Artificial v Organic.
Soil is a huge subject and to try and cover every aspect of it here would be impossible.
To keep it simple I will outline why I choose the soil mixes that I use and give a rundown on other soil components and why they should or should not be used in a bonsai mix.
I will refrain from filling the post with technical figures and mind numbing botanical blurb. The tech stuff can be found elsewhere on the internet and I encourage everyone to do their homework. Of course there will always be those that will have a different opinion about some of the things I am about to say.
In the past couple of decades it has been the fashion here in the UK to try and make the most "free draining" soil mix possible. I know some will totally ignore the basics of soil composition to favour the 'free draining mantra'.
I feel this has been to the detriment of the overall function of soil in the bonsai pot and what it is there to achieve.
Many modern mixes are now so free draining and made of particles that the roots cannot penetrate that the reason why soil ingredients are there is being overlooked.
A mass of thick white roots can often been seen in the bottom of such bonsai soil filled pots and while this might give the impression of a healthy tree growing vigorously, it comes at the cost of finesse which is shown by the lack of fine twiggery being achieved in the tree above ground. There will also be a lack of roots in the main body of the pot soil.
It also results in trees needing to be re-potted far to often than is good for the tree which can delay the overall progress of a trees development.
The use of 'molar' or 'cat litter' & 'Seramis' has prevailed over recent years but many are using it without knowing its properties.
It was originally invented to soak up chemical spills which it does very effectively. It will soak up a lot of water too, in fact it holds more water by weight than all the other soil ingredients that most people will encounter such as akadama or kanuma.
It changes colour so it is easy to see if it is damp. The problem is that it holds on to the water it soaks up and doesn't give it up easily.
In a situation where it is used in soil mixes it will actively suck moisture out of other ingredients as the pot soil dries out. At the same time it makes the soil 'look' as though it has water in it when it is the only thing in the pot with water soaked into it.
Additionally, the trees roots will not penetrate properly into molar or Seramis particles.
I have seen many trees that have been potted into these 'molar-based' mixes and I have noticed that the trees roots do not interact with the particles in the soil. Instead they just passed by the particles in a race to the bottom of the pot where all of the water and nutrients are ending up because of the 'over' free draining properties of the soil.
These mixes might be okay while a tree is in development and course, vigorous growth is wanted but when it comes to a more finished tree and long term trying to build up fine roots and twigs, my opinion is that they just do not make a good substitute for the more traditional akadama based soil mixes that the Japanese have used for decades.
They may also have other chemicals in them that are detrimental to plants such as disinfectants.
Over the past 40 years I have tried dozens of different soil mixes and different components in the soil.
I have carried out many experiments on various soil components over the years regarding
water content, water holding properties, root penetration etc. and how these things affect the growth of the plants that are put in into them.
I always noted that plants that were grown in akadama put on more physical weight than the plants grown in "artificial" soils. This leads me to conclude that the wood is more dense and therefore stronger and more able to put up with the various pathogens that affect softer growth on trees. When plants reach this level of health, many pests simply cannot get their teeth into it.
Colin Lewis wrote a blog a few years ago concerning the properties of molar and it's use in soil mixes. He appears to agree having conducted his own experiments. I will try and locate his blog again and put the link up.
Bjorn Bjorholm explains soils very well without too much Blah blah blah!
If you can tolerate the Blah blah blah then Ryan Neil has produced a good video all about soil and how it behaves in a pot.
All of these are qualified bonsai professionals and are worth watching.
Akadama is a special kind of volcanic clay that has been used in horticulture for decades.
Native to Japan, this soil offers a wide range of benefits to bonsai enthusiasts all over the world.
Akadama has a very unique structure and contains such a high concentration of minerals that it is often referred to as “the king of potting soils”.
It is mostly composed of minerals such as iron, magnesium, and potassium as well as many other trace elements.
Although it is not as acidic as peat moss, it is slightly acidic and is still able to provide essential nutrients for most tree species plants.
It's granular nature allows for lots of air in the soil which is essential for healthy root growth. (Never add compost to akadama as doing so will destroy most of its beneficial properties.
As a result, it has been favoured by bonsai growers for its unique characteristics, for decades.
Akadama can quickly absorb and release moisture depending on the environmental conditions.
It is also renowned for its buffering properties as it lowers pH levels of some hard water. This is because minerals, such as iron and magnesium, are able to break down and release more alkaline elements as the substrate begins to break down.
This provides a much more stable environment for bonsai.
Also, Akadama can retain more moisture than most ingredients and can help keep the substrate moist for longer.
Akadama based soils are free draining so I don’t think it is that that is causing the movement against akadama.
The argument of price is often brought into the conversation. My experience is that using akadama is actually cheaper in the long term because repotting does not need to be done as frequently compared to artificial soils.
Having to re-pot a tree because it’s pushing itself out of the pot every couple of years compared to the trees in akadama mixes where the tree can be quite happy for 5-10 years and even longer. Repotting too frequently doesn’t do the tree any good either and gradually weakens the tree.
The predominant argument against akadama that people use is that it ‘breaks down’ too quickly.
I don't know what brand of akadama they are using but I have not found this to be the case either. I have had trees in akadama based mixes for over 10 years and when it comes to repotting them the akadama is still in good condition, with of course some breakdown but nothing as catastrophic as people are making out.
It is here that the brand of akadama is important.
Cheap akadama is not worth the bother. I have used Ryusen akadama and I've found that that does indeed breakdown very quickly. It almost dissolves instantly on contact with water.
Personally, I use Iberaki akadama, double line which is a hard akadama of top quality and I will not use anything else.
There is a triple line akadama available although we very rarely get it in this country and I have been speaking to some importers of akadama to try and get them to bring over the triple line as it is harder than the double line. I think it would probably silence the critics that say akadama breaks down too quickly.
There are some caveats to what I’m saying.
Akadama used on its own can break down quicker than akadama that is used in conjunction with grit or pumice.
(This is where adding grit comes into it. Adding a grit that also provides moisture like Leca or pumice, is beneficial with our hotter weather.)
I think this is because the grit/pumice content allows movement of soil particles
within the soil when the temperatures reach freezing point. When the soil defrosts the particles return to where they were in the soil without having broken down too much if at all. Whereas, pure akadama freezes as a solid block and therefore seems to suffer more breaking down than it does when mixed with other ingredients.
I use Akadama based soil mixes for trees that are in development and for the more finished specimens.
Simply put, the Japanese have been using akadama for their bonsai for decades and they are the masters of bonsai culture so who am I to argue with them.
Akadama works well with the natural process of trees growing in containers. The roots penetrate the particles and over time this interaction results in finer roots and a better structure in the tree overall.
In conclusion, using artificial soils such as molar and similar products may provide a free draining soil but there is little interaction with the trees roots. I have observed that the roots will pass by the particles in the soil and just end up at the bottom of the pot going around in circles until they build up such a mass that they physically lift the tree out of the pot.
They are basically ‘too free draining’ and we have sacrificed soil complexity for the sake of having a very free draining mixture made of artificial soil particles, which in the long term does not produce top quality bonsai like the Japanese are able to produce.
The only thing that seems to matter anymore is ‘Free draining’, but at what cost? I have seen a dawn redwood group planted in one of these modern mixes where the roots were
all at the bottom of the pot pushing the tree out of the pot. In addition there was not a single particle of that soil where the roots had penetrated. They just pass by on the way to the bottom, where the last remnants of moisture and nutrients are.
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