Soil Basics

Adapted from material by John Planting

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 decomposed granite instead of sand or pumice instead of Kanuma). These recipes are intended as a starting point.

Soil PH

Soil pH is a measure of the acidity or alkalinity of the soil on a scale of 0 to 14. Neutral is 7, while less than 7 is acidic, greater than 7 is akalai. Soil pH affects the amount of nutrients that are soluble in soil water and, therefore, the amount of nutrient available to plants. Some nutrients are more available under acid conditions while others are more available under alkaline conditions. Most deciduous trees (e.g. maples, elms) prefer soil that is nearly neutral – 6.5 pH. But conifers prefer a slightly more acidic soil, around 5.5 pH.

Akadama and sand are used in our club soil recipe for moisture retention. Akadama is slightly acidic. Lava and pumice are used to improve drainage. Lava has an average porosity of 70-90% (depending a bit on type and size) and is listed as an inert material so no influence on pH values. Pumice has an average porosity of 90% and has a tendency to increase pH values slightly. It has the added advantage of weighing much less than lava. Pines and junipers prefer a slightly more acidic soil, so using Kanume (4.5 – 5 pH) instead of punice is recommended. Sierra and Utah Junipers definately need this.

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 Kanume - and is a lot less expensive. Many people successfully use Turface instead of Akadama

Soil Size

The size of the particles used is important. For example, a mixture made with larger particles will drain faster than a mix made using smaller particles. A small pot will have less soil and tend to dry out faster than a larger pot. We can keep a small pot from drying out too quickly by using smaller sized soil particles in the mix. The table below shows the size of particles recommend for different size pots.

Pot Size

 Particle Size 

Shohin (e.g. 4" diameter)

1/8"

Shallow (<1.5" deep)

1/4"

Medium (1.5" to 3" deep)

3/8"

Deep (> 3" deep)

1/2"

Soil mixture is made by first sorting the various materials (see tables below) into the proper size and then combining them. Sorting is usually done using a sieve such as shown below. Sieves typically come with three screens that allow you to sort your materials into the proper size according to the table above.

Sieve for sifting soil

Sieve for sifting soil
Image courtesy of Joshua Roth tools

Making Soil

Here's the procedure for making a "soil mix." 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 keep this dust to make Muck (described below) or discard.

Now that you have each of the components sorted into three sizes, combine them in the rations shown in the tables below. For example, if you need soil mix to pot a black pine follow the ratios for Conifers.

Deciduous (e.g., maples, elms, flowering trees) and Broadleaf Evergreens (e.g. boxwood)

Material

 Measures 

Akadama (use coarse for large trees)

5

Pumice

2

Fir Bark (aka Orchid Bark

1

#3 River Sand

1

Fine Lava

1

Turface

1

Conifer (e.g., pines, junipers)

Material

 Measures 

Akadama (use coarse for large trees)

6

Pumice

2

Fir Bark (aka Orchid Bark

1

#3 River Sand

1

Fine Lava

1

Turface

1

Azaleas

Material

 Measures 

Kanuma (use coarse for large trees)

All

- or -

 

Kanuma

2

Pumice

1

Fir Bark (aka Orchid Bark

1

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 use 3/8" size soil. However, when pots are deeper than 3" it seems to be valuable to use a layer of larger soil in the bottom to make the soil drain faster. This is shown in the figure below.

Pot with soil

How to layer soil

BTW: 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. Simply use a chopstick or small rake and gently rake the surface over the edge -- be careful not to damage any roots. Then put fresh soil on top and use a chopstick to settle it in around the roots.

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/or sand 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 and sand.

Suppliers

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

Material

 Supplier 

Akadama (fired clay gravel from Japan)

Grove Way Nursery
1239 Grove Way
Hayward, CA 94541
(510) 537-1157
Open: Thurs-Sat 9am - 5pm

Kanuma

Grove Way Nursery

Turface

Turface (in 50 lb bags) can be found at:

Sierra Pacific
510 Salmar Avenue
Campbell, CA 95008
408.374.4700

Pumice

Sold as Dry Stall (in 40 lb bags) at
An Jan & Feed located throughout the Bay Area.

You can buy larger quantities from:
TMT Enterprises
1996 Oakland Rd.
San Jose
(408) 432 9040

The minimum charge is $20, but you can fill up 5-6 five-gallon buckets for that price. This is a commercial site with large trucks so be careful. Open Mon thru Fri, 7am-4pm

Fine Lava (Red)

TMT Enterprises

Fir Bark (aka Orchid Bark

Most nurseries, e.g. Orchard Supply Hardware

#3 River Sand

Cemex
Lapis Road
Marina, CA 93933
(831) 883-3700
This company sells truck load lots, but will sell a few bags to individuals. But you must order several days before driving down to pick it up.
Note: Marina is almost as far south as Monterey.

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.