moisture to sustain the tree.

What Nutrients Are In Bonsai Soil?

The question of nutrient levels and fertiliser requirements are always asked about. Organic compounds will contain some nutrients that are released into the soil. Especially components like compost.

So should we use more organic compounds in our soil to get more nutrients for our trees? What about trees that use only use non-organic soil, how do they survive with no nutrients?

Factoring in the soils nutrient density is not required when you are creating any form of soil mix. The fact that our trees are in small pots and the water can be fully controlled by us means we can add whatever nutrients we want, whenever we want.

We can either dilute the fertiliser into the water, or we can add fertiliser on top of the soil so that it is slowly flushed down into the soil when we water.  

We can always provide the optimum level of nutrient for our trees throughout the growing season regardless of what components our trees are growing in.

Bonsai Soil PH

The PH of the soil is equally important, as it is not important. I honestly don’t think you should be getting to worried about it. Some people buy test kits for the soil and know exactly whats going on.

While I think this is good knowledge to have, I also think it is over the top. Most components and mixes are going to be suitable for your trees and give them the PH they need. If something was terrible, there would be plenty of information online saying “don’t use this soil component”.

The only exceptation to this if you are growing trees that love an acidic soil. So, azaleas and anything in their family. However, even this isn’t a big worry, just use ericaceous composts or kanuma, which are acidic. This little simple change is going to be suitable enough to keep them happy.

I know what I’ve just said will annoy some people, but let me explain why I don’t think it’s something you need to worry about. Most bonsai soils will break down over time and the PH will start to change. Other elements such as water and fertilsier can also change the PH over time.

You also need to remember that bonsais are also repotted fairly regularly. I believe that most trees are repotted long before the PH becomes a noticeable and concealable problem.

If you are leaving your tree in the same soil for a number of years, or you tree is very delicate to PH, I would only then concern yourself with looking at the soil PH. Up until then, I think regular repotting is going to keep your tree healthy enough.

What Do You Want In A Bonsai Soil?

Bonsai soil mixes can be made up of Organic and Non-Organic components.

You need to find a balance between the amount of organic and non-organics. As discussed above, different ratios will give you different results. This can allow us to alter the mix so we can cater for different trees requirements and environments.

Typically organic compounds used to be living matter and they are often very porous so they are going to be able to absorb water and will give your soil mix water retention. They will also slowly break down and decompose over time.

Non-organic components are usually are from the ground and do not absorb much water. They also usually do no break down over time.

Obviously there are acceptations to both of these, but understanding how each component works will really help you put together your own soil mix more effectively

Common Components In Bonsai Soil

This is not the complete list. I have tried to name as many as possible. Every different country may have different access and availability of these and other components may be more popular in other places.  

Organic

• Garden Compost
• Coco Fibre
• Peat Moss
• Sphagnum moss
• Bark chips

Non Organic

• Grit/stones
• Perlite
• Turface
• Vermiculite
• Molar Clay
• Akadama
• Kanuma
• Kiryu
• Kyodama
• Kurodama
• Pumice
• Lava Rock
• Diatomaceous Earth

Can You Just Use Fully Organic Or Fully Non-Organic Bonsai Soil Mixes?

Yes, you could do that. The only thing is, you will probably be giving yourself a lot of work and you will have to manage your trees very carefully. Your soil is either going to potentially dry out too fast, or stay wet for too long. Both of which can result in trouble.

You can also just use one of the components on their own, so not really a mix. However, some of these will work a lot better than others. Most people are probably aware of using only garden compost to grow plants in. This will work, but it will not fully meet the 3 key requirements required for optimum health. You will find garden compost holds too much water and can also be compacted at times so air flow can be a problem. This is why you will often see gardeners mix compost with perlite. It allows it to be a bit looser and drain easier, as well as allow more oxygen into the soil.

Using perlite on its own would probably not be a good idea. It is very light and does not hold any water, so your pot would dry out very quickly.

Akadama is a Japanese volcanic clay and it is often used on its own. Although it is non-organic, it can still absorb a good amount of water while letting excess drain away. Also due to its particle size it allows for good air flow.  This means akadama is really good at meeting those 3 key requirements needed in a soil.

However, I would still suggest that having a mix of various components, both organic and non-organic will allow you to be able to create a more balanced soil that achieve all the requirements you need.

Other Important  Factors To Consider For Bonsai Soil

When trying to work out what components and what ratio you want to use, there is a number of factors you need to consider.

It is not as simple as wanting your soil to stay wetter or drier. The amount of water retention is obviously very important buy you need to think about the larger picture and how it will affect a number of things.

Species

You need to access what your trees water needs are like. Some species, like willows, are pretty happy to be sat in water at all times. Where as a succulent tree, like a jade, is going to want to go bone dry in between watering. 

Some species also just drink up the water a lot fast than others, so the time it takes for the pot to dry out, along with how wet it can handle, all need to be considered.

How Much Time You Can Dedicate To Watering Your Bonsai?

If you are going to be away from home quite often, or you just in general have a busy work life, can you commit to watering your trees a lot? Some nurseries in Japan have their trees in 100% akadama and will water the trees 3 times a day.

However, they have a team of apprentices who go around every few hours and check what needs watered. This might not be a viable option if you work long hours or something similar.

So maybe adding more organic matter might reduce how often you need to water. I myself have done this and have found I can usually go 48hrs in-between water at times. I wouldn’t be able to cope if I had to water multiple times a day. 

Weather

The weather plays a huge role in watering, so it should definitely be taken into consideration when making a soil mix. If you live somewhere warm, you’re probably going to need a little extra organic matter in your soil to help keep it wetter for long and stop it from drying out to fast.

The opposite is true for me. I get rain nearly every day. So I have actually reduced my organic matter and increased my non-organic levels. My soil mix was staying too wet as it did not get a chance to lose enough moisture in between the rain showers. By adding more non-organics, it now holds less water in general, but the constant rain keeps it always topped up with fresh water.

The Changing Seasons Of Year

The change of seasons is a difficult one to access, but still very important. As I was saying above my soil was holding too much moisture. This was actually only the case in winter. As my trees where dormant along with the lower temperatures/worse weather, there just seemed to be more water staying in the pots over winter. So I reduced my organic matters and increased my non-organic matter. The problem was now solved, the soil didn’t stay as wet throughout winter 

However, this made a new problem. The soil now dries out very quickly in summer. When the trees are growing and using more water, along with the sun shining all day, the pots dry out quickly.

Luckily I had thought about this before and was ready for this.

Previously my soil was perfect for the summer. It would hold the water for a couple of days before I had to water again which suited my busy work life.  (on a side note, I actually lived in London and this soil mix was perfect all year round as it’s a bit drier there, the problem only arose when I moved further north).

When I accessed changing the soil, I just took in to account that I would probably need to water daily through the summer. This was fine with me I could live with this change. I had the time and also now had access to a hose (hand watering before was tough).  So a little bit more work from me in the summer but now my trees don’t drown throughout winter.

Having to balance your soil to cope with the change seasons is tricky and it may take you a few seasons to find a balance in your soil that works for you or you may need to make compromises like me. This is all part of learning bonsai.   

Pot Size

The size of your pots will also affect how you make your soil. The smaller the pot, the smaller your particle size should be. This will make more sense when you have it your hand and can see the different sizes of the particles. Logically it makes sense that you would want lots of smaller particles in a smaller pot, rather than a few large ones.  

(You can read more about – What Is The Right Size Pot For Your Bonsai?)

Particle Size Of Bonsai Soil

When you buy your soil components you should sift them. This is basically just separating the larger particles from the smaller ones. This will also get rid a lot of the dust that can be in created from some of the components being crushed in the bag. 

When you sift your soil you can group components of the same size together and make an even soil consitency throughout and will help with how water is held/ passes through your pot.

Should You Make Your Own Bonsai Soil Mix Or Just Buy It?

This honestly comes down to personal preference. I think it’s easier and cheaper to just make your own soil. However, if you feel really lost in how to make your own, just buy some. Try it out. I’m sure there is someone living nearby who can recommend what soil to buy.

You should get a feel for it pretty quickly. Maybe it’s drying to fast. Maybe it’s holding watering too long. Maybe it is totally perfect. If you’re happy with it, try to recreate it in the future, or adjust it to suit your needs and conditions.

(If you want to know if you can reuse Bonsai soil, you can read about that read Here)

My Personal Bonsai Soil Mix

I get asked this a lot. To be honest, how I make my mix doesn’t really matter.

If you go through some of the points above, you will quickly see that just copying me may not give you the exact same results. For example, my mix is going to be dreadful for someone living in a hot climate. 

But since you all keep asking, I will tell you.

I use equal parts Molar Clay, Compost and Perlite. As already discussed I have changed my mix since moving house. I just held back on the ratio of compost in exchnage for a little more molar clay and that seemed to solve my issue.

I think my mix is decent and does the job, but it’s not the best. I would like to experiment a bit more, but some components can be quite expensive here. My molar clay is about £14 for 30L. Akadama is about £28 for 15L. So twice the price, for half as much.  I also struggle to find some components all together which can be frustrating.

I think this is a common issue around the world, but if you use what you can find, you should be able to make a decent mix.     

My Bonsai Soil Video

I actually uploaded a video on Youtube pretty much covering everything I have covered in this article, but if you want to watch it, you certainly can.

Common Bonsai Soil Mixes

Here are some popular mixes you will a lot of people using:

-100% akadama, this is very common, especially in Japan.

-A very popular mix and what is often seen as one of the “best” is: Akadama, Pumice and Bark chips. This is often a professionals go to. Depending on the tree, the bark chips may not be added.

-Akadama and Kanuma. This is used for trees that love acidic soil like azaleas. The ratio can change and sometimes only 100% kanuma is used (this is the acidic component).

-The list is endless. I could name combinations for days, the above are very common, but by no means the only options.  

Conclusion

Hopefully now you can appreciate that the topic of soil can be vast and complicated and understand why it can cause such heated discussions. So long as take a step back and analyse what your tree needs, in conjunction with what you have available to use, as well as remembering that the trees health is the overall goal, you should end up with a good mix.   

Just keep experimenting and you will end up with something that works for you. Most importantly remember that just because your soil is different to someone else’s, it doesn’t mean theirs is better. So long as your tree is healthy, you’re doing it right.