Let us begin this article regarding Bonsai soil with a clarification of terms. Whilst the phrase Bonsai soil is what most people will search for to reach this point it is a term that makes me think of vegetable gardens and the stuff that you get in the borders. Similarly the terms compost, loam, soil mixture and other such earthy phrases tend in my opinion to give the wrong impression. What we need for our trees is better represented by the phrase "Bonsai Potting Medium". It makes no assumptions as to its content and describes perfectly what the function will be.
There are countless ingredients that can be used in the make up of a potting medium suitable for Bonsai. Everyone has their personal favourites and will swear by the success of their own mixture. This suggests that what you use is not that critical and the differences are not that great. However, we all live different lives and when considering the right medium or combination of ingredients you need to consider what that suits your trees requirements, your lifestyle and local climate.
Lets attempt logically to build the "ideal" potting medium step by step by examining certain desirable qualities.
Your Bonsai growing medium should be free draining. That means that when you water the water runs through the pot and out of the holes in the bottom. This makes it almost impossible to over water. Pure grit drains perfectly. However you don't need me to tell you that if all the water drains away then the roots are going to be pretty dry. Which brings us neatly to our next quality.
Right then, we have our lovely free draining grit but all the water is running away so we need to add something that will hold on to some of that moisture so the roots can take up what they need. Peat or its environmentally friendly alternatives is pretty good at holding onto water so we could use that. For many years when I had more trees than I knew what to do with and a limited budget, this is exactly what I used. Bog standard, multi-purpose potting compost mixed 50/50 with fine 2-3mm builders grit. It worked fine. The trees grew quite happily, and there was enough water retained to keep them going while I was out at work. I used this mixture for at least 10 years but as the quality of my trees improved I noticed how the surface got crusty towards the middle of summer and became aware that the root growth whilst abundant was not as fine as I would have liked. There was also a tendency for root growth to be mainly at the outer edges. So our perfect medium will need to be water retentive to a point. Clay is pretty good at holding on to water but a solid lump of clay is not exactly what we need. We're still lacking that magic ingredient. Lets think about that for a moment and see what else we need.
A much-overlooked requirement for roots is a ready supply of oxygen. The tree takes in oxygen through the roots as part of all that technical stuff that goes on to make it grow and if the mixture around the roots is too dense then it will not be able to penetrate as easily. This explains the disappointing inner root growth with my old peaty/gritty mix. The potting compost would clog up the gaps between the grit and would compact over time making it difficult for oxygen to penetrate. Another important point to consider is the means by which oxygen is drawn into the root ball. This happens as the potting medium dries out so it is worth remembering that a tree that is kept constantly wet will be suffocating. The trick is to water just before the supply of moisture runs out and give a thorough soaking. This cycle of wet/dry is one of the things that the books don't tell you and will greatly improve results.
For many new Bonsai growers their experience of gardening will have involved planting trees and shrubs, heeling them in well with the rear end of a size nine Wellington boot or planting up cuttings by firming them in well with big green thumbs. Forget that you ever did that. The approach to Bonsai is totally different. The potting medium is not compacted between the roots and the tree is not held in the pot by the potting medium. Trees are fastened into pots by wire through the base of the pot and the potting medium is introduced around the roots and flows between them. Flow is a very good way of imagining the perfect mix because to do so it must be granular. Granularity is a very important concept and another key to success. Analogy time...Let's imagine a box of marbles. They can be considered granular. Sitting there in the box there is loads of air in between and plenty of drainage.Now lets take a bucket of sand which can also be considered granular and add it to the marbles. Give it a bit of a shake. What do we have now? Our lovely granular box of marbles with nice gaps between has been clogged up with sand. The point I'm trying to make with this regression to childhood pastimes is that the grain sizes need to be the same if the mixture is to remain open. Every time I mix soil I regress to the Homepride adverts that ran for most of my growing years. The slogan was "Graded grains make finer flour". Words of wisdom indeed and worth bearing in mind. I digress. Let me run through the thought processes that led me to my current mixture that takes into account all the stuff above. You may take a different approach but here's how I did it.
I started off with grit but just about that time a wonderful product came onto the market that goes by the name of Kyodama. It looks like crushed up lava and even though I call it grit the makers claim that it can be used on its own. I was sceptical that anything so grit like in appearance could hold enough water so planted a Juniper in pure Kyodama and waited. Three years later the tree is thriving and has never looked like it might dry out even in mid summer in a green house.
In the meantime I needed a mixture that I had confidence in and so added Akadama to the mix because I knew that it was tried and tested. The quality of Akadama which I dislike is that over time it will crumble to dust. Not necessarily a bad thing because it allows the roots to travel wherever they wish but I prefer to limit this crumbling quality to a third of my mixture. Many people use Akadama on its own which is fine but it has to be sieved and graded through the pot - large granules at the bottom and finer ones on top - to get the best results.
I have more than a few trees and not enough time to sieve all my soil components. I have found that by mixing Akadama with Kyodama, the need for sieving is removed. Not ideal but an acceptable compromise. This mixture of Akadama and Kyodama is a perfectly sound mixture that will suit just about any tree.
However, there was one more component that I chose to add. Cat litter. This is the point where you frown and mutter to yourself in an unbelieving way "cat litter?" Yes, cat litter. There was a product on the market called Biosorb - outrageously expensive baked clay granules - Cat litter by any other name. Not every cat litter is suitable. It has to be a moler clay product the most popular known to Bonsai growers being "Sophisticat Danish Pink". This is available in large sacks from pet superstores and is added to my mix in the same proportion as Akadama and Kiyodama.. The advantages are obvious. It absorbs water readily as you would expect (and makes a pleasant fizzing noise while doing so), it is of a nice granular size and best of all it does not break down. The cat litter stays solid and keeps the mixture nice and open. It also works out cheaper than the other alternatives. Last but not least it has a pleasant deodorising fragrance which has not been found detrimental to trees in any way but at the same time reportedly keeps greenfly away. Furthermore, when repotting, this mixture will easily be removed from roots limiting damage with a root hook, making the job both easier, quicker and eliminating mud from the equation. Apart from a little dust when fresh there is very little mess, it can be reused, contains no undesirable fertiliser and is neutral. This means that you know exactly what you are giving your tree - nothing. Therefore there is no danger of burning roots with unwanted fertiliser and nasty things tend not to make their home in it. Knowing that you are starting with a clean sheet means you have total control over what you subsequently feed your trees. So there you have it, the perfect mixture? Well perfect for me. I mix it a third of each but adjustments can be made without affecting performance since each of the ingredients possess the necessary properties that we have talked about. The horticulturists out there will be up in arms at this point and use words like loam, sphagnum moss and leaf mould. I have limited knowledge and experience using these traditional growing mediums so will neither try and extol their virtues or dismiss their value. I'm sure they work fine. All of which proves that there is no such thing as a perfect potting medium and finding the right one for you is a matter of trial and error.