Bonsai trees are usually grown in pots of various kinds. Since the pots are usually quite small relative to the tree, the amount of soil in the pot is also quite small.
This means that the composition of the soil can be more important than for other plants and trees since the benefits need to be provided in a smaller volume of soil.
What is the best potting soil for bonsai trees? The important things for bonsai potting soil are the drainage versus water retaining properties of the soil, providing a sturdy anchor for the tree roots, plus the nutrient content and acidity. Commercially available bonsai potting soil will have considered each of these factors when developing the soil mix. If you want to mix your own potting soil you will need to pay attention to each of these factors.
The key factors seem to be drainage, acidity and nutrient content, as well as providing a sturdy anchor for the roots. We’re going to look further at each of these and see what can be included in the potting soil to provide the best potting soil mix.
Why Is Drainage Important?
As with the soil for most pot plants, you need bonsai soil to retain moisture but not cause the soil, and the roots to become waterlogged. A common problems with bonsai trees is rotting of the roots, commonly know as root rot. The symptoms of root rot are soft roots that fall apart easily.
Root rot is caused by pathogenic fungus growing and feeding on the roots of your tree. Poor drainage and waterlogged soil can encourage this fungal growth, just like not drying between your toes can lead to athlete’s foot.
Another problem with waterlogged soil is that it can deprive the roots of oxygen, causing them to become damaged. Bonsai tree roots need to absorb oxygen from the tiny spaces between soil particles, and if the spaces are all full of water there is no room for the oxygen.
Because of the risk of fungal root rot and oxygen deprivation, drainage is usually considered more important in bonsai soils when compared to soil for any other purpose.
Soil Particle Size Is Important for Drainage
One of the most important things for good drainage is the size of the soil particles. The larger the particles are the better the soil drains, as this allows the water to pass through due to gravity. Obviously, if the soil particles are too big the soil won’t retain water at all.
Bonsai soil mixtures contain particles that are big enough to encourage drainage. These particles range from the smaller ones, which you can find in sand, to larger ones, which you would call gravel. Neither of these retain water very well, especially gravel, so will need to be combined with smaller particle material.
Commercially available bonsai potting soil normally contains the right amount of each sized component, but if you are going to make your own soil mix you can experiment to get the best results. For potted bonsai trees it’s important to use horticultural sand as the type builders use can can contain chemicals that are harmful to plants.
One of the most interesting of the large-particle material used for bonsai soil is Akadama. Akadama is a naturally-occurring clay-like material that is mined in Japan, and it comes in a variety of particle sizes.
Akadama is expensive when compared to other potting soil components, but bonsai growers seem to be prepared to pay for it. This is because it is claimed to help provide an effective balance between drainage and water retention.
It is also claimed that roots grow through the Akadama particles, not just around them, and that it supplies important minerals. Another factor could be that, in addition to the supposed growth benefits, using it makes bonsai growers feel that they are following traditional Japanese methods.
But the Soil Needs to Retain Water Too…
For the soil to support growth of the tree it needs to be able to hold water. As you know, plants absorb water through their roots, and this depends on water being available by being held between small particles in the soil.
Salts and nutrients are usually absorbed in the roots together with the water, so having enough water available is important for this process too.
As with drainage, particle size is an important factor in the ability of soil to retain water.
Small soil particles create spaces between them called micropores, which are small enough to hold water against the force of gravity. Clay soil has the smallest particle size, and if your garden has clay soil you will know how easily it can become waterlogged.
These small particles help to keep the soil moist, but by mixing with the larger particles, as outlined above, you can avoid the soil becoming saturated with water. Saturation, or waterlogging, is one of the things that can lead to root rot. This essential need for effective water drainage is one of the things that distinguishes bonsai potting soil from other types of soil.
Organic and Inorganic Soil Components
In addition to particle size, the other main way of categorizing the components of bonsai potting soil is by dividing into organic versus inorganic material. The recommended proportions seem to be roughly 75% inorganic material to 25% organic material.
Organic Soil Components
Organic material comes from living things, and for potting soil this will usually be plant-derived.
Various types of bark can be used in bonsai potting soil. One of the most popular types is conifer bark, particularly pine bark. The pieces of bark are usually selected to be around 2mm to 4mm in size so that they help to optimize drainage.
Peat moss is another type of organic material that is often used in potting soil. Moss can retain a lot of water, so is often used sparingly in the mix.
One of the reasons I wanted to write this article was because I wanted to know why I kept hearing that I couldn’t just use the regular potting compost that seems to be suitable for every other type of plant that I put in a pot.
From the research I’ve done it seems that it is okay to use standard potting compost so long as you mix it with some larger particle material to encourage soil drainage and air content. These particles can be the bark mentioned above, and the inorganic material outlined below.
Inorganic Soil Components
The inorganic components include some of the materials we looked at earlier when talking about the importance of soil particle size for drainage and water retention.
An important inorganic component for bonsai potting soil is clay, and the clay plays different roles depending on the particle size. If the clay particles are very small they encourage water retention, and if they are bigger they encourage drainage and aeration of the soil.
We already looked at Akadama, which is the traditional, naturally occurring volcanic clay that is mined in Japan. Another type of clay that can be used as an alternative to Adadame is Arcillite.
Arcililite seems to have similar properties to Akadama, and there is research that has shown it can improve nutrient availability when used in combination with pine bark.
Other, large particle inorganic substances include various types of granite, grit and sand, all of which provide the soil mix with larger particle size, which is important for drainage.
Soil Nutrient Content
We took a detailed look at essential nutrients for bonsai tree health in another article, and you can find more information over there.
The key nutrients for plant health included in various fertilizers are nitrogen, phosphorus and potassium, so it’s going to be important that the bonsai potting soil provides a good supply of compounds containing these chemicals.
Organic material in the soil mix will contribute to the supply of these major nutrients. It will also provide a supply of other essential chemicals such as calcium and sulphur, and trace elements, which are usually metals.
The final thing we’ll look at here is soil pH. This is another factor that can be particularly important when growing bonsai trees due the the small amount of soil in most bonsai pots.
From scanning various bonsai forums it appears that most types of bonsai tree do best in soil that is neutral to slightly acidic. In terms of pH values, this seems to relate to a pH range of around 5 to 7.
There seems to be a variety of ways to assess the pH of the soil, but I’m having a hard time finding good information on what to do if the pH is outside the ideal range, so I think I’ll have to come back to that one.