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Soil Basics

There are many different soil mixtures used to plant bonsai. Bonsai people in Brooklyn, Scotland, Holland, Japan, Canada and California all have different growing conditions and different materials available to make soil mixtures. Most mixtures have one thing in common — they don’t use garden soil. This is because shallow pots don’t drain as well as soil in the garden and as a result the roots can stay too wet and rot!

These recipes describe “soil mixes” that we’ve found works in the micro-climates around Palo Alto, California. If your weather is warmer or cooler than Palo Alto then you’ll need to modify the mix slightly. This is explained later.

While these recipes were created by several of our club members working together, these aren’t the only recipes being used. Some club members use different proportions or substitute materials (e.g. using Turface instead of Akadama). These recipes are intended as a starting point.

Development vs Refinement Phases

The earliest phase of a bonsai’s life is the Development Phase. This is when you encourage rapid root growth and therefore rapid upper tree growth. During this phase use a soil that ensures that the tree roots get as much fresh air (oxygen) as possible. We recommend a soil mix that is mostly pumice and lava with small amounts of Akadama and fir bark.

Normally a tree will be in Development Phase for 1 to 3 years after being transplanted from a nursery pot, but some trees may require additional time. Once your tree has grown to the size you want and the surface roots (aka Nebari) look good, it is time to shift to Refinement Phase. In this phase you want the tree’s growth to slow down. This will give you shorter internodes and finer growth. We do this by reducing the amount of air available to the roots. The soil mix will use less pumice and lava and more Akadama. Over time the Akadama will break down and fill up the air pockets a bit, and thereby reducing the growth rate further.

Development Phase Basic Recipe

The following recipe will work for trees in Development Phase.

  • 4 parts Pumice
  • 4 parts Lava
  • 2 parts Akadama, a fired clay particle imported from Japan
  • 1 part fir bark and/or horticultural charcoal

Turface can be used instead of Akadama but will drain more slowly. If it dries out it is harder to re-wet.

Horticultural charcoal, also known as Orchid Charcoal is similar to Activated Charcoal used in aquariums, but is much cheaper. It is similar in size to pumice. Note, charcoal briquettes such as used in a BBQ grill are very different. Don’t use that in your mix.

Refinement Phase Basic Recipe

The following recipe will work for trees in Refinement Phase.

  • 4 parts Pumice
  • 4 parts Lava
  • 4 parts Akadama
  • 2 part fir bark and/or horticultural charcoal

Making Soil

Sieve for sifting soil.
Photo courtesy of Joshua Roth Tools.

Soil mixture is made by first sorting pumice, lava, akadama and fir bark into the proper sizes. Sorting is usually done using a sieve such as shown here.

You’ll be sifting each material three times. First, sift using the largest screen and keep the material that doesn’t fall thru the screen into one pile or bucket. Now use the medium screen and again sift the material that went thru the largest screen — keep the material that doesn’t fall thru the screen into another pile or bucket. Now use the finest screen and sift again — keep the material that doesn’t fall thru the screen into a third pile or bucket. Only very small particles or dust will go thru this screen. You can keep this dust to make Muck (described below) or discard it.

Now that you have each of the components sorted into three sizes, combine them in the rations shown in the Development or Refinement recipes. But first, decide which size of soil particles you need to use. And then adjust your mixture (slightly) according to what you’ll be planting in this soil

Soil Size

The size of the soil particles will influence how fast a pot dries out. For example, a mixture made with larger particles will drain faster than a mix made using smaller particles. A small pot used for shohin will have less soil and tend to dry out faster than a larger pot. Trees larger than 24″ will be grown in big pots and dry out slower and so larger size particles will help them dry out faster. Taller pots drain faster than shallow pots. The table below describes the size of soil to use for different size and shape of pots.

Pot SizeParticle Size
Shohin (e.g. 4″ diameter)1/8″ / 3mm
Shallow (<1.5″ deep)1/4″ / 6mm
Medium (1.5″ to 3″ deep)3/8″ / 10mm
Deep (> 3″ deep)1/2″ / 12mm

Adjusting Recipe for Junipers, Pines and Azaleas

The two recipes are good for most trees, but Junipers and Pines will grow better in soil that is slightly more acidic. We adjust each recipe by adding one additional measure of Fir Bark or by using Kanuma (a Japanese pumice that is more acidic) instead of pumice.

Azaleas need an even more acid soil and so are frequently grown in a mix where Kanuma is the primary or even only component.

Adapting for Other Micro-climates

If your micro-climate is warmer than Palo Alto (e.g., San Jose or Gilroy) you’ll want to modify the mix so it holds more moisture. For example, by adding more Akadama and less lava and pumice. But, if your micro-climate is cooler / moister (e.g. Seattle) you’ll want to modify the mix so it holds less moisture. In this case use more lava and/or pumice and less Akadama.


Using Soil

Now that you’ve made your soil mix, here’s how to use it.

Putting Soil in the Pot

Generally you will put a shallow layer of soil in the bottom of the pot, put your bonsai in the pot and add the same size soil on top of the roots. For example, if you are planting a black pine in a rectangular pot 12″ x 8″ x 2″ deep, you would start with a layer of 3/8″ size soil that’s about 1-inch deep.

How NOT to layer soil

Don’t Mix Different Sizes of Soil. Some people believe that using a layer of larger soil on the bottom will allow the soil drain faster. However, research shows that this isn’t true. In addition, once you start chopsticking the soil to settle it around the tree roots you will mix some of the smaller particles into the larger soil layer. And, over time, as the soil is jostled by watering, etc. the larger particles will “float” upwards – at least until roots are well established.

Maintaining Your Soil

Kathy Shaner recommends replacing the top inch of soil once or twice a year. This gets rid of a lot of weeds and salt build-up. Use a chopstick or small rake and gently rake the surface over the edge of the pot. Be careful not to damage any roots. Add fresh soil on top and use a chopstick to settle it in around the roots.


Sources for Soil Components

Some of these materials are difficult to find, so here’s a table of reliable sources.

MaterialSupplier
PumiceEncinal Nursery carries large bags of sifted pumice.
Jonas Dupuich (owner)
2041 Encinal Ave
Alameda CA 94501

TMT Enterprises
1996 Oakland Rd.
San Jose, CA 95131
(408) 432 9040
The minimum charge is $20, but you can fill up 5-6 five-gallon buckets for that price. The pumice (and lava) are in large piles that you will need to sift. This is a commercial site with large trucks so be careful. Open Mon thru Fri, 7am-4pm

Yamagami’s Nursery in Cupertino has small bags of pumice.
1361 S. De Anza Blvd., Cupertino, CA 95014
Summer Winds Nursery and other nurseries sometimes have small bags.
LavaMountain View Rockery has been carrying multi-colored lava – brown, tan, red. It looks really good in a soil mix. 
50 Centre St,
Mountain View, CA 94041
TMT Enterprises 
carries red lava
Fir Bark (aka Orchid BarkYamagami’s Nursery has Orchard Bark in large bags. Sometimes there are two sizes, be sure to get the smallest sized particles.
Akadama (fired clay from Japan)Encinal Nursery 
address above

Grove Way Nursery
Johnny Uchida (owner)
1239 Grove Way
Hayward, CA 94541
(510) 537-1157
KanumaGrove Way Nursery
address above
TurfaceTurface (in 50 lb bags) can be found at:
Sierra Pacific
510 Salmar Avenue
Campbell, CA 95008
(408) 374-4700

Horticultural or Orchid Charcoal
Yamagami’s Nursery
address above

Substitutions

Turface is most commonly used as a soil additive to help improve drainage. Turface absorbs its weight in water while decreasing soil compaction, which is what you want from bonsai soil. So in that respect it acts like Akadama or Kanuma – and is a lot less expensive. Many people successfully use Turface instead of Akadama.

What About Sand?

Previous versions of our club recipe used #3 sand. The idea was that because sand doesn’t hold water it would improve drainage. However, we now know that’s incorrect. Because the sand is smaller than the other components it actually winds up reducing the size of the air pockets and increases the Water Holding Capacity of the soil mix- just the opposite of what we think it would do.

Nurseries use sand in their mix to make the plastic pots heavier and less likely to blow over. That’s not usually a problem for us.

Muck

Muck is another important “soil” mixture. It is a paste that’s used as a binding agent to hold soil or objects such as rocks stable in the pot until the tree roots are established. When we use a rock slab (e.g. slate) instead of a pot, we often use Muck to create an edge that will prevent the soil mixture from sliding off. To do this, roll muck between your hands until it looks like a rope, perhaps 1/2 inch in diameter. Then lay the rope on top of the rock slab and arrange it until you like the shape. It usually should be irregular in shape. Now press it down until it sticks to the rock slab. Now add your trees and soil mixture.

Recipe: Sift adobe soil, keeping only the fines (dust to 1/8″). Use a screen that’s the same size as window screen or slightly smaller). Some peat moss is long and fibrous. If this is yours, cut it using scissors into lengths approximately 2″ in length. Now mix half adobe and half peat moss. Add just enough water that it can be kneaded to a dough-like consistency. It’s easy to knead if you put the mixture into heavy-duty zip lock bag.

Hint: If you can acquire black adobe that will make the best looking muck, but it’s difficult to find. In fact, depending on where you live any kind of adobe soil may be difficult to find. If you are using Akadama in your soil mix you are probably throwing away the dust when you sift it. Don’t — use it in this recipe instead.

Storage: If you have any left over, store it in the zip lock bag in your freezer. (Yes, your freezer. This will keep it from becoming moldy.) When you need it for your next project just remove from the freezer, let it thaw, add a bit more water if needed and then knead it a bit to get it flexible and ready for use.

Going Deeper

During our February 2017 club meeting Dave Curbow and Michael Greenstein did a presentation that explored the science behind Bonsai Soil. The slides for that presentation are available here.

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