Bonsai soil is a topic that may seem basic, but it can be quite a deep subject. No pun intended.
Quite obviously, bonsai soil is the medium by which the bonsai is provided water, fertilizer and oxygen. In order to do this most effectively, however, it requires varying types and sizes of soils. A bonsai tree requires these different particle sizes and types of soil because it helps them to grow better and emulates the soil the large cousins of the bonsai grow in.
Soil Science 101
Imagine a cross section of the Earth like you see in science books. At the very top, the surface of our planet, there are two levels: bedrock and sediment.
Bedrock is the farthest you can dig with a shovel before you need drills to pierce the rock. On top of that is the sediment layer. This layer is really not that deep comparatively. In some creeks, it’s a mere 2 – 3 feet down. Typically, however, the depth of sediment is 18 – 60 feet deep.
As we look at this layer of sediment, no matter how deep it is, we find that it is made up of various layers of material that increase in size as you go deeper.
On the top layer of sediment, you find silt, which is like powder. This silt most often has organic matter mixed in with it. Going deeper, you find rocks that were once pieces of bedrock. On the top most layers these rocks are pea gravel sized. Then they increase to gravel, then stones, cobbles, then at the very bottom, boulders.
This layering effect is a natural byproduct of gravity. The resettling is a natural result that occurs over thousands of years because the Earth is spinning and going around the sun and water frequently saturates the sediment.
What we find when we look at the large trees is that the root zone is primarily in the mid level of the sediment. This level quite frequently also retains the most moisture as well.
Every time it rains, the water leaches into the soil. As the water percolates down, it draws oxygen into spaces and gaps between the soil particles. The water is held the longest in the mid layer because the top layer is warmed by the sun and blown by the wind, and the bottom layer has mostly larger rocks which don’t hold a lot of water.
Types of Bonsai Soil
Our goal when choosing bonsai soil is to create the same type of environment that the tree would naturally live in out in the wild.
In order to do this, we will need to duplicate the various layers in nature in the bonsai pot. We would just do it on a smaller scale. The top layer particles would be around 1/8 of an inch, the mid soil layer would be ¼ inch sized particles and the bottom layer would be ½ inch and larger pieces.
We can use several different types of soil. Namely, we can use akadama, kanuma, haydite, pumice, grit, gravel, pine bark, peat moss, calcined clay, river sand, top dressings and zeolite.
Basically, these materials comprise types that retain water, types that don’t, and types that do both –they hold some water and they fluff, and aerate the soil.
Putting the Soil To Use
Some species of plants require a soil that retains more water and others enjoy soils that are more porous and dry. This means that you can use the various soil materials to create specific versions of soil for specific types of plants and specific climates.
The mixtures of soils can vary from heavy moisture retaining, to arid soils. Since Akadama or Zeolite are excellent bases for soil mixes, I suggest using various sizes of these and mixing in percentages of other material depending upon layer and species. Note: If your plant is acid loving, substitute Kanuma for Akadama or Zeolite.
For example, Maples and Beeches thrive in 100% Akadama that has been screened by size. Junipers like 50/50% mixes of Akadama to calcined clay, haydite/expanded shale or grit. Most every other tree falls in between these mixtures.
While it might sound more complicated, most trees, if not all, would thrive using the layered approach found in nature. This means that the soil from bottom to top is made up of several different materials in differing sizes.
Starting with the bottom layer as if potting a plant, add large pieces of akadama, calcined clay, pumice and haydite in pieces ½ inch or larger to the pot. On top of this layer add in ¼ pieces of Akadama, pumice, pine bark chips, and river sand. It is into this layer that you will plant the tree. On top of this, the top layer would consist of 1/8 inch pieces of akadama, pine bark chips, and pumice. To top it all off, you could then apply a top dressing of sand which complements the pot and the tree.
Hopefully you now see that there is a science to soil which takes it out of the realm of mere dirt. As you purchase different materials and spend some quality time with your bonsai, you will start to see how all of these materials can work together for you. Every mixture can be tweaked depending upon the climate your plants live in. If it’s extremely rainy, windy, cloudy, or sunny throughout the growing season, you may need to tweak the soil depending. If you live in Arizona and want to grow bonsai, you are probably going to have to add peat moss to your soil. If you live in Seattle, you are probably going to want to add more pumice, calcined clay or grit.