Is there a special soil mix for bonsai?
Yes and No, there are special substrates we use to create different mix’s that give us different results. If you were to go to a garden center or hardware store and buy “Bonsai Soil” you are likely just buying regular potting mix with a picture of a bonsai on the bag and a slightly higher price tag. Soils are argued over a lot in bonsai but it is up to the individual what they want to use and how far they want to go with their bonsai trees. Lets take a look at some details of bonsai soils that most people don't talk about, usually they will tell you an exact mix that they use but this could be harmful to your trees.
Bonsai soils are goal based
The first thing we need to understand and look at is the results you are trying to achieve with your bonsai, the most obvious example is development and refinement. The goal for development is growth and thickening, in development we take a young piece of material and grow it into a more mature piece of material building base structure along the way. In refinement on the other hand we are now putting the icing on the cake. We take that structure that we built and create fine ramified branching and small leaves which we can wire into pads to create the delicate on top of the structure. So with essentially 2 goals that are opposite to each other we really need 2 soils that are opposite to each other. With development we want lots of growth and we aren’t paying much attention to things like leaf size or internodal length, we want as much growth on the tree as possible to speed up the thickening and growth. So in this case we can use organic soils which gives us density, high nutrient retention and a large root system when put in a nursery growing container.
With refinement we want the opposite, we are now concentrating on smaller slower growth and finer leaves and branching. To achieve this we need to have a smaller restricted root system, finer roots and more open soil.
Now this is a very basic explanation of this as there is much more to developmental and refinement soil mix’s but the message I am trying to get across here is choosing your soil can be first of all based on your specific goals.
Different species have different requirements
The next thing that can change a mix is the species of tree you are working with, not all trees like to live in the same environment so each tree will need special treatment in terms of the soil mix. This can be true for both developmental mix’s and refining mix’s. For example most deciduous trees will require more moisture content to be available to the tree so we will create a mix that provides that to the tree, where as things like pines and junipers appreciate a more free draining mix that holds less moisture. So for a maple you may have a soil that is a little more dense in development or has a higher akadama ratio in refinement, where as a juniper you may start adding things like perlite in development to create a less dense mix and a more open free draining mix, or in refinement you may add more pumice with your akadama to reduce the moisture available. BUT this can change as well depending on other factors which we will talk about now.
Your environment and your soil mix
Here is the big factor that changes dramatically from town to town, from state to state from country to country. Soil mix’s cant be taught as a simple method of use X & X & X mixed at a ratio of Y:Y:Y.
Why? Because what might be good in 1 environment could be a death sentence in another. Lets use a really dramatic example just to help make this point. We are going to compare outback Australia with The UK. In outback Australia it rarely rains and is hot and arid for most of the year, In the UK ( From What I’ve heard ) it rains a lot, almost too much. It is also cooler there. So lets say someone in the outback of Australia uses a mix that is a little more dense and holds a lot of moisture because it is hot and arid and they don’t want to be watering 10 times a day because they have a mix that is too open and free draining, They now share their mix that they get good results with to a person in the UK. The person in the UK is now suffering from root rot because they have a mix that isn’t very free draining and holds a lot of moisture, they live in an area that gets a lot of rain and not much transpiration happens in the tree as they don’t reach the temperatures we do. So for one person they get good results and the other has a tree that is dying but it’s the exact same mix, what gives? It’s the environment. This is a big factor in helping decide what mix you are going to use. So keep in mind when getting advice on soils is the person you are getting the advice from in a similar environmental situation as you?. Once again this is just a basic overview to show you how things can change quickly. On top of the example above which was just of temperature and rain fall there is also humidity, wind and positioning that can also change what kind of mix you need.
So what are the special substrates used for bonsai?
You have probably heard people talk about Akadama, Pumice and Lava Rock, you have probably also heard people very passionately claiming that these substrates are useless and are only around to make money. I will first start by saying, do some research on some of the best bonsai artists around the world. I’m talking about true professionals, not just the guy in your club that has the best trees. You will find these artists all have 1 thing in common besides having the best quality trees in the world, They all use in-organic substrates such as Akadama, Pumice and Lava Rock. They don’t use them because they like paying more for their soil then everyone else, they use them because they get the best results.
I find that 9/10 that if someone says “ I use regular potting mix and its just fine, I don’t need that expensive crap “ if you ask them, have you ever actually tried In-Organics to see if the results are actually better the answer is usually no. Sure their trees have survived and probably even gotten half decent results but what they don’t realize is they could have gotten even better results with half the effort with In-Organics.
I wont fully go into depth here on in-organics and the advantages because you probably don’t want to read a 500 page essay and my fingers will get tired from typing. But just understand that there is another level of soils once you reach refinement and it is up to you whether you take that next step or not.
But lets take a quick look at those substrates
Akadama : This is a special clay mined in Japan, unlike most clays it is unique. Akadama is the only one of the 3 substrates mentioned that has a Cation Exchange Capacity “CEC” which is the ability to hold nutrients, it only holds a small amount though unlike organic soils which helps us better control our fertilizer application. Akadama also scales, as the roots grow into it the akadama splits which also splits the root in 2, what you now have is 2 finer roots. This happens again and again until you have a really fine root ball. Essentially you have increased your surface area of roots in the limited bonsai pot environment which can increase water uptake, nutrient uptake and strengthen the trees natural ability to fight pests and disease. As the roots get smaller so does the akadama so it self scales to match the roots. Akadama also has a good amount of moisture retention which can be used in different ratios to increase or decrease the amount of moisture needed in a mix. And lastly as mentioned akadama scales which means in breaks down, it is the only 1 out of the 3 that does break down meaning special consideration will need to be taken as to how often your trees are repotted. If you are using 100% akadama make sure your trees are fine with a 2 year repotting cycle. If the repotting cycle is longer you will need to add pumice or lava or both to prevent the complete break down of the soil system as akadama usually fully breaks down in 2 years ( I say usually as ive had higher quality akadama last longer ).
Pumice : Pumice is a volcanic substrate that holds a little bit of moisture a little bit of oxygen and doesn’t break down. It also doesn’t have a CEC. Pumice can help stop an Akadama based mix from fully breaking down over time but it also adds a little extra moisture retention to the mix while also making it more free draining.
: Lava rock can also be called Scoria or Kiryu
, This substrate is used purely just to add more oxygen to a mix. It doesn’t hold much moisture and it also has no CEC. It wont break down over time either.
So as you can see you can use these 3 substrates to increase or decrease things like nutrient retention, moisture retention or Oxygen.
I hope this has given you a little bit to think about when it comes to soils. This is a very deep subject so there is a lot more to it then what I have written here, but what I hope I have given you here is the right mind set on how to figure out what soils to use, when to use them and how to use them rather then just listening to what some random person uses and copying that because they have nicer trees.