Characteristics of good bonsai substrate

When growing bonsai, there are few subjects that are more controversial than substrates. To some the concept of not using the Japanese substrates is near blasphemy. Everything that is used has to be approved by the Japanese masters. That is of course not a bad idea. However, not all climates are like in Japan. As such, not all Japanese techniques are suitable for my local climate. Maybe that is the most important lesson to learn.

Everything you do in bonsai, and as such, also the choice of your substrate, should be based on your local growing conditions & your personal care skills & abilities.

In any case. In the very first year I started bonsai I had a few plants from other growers that were planted in Akadama, and I was disappointed. All of them died within 2 years of purchase. Upon checking the plants afterwards, the roots had died and turned mushy. In my garden Akadama broke down to airless clay within one year. The repeated frost-freeze cycles in winter just did too much damage to the structure. As I am not convinced a good substrate has to come from the Orient, and be shipped around the globe in order to be suitable, I started looking for alternatives. The first thing I wanted to know are the qualities a good substrate must have. From my trawling the internet combined with my own insights in plant physiology I came up with a shortlist of things a good bonsai substrate should have.

Qualities of a good bonsai substrate

In order to grow bonsai, substrate needs to meet a few simple criteria.

1. Bonsai substrate should be water retentive
2. Bonsai substrate should be able to retain & release nutrients
3. Bonsai substrate should allow air circulating to the roots
4. Bonsai substrate should be free draining
5. Bonsai substrate should be stable

Let us look at these individually, shall we?

Bonsai substrate should be water retentive

This is logical right? A plant needs to have a certain amount of water near the roots. Some of them a lot (like willow) and others prefer the substrate on the dry side (Pines). But, fact is, none of them survives if the substrate is too dry for too long a lime. Water is the means to moving nutrients into a plant, moving it around and for evaporation, essential to cooling the foliage. So a good substrate is capable of retaining enough so that the roots have access to enough water between watering.

Bonsai substrate should retain nutrients

Similar to water retention. We fertilize our plants in order to give them access to nutrients. The substrate should be able to keep this and not let it all flush away as soon as a bit of rains passes by. The way this is measure in cation exchange capacity: The capacity to exchange ions (Charged particles). In effect, this binds nutrients until the concentration in the water gets below a certain point, and slowly releases the nutrients back in the substrate.

Bonsai substrate allow air circulation to the roots

Contrary to intuition, roots do not live long in situations where there is a shortage of air. They need air to stay alive, grow and function well. So a good substrate is open enough to allow air to come in. Especially when watering, a lot of air is pulled through the substrate. Typically, a good substrate will have a grain size of at least 2 mm (~1/10^th of an inch) to allow the fast drainage and aeration needed, bringing me to the last characteristic.

Bonsai substrate should be free-draining

Standing water is death for trees. So all substrate should drain freely. Any water should go through it without too much effort. As mentioned before, a grain size of 2mm (1/10^th of an inch) is a generic minimum to have for your substrate. Everything smaller should be sifted out.

Bonsai substrate should be stable

As can be concluded from my own experience with akadama, and the reasons why coarse substrate is used: You want the substrate to remain intact for the period that a plant grows in it. Pines may be repotted every 10-15 years once mature bonsai. So ideally you have a substrate that lasts up to 10, 15 years. Otherwise the substrate, and not the tree, dictates the time of repotting.

What do I use?

Cat litter

Based on these characteristics I started to look for solutions that were in use. One of the first I found was actually through the website of Harrie Harrington in the UK: bonsai4me. He grows his trees in pure baked diatomous earth. This product is sold in many European countries as a non-clumping cat litter. If you read the packaging it is actually recommended as a soil improvement addition. Note: This is a very specific type of cat litter. Most brands of cat litter are NOT suitable.

After growing everything in pure diatomous earth for some 2 years, I realized it stayed quite wet. In winter the water stored in the granules when frozen produces a number of icepeaks on the granules. And in summer the substrate did not dry out quick enough to stop algea to form. Watering only every other day was not an option: The substrate would be dry b the time I would rewater. Not ideal, unless you can use a sprinkler system on a 36 hours cycle.

Pine bark

I started to look for additives that could work. I started with adding ground up pine bark, which helped a bit, but did not make a big difference. I do keep it in there as I am convinced it helps with bacterial colonization. This in turn helps with the breakdown of organic fertilizer pellets. But.. I cannot find any real confirmation that this indeed helps.

Not lava

Then I looked at lava. But although I could find it at the right size, it was very heavy. Because of the solidity of the material, it warms up very slowly, which in Spring for me is a downside. So I left that track quickly.

Expanded shale

So I was still stuck for a solution, when I was buying some construction materials for the renovation of my house. And that is where I came a across a product called expanded shale (In German: Blähton). I found them as a product made to level floors, in a good coarseness for the pots. The kernels are broken and sifted to size. This material drains perfectly. Has a large surface area due to the broken nature of it. And dries quickly. This is now the core material of my substrate. On its own it is too dry. So I mix in kitten litter and bark to keep it more evenly moist without having a constantly wet surface area.

Cactus mix

Besides that, this year I am going to experiment with a mixture I found consisting of 1.5-5mm graded mix of Lava, Bims and Zeolith, either mixed in the original substrate I was using, or separately.