Repotting a Beast!

Updated: Oct 8, 2020

Repotting a Beast


The first tree I noticed when I came to Aichien a year ago was this huge monster Black Pine!  As I wondered around the yard I kept coming back to this one and snapping pictures of it with the camera.  The reason why this Black Pine intrigued me so much (yeah, who wouldn’t be) was that it wasn’t a finished tree.  This tree was a perfect example of a tree in development.  As I looked over the tree, I thought to myself, “how great is it if I can play a small part in the development of this tree.”  Well, a few weeks ago, I got to do just that.  As you can see from the photo, the branches are growing out and sections of the canopy is still empty in some places.  The trunk was collected decades ago and all of the branches were grafted.  From the looks of the branches, the grafting was done about 8-10 years ago.  In this post, I’m going to be sharing some photos of this tree being repotted, discussing the soil mix we use and the importance of understanding how soils works.

There Are Better Things to Do

Soil mixes can be a hot discussion/argument among many Bonsai enthusiast.  Different people use different mixes and material for various reasons and it seems that some will even fight to the death defending it.  In this post, my intention is to only talk about the soils we use at Aichien and understanding why we use them. Overall, I’ve come to realize that the soil components used doesn’t really matter and that understanding how the components retain water and its effects on the  tree is more important.  Depending on the mix you are using, think about how they affect your trees and ask yourself, “is there something I need to change or adjust to better develop my Bonsai?” There is no universal soil receipt that will work for every tree at every developmental stage in every environment and everyone.  Since there are so many variables in Bonsai including the artist themselves, there would never be just one way to do anything.  So instead of trying to defend the soil we use, I thinks a better approach would be to understand how much water a soil mix holds and how that affects the tree.  This way, no matter what components we use in our mix, it’s focuses towards the tree and it’s water needs.  Then we  can move on to better things like creating great Bonsai!  There’s going to be lots to talk about so lets get started!

The Tree

This Black Pine hasn’t been repotted for over 5 years.  It just happens that I was lucky enough to be here when it was time.  Mr. Tanaka said that if we waited one more year, the pot might not stay in one piece!  I’m not sure if this tree is the oldest in the yard, but it’s definitely has the biggest trunk in the yard.  Take a look at what we did.






It’s About Water!

Understanding the individual soil component is all about understanding how much water it holds.  How much water a soil mix holds will affect how the tree grows.  Types of soil and sizes dictates how much water they hold.  The soil mix then in turns, affects how we adjust our watering schedule.  Forget your preconceptions of what bonsai soil should be made of and think more about how much water a tree wants, water retention of the soil and the effects it causes.  In this case, we’re working on a Black Pine.  Black Pines grow best in drier conditions and though it can take a good amount of water at one time, it needs to dry out for the roots to grow well.

Basic Laws of Bonsai Soils affects on Trees – 1. Trees will grow slower if the soil is wet for a long time (wet mix).2. Trees will grow faster if the soil is we for a short time (dry mix).

 Of course there are exceptions to everything but for the most part, these statements are true. Bonsai Soils We Used for This Tree


Akadama is a clay that has been fired. Depending on the temperature at which it was fired, the hardness of the clay will change.  Depending on the hardness of the akadama, it will break down at different rates.  Soft akadama breaks down faster and hard akadama breaks down slower.  This material is what holds most of the water in the mix.

The reason why akadama is a good soil medium for bonsai is it evolves with the root system by breaking down.  When a tree is first repotted, there are not many roots so fresh new akadama holds less water.  As the tree grows more roots and demands more water, the akadama will break down and start to retain more water.  There is a point though where akadama will breakdown so much that it will bind together and form a water repellant block that stays very dry.  Depending on the tree variety, this may be a good or bad thing.

Akadama in Japan is very cheap.  It available at the local garden center and is about 500yen per bag ($6.00US).  Shipping this stuff around the world makes this material much more expensive and have turned away bonsai enthusiast from using it.  When the price of akadama becomes 30 dollars a bag, I understand.


Coarse river sand is just that, large sand particles.  This medium is used to help in the drainage of the soil.  Other then surface tension of the small granules, this medium doesn’t hold water at all.  It’s not important though that it’s coarse river sand.  The important part is that it doesn’t hold a lot of water.  Are there other materials out there that will do the same?  Of course!


Hyuga is a type of volcanic rock.  It doesn’t normally break down though if compressed hard enough will crumble (everything breaks down with enough pressure).  The average piece of pumice will not normally break down.  Hyuga is light and holds some water though is considered a fairly dry component.  It’s mainly used to dry up a soil mix just like coarse river sand.  In Japan, a bag of hyuga normally has a small amount of granite mixed in as well, which are the darker pieces in the photo above.


The last component that we add to the mix is crushed charcoal.  It’s less then 5 percent of the overall mix.  Charcoal is used as an absorbent of any toxicity that is found in the environment.  Pollution in the air and water could potentially have an ill effect on our Bonsai so a little bit of charcoal can help limit their amounts in the soil.  Unlike full grown trees in the ground, our Bonsai are limited in space to grow so keeping that environment clean is a plus!




To Sift or Not to Sift

Sifting soil is also another hot topic that people argue back and forth about.  Again, I’m not taking sides one way or the other.  The important part is understanding why or why not to do it.  Small particles and dust holds a good amount of water compared to large granules.  Since this tree needs drier conditions, we removed a component that holds a lot of water.

I talked to Mr. Tanaka about sifting and he said we don’t really have to sift the soil and that his father never sifted his soil mix.  The consequences to this is that he had to be more careful in allowing trees to dry out before watering.  If the conifers never dry out in between waterings, the roots will slow down and start to rot and die off.

On the other hand, removing the dust and small particles will cause the soil to dry out faster.  Now it’s not a matter of allowing the soil to dry before watering, it’s a matter of not allowing the soil to stay too dry for too long between waterings.  Roots always grow faster in drier conditions, but when the roots are completely 100 percent dry, roots will not grow at all and start to die off.

So before you decide to sift or not sift your soil, think about the effects it will cause to the roots in the soil and if you’re able to adjust your watering habits to it.  The decision is yours.

*Secret side note* (well, not so secret anymore): this year, we sifted all of our conifer mix and didn’t sift our deciduous mix. Guess what, the trees are growing fine.  Since the conifers needs to dry out in between waterings and deciduous trees like being wet all the time, it worked out just fine.


We use this large size pumice for the bottom layer of the soil.  Using this large pumice helps keep the bottom of the pot drier so that the roots don’t stay wet too long.  It also helps protect the drainage holes from plugging up when the akadama starts to break down.  We normally only use a drainage layer on conifers.

The bottom of the pot is always the last area to dry out.  Heat and evaporation dries the top soil first and slowly works its way down.  By the time the bottom of the pot dries out, the top layers may be too dry and roots may have already died off.  If we water the tree correctly and not allow the majority of the soil to dry out completely, then the bottom layer will stay wet all the time.  The use of pumice is a great way to keep the soil a bit more uniform in holding/losing water.

Back to the Tree



The pot!


This is a Japanese Antique pot, which means it’s over 100 years old.  For those that are observant you already see the crack repair on the bottom left of the pot lip.  This was actually a factory crack from the firing process.  Creating large pots is much more difficult than small ones.  There’s so much more clay and weight to it.  Have you noticed that the larger the pot gets, the price increases exponentially?  Risk of deformity and cracking during the firing process is higher so sometimes a maker has to make several just to get a good one out of it.  I have yet to see a Japanese Antique pot this large without some sort of factory crack.