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Bonsai Soil Mediums - How they work and what they do

Last time, we discussed our fertilising guide. Now, we look at one of those topics that all bonsai growers love to discuss. If you get a group of bonsai growers together, they will soon start discussing their preferred soil mixture.

The basics

Let’s start by defining soil medium. Conventionally, a ‘soil medium’ is a natural component in which plants grow. Soil can also be defined as a natural body consisting of layers that are composed of weathered mineral materials, organic material, air, and water - However, in bonsai, we use various other elements that start out being natural but that are processed to improve their functions. A few examples are perlite, akadama, and vermiculite.

In order to break it down even further, we need to think of what the trees need. Any soil medium must be able to do two things: supply a sustainable foundation for the plant to anchor itself into; and create a balance between moisture content and aqueous vapour. In other words, the soil needs to provide sufficient drainage while still being able to hold enough moisture to sustain root health.

The soil medium that we use was started 20 years ago by Duncan Wiles to get a medium that covers all or most of the basics. Over time, we have changed the mixture to maintain the high quality of the medium we provide. The mix that we make has been thoroughly tested and updated to incorporate better media.

Our current mix was revamped two years ago to incorporate two additional elements: perlite and a better graded stone. Since the change, we have dropped the weight of the soil by 3.5kg in a 9-litre bag. This helps a lot when having to use 5-10 bags per large bonsai pot. Plus, we have improved the soil texture for use in small and large pots alike.

The elements of our soil

Our mix has the following elements:

  • 4-6 mm stones: these are the basis of our mixture and supports the drainage and root development of the tree. Our mix drains in 5 seconds from when water is first applied. The more grit or stone in a mixture, the more the roots must work to get at the moisture, creating finer roots. These finer roots are more capable of searching for the moisture in the soil, which creates healthy trees that can take up nutrients even better.

  • Perlite: this is added to keep the weight of the medium down but, most important of all, to aid in the absorption of nutrients and some moisture. We do not want to hold onto too much moisture as this will make the roots lazy and may lead to root rot.

  • Pine bark: this is a naturally acidic medium, and it aids in keeping the soil mixture at an optimal pH level. The pine bark we use is finely sieved for better decomposition and release of nutrients.

The most-important factor in soil mixes is to use the same mix throughout. Do not layer it with another type of soil mix as this will hamper drainage, and root development will not be balanced in the medium. Our mix is made to be used as is.

Our soil medium can be used on all species of bonsai trees. The reason for this is that, despite trees having different nutrient requirements, our soil provides excellent anchorage and promotes root development while correct and regular fertilising will deal with any nutrients that each tree may require. Correct placement of the bonsai tree in the garden will also make a huge difference on the growth.

What is your optimum soil medium? Do you make it yourself? Let us know what you think about our soil medium.

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