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The Problem With Garden Soil For Bonsai

Updated: Jan 7, 2023

As i look through Facebook pages i see just how little of an understanding there is about the role that Bonsai Soil plays in Bonsai in particular. So for those of you who are apart of this blog i am going to try and break down why we say not to use regular potting mix for bonsai ( if you want quality trees ).

Soil For Bonsai

We all know by now there is a million different opinions on Bonsai soil and you could argue this is my opinion, Except my opinion is about to have a lot of explaining why after it.

When we talk about Bonsai Soil one of the first things we need to clear up that there is a distinct difference between a tree surviving and a tree thriving and achieving the goals we are chasing. I've seen the argument before where people use garden soil in their bonsai pots and their tree is still alive, and to them this is proof that people who use inorganics and say potting soil is bad are wrong. So why on earth would me and many other Bonsai Practitioner's say not to use the regular cheap garden mix?

i will give a few reasons here in dot points then we will explore each one of them;

  1. Organic soils create a water table in your bonsai pot leading to root rot

  2. Organic soil holds a lot of nutrients which makes it hard to control your growth in refinement

  3. Organic soils promote larger more elongated root growth which will cause the same growth on the top of your tree

  4. Organic soils can become Oxygen deprived quick in a Bonsai Environment

Ok so lets start with point 1. Organic soils hold a lot of moisture and don't have a lot of oxygen flow compared to a more open inorganic bonsai mix. In bonsai pot it is important for us to have a balance of water and oxygen in the root system and if that balance gets thrown off we can slow down our trees progress. This is even easier to achieve an imbalance in a Bonsai Pot because of how shallow it is and how little of a gravity column the bonsai pot has, every time you water and the water moves through the soil that is gravity pushing the water down through the soil, following behind the water is oxygen that is getting pulled through as well. The taller the pot is the more gravity column you have so the water moves through faster, the shallower the pot the slower the water will move through the pot. So in regular gardening generally they use very large pots to put plants in so garden soil isn't such a problem, they are also not trying to achieve very specific results with their plants but we will get to that later.

So now that we know that we have a very little gravity column in our bonsai pot and organic soils hold a lot of moisture and not a lot of oxygen you can add these 2 things together and see what that will equal.... This is going to give you a water table in your bonsai pot, the bottom layer of soil will likely stay very wet and wont dry out between watering's so every time you water your tree you will just keep adding to that water. A common problem is when the top layer of soil dry's because of heat and wind flow making you think that the tree needs another water when below the surface layer your soil is probably still fairly wet. Inorganic soils drain very well and usually just hold onto the moisture that will be needed while the excess drains freely out of the pot, You can also layer your inorganic soils by particle size to achieve different results throughout your mix, for example you would have a larger particle size on the bottom layer to allow free draining of the bonsai pot to avoid water build up and also this allows air to enter the soil from the bottom as well. you use a medium particle size in the middle as your body mix to help cultivate a healthy root system and a fine particle size as a top dressing to help stop rapid evaporation of your body mix below.

Now lets address point 2. Once we move into a Bonsai Pot we should be ready to refine the tree, The development process should be over so we now no longer want big thick growth and elongation. We no longer want large internodes or big leaves to help thicken the tree, we want the exact opposite. We now want to focus on reducing leaf size, reducing the internode lengths and getting fine twiggy branching. To achieve this we need to both slow the trees growth rate which we can do by restricting its root growth in a small bonsai pot, but we also need to be able to give it enough fertiliser that the tree gets the nutrients it needs but we need to be able to control that application so that the tree doesn't have access to large amounts of nutrients causing the growth to take off and become larger. Organic soils are very good at holding nutrients which makes it difficult to control the application of fertiliser, you may get a build up of nutrients without realising it and if you keep applying more then that is only going to get worse. This is why organic soils are great for the development process because as the tree grows it always has a good amount of nutrients available to it to help it grow. When we use inorganic soils in refinement if we don't fertilise for a month then chances are at the end of that month there will be no nutrients left in the soil system depending on what you are using in your mix. This is helpful for deciduous trees moving into spring, we can ensure there is no nitrogen available to the tree as it puts on its first flush of growth allowing us to get shorter internodes and tighter branching, the same for black pines, a month before de candling we can remove all fertiliser so that after we de candle there is no nitrogen available to the trees second flush and we can get smaller growth, we can also reduce the amount we fertilise moving forward after that. So as you can see with organic soils it is very hard to control the growth on a tree in refinement which is very important if you want to practise bonsai. i get that for some just having a tree in a pot is bonsai too them and that is what they are happy with, but for those wanting good quality trees we need to reduce leaf size, internode lengths and get fine ramified branching which is going to be very hard to achieve with organic soils.

now lets look at point 3. In an organic mix the tree will grow large thick roots and they will be very elongated due to the dense nature of the soil and the lack of oxygen available, We know that what ever happens in the root system is mirrored in the trees growth. When a tree moves into refinement to achieve smaller finer branching we need to achieve smaller finer roots. This is where Akadama shines, the structure of the Akadama particles is special as the root can grow into the akadama which will cause the akadama to split and so does the root, The 2 splits then continue to grow and the same thing happens again and now we have 1 thicker root that has split into 2 finer roots which split into 4 even finer roots. this continues to happen until eventually the Akadama fully breaks down at which point we need to Re-Pot the tree if we notice that the growth on the tree begins to get larger again. But as the roots are getting finer you should notice that each time you get new growth on the tree it will start to get finer which is great for branch taper and ramification. If you use organic soils the growth will continue to be large and your branching wont have much taper or fine growth at all which takes away from the overall Bonsai look of the tree. After all we are trying to achieve fine twiggy branching on a nice thick barked up trunk. The higher amount of oxygen flow that inorganic particles allow also helps make a root system finer resulting in the growth we need for a quality bonsai.

And last lets have a quick look at point 4. This kind of plays on from point 1 a little bit but organic soils will break down and become compact fairly quickly which will cause percolation issues for your tree meaning the top layer will become so compacted that it wont take on water. In this situation what usually happens is the person watering the tree wets the surface and thinks that the tree is taking on the water but what is actually happening is the water is running down the sides of the root ball and not soaking into the centre leaving the heart of your root ball dry which will eventually die off. The more of your root system that dies of the more your tree will struggle with things such as heat or cold stress and it will also suffer from pests as they usually attack weak trees.

So to summarise everything we just spoke about if we under stand that putting a tree in a Bonsai Pot means we have finished development ( growing ) and now want to start refinement ( Bonsai Work ). Refinement is that act of now growing smaller finer branches, leaves and internodes. Organic soils promote larger growth on trees, they hold a lot of nutrients that promote larger growth on trees and they are also not suited to a shallow environment. Inorganic soils promote smaller finer roots which promote smaller finer growth, They hold little nutrients giving you more control over feeding so you can back it off when the tree is about to put out new growth leading to smaller tighter branching and they also work very well in a shallow environment holding enough moisture for the tree to be happy while draining any excess out of the bottom.

I hope this article today has helped you understand a little bit better why you may see some people out there saying to never use regular garden soil in a bonsai pot, If you want to practise proper bonsai then this will be a big help for you moving forward. There is a good reason that all professionals use inorganic components such as akadama, pumice and lava rock, it isn't just because they enjoy paying 10x more for their soil then everybody else.

You can purchase Akadama here

1-3mm / Shohin Bonsai or Top Layer

3-5mm / Medium size Bonsai or Body Mix

5-8mm / Large Bonsai or Drainage Layer

Until Next Time, Enjoy Your Bonsai Journey.

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