Bonsai Soil Particle Size

I researched the hell out of bonsai soil components. Then I spent more than I’d like on said components under the promise of growing better bonsai. Then I did the thing that stops me up every time. I actually did the thing I just researched.

Reformed from using potting soil and any premade mix called “bonsai soil,” I took out my components and carefully sieved them out. That’s where I ran into the problem.

About half my material was filtering through the finest sieve. So not cool.

 

I knew it was important to sieve. And bonsai soil components should be roughly equal in size. I was going to toss what came through the smallest sieve, but I didn’t know there would be so much.  My soil sieve set comes in 0.25, 0.20 and 0.10 inches (that’s 4, 5, or 10 wires per inch).

The material I sieved was specifically sold as bonsai soil mix. Surely the smallest stuff, and so much of it, couldn’t be worthless? It also looked nice and uniform. It felt clean and piled like coffee. I was sieving Turface, pumice, a peat mix, and some perlite.

Giving a colossal, exasperated sigh and a few choice words I crossed my arms, sat back and glared at it. I tossed it around in my mind awhile.

The stuff on the right is what didn’t pass through the smallest filter on the sieve.

I figured it was same size components. The stuff that didn’t go through the filter looked pretty big to toss. That’s when I did something dumb. I used the fines on a regular pre-bonsai tree I was repotting.

Which is why my tree looked the very next day like I killed it.

Given it looked like I killed my bonsai, I did more research. I thought I had researched it all but search the phrase “bonsai particle size” and you’ll get a whole new batch of articles kicking it up a level.

 

The Question

The rough answer I knew before I started.  Particle size depends on what size bonsai you’re potting. What I didn’t know was how to quantify that. I figured I’d sieve everything and discard anything that passed through the smallest size, and use the larger size steps for larger bonsai. This isn’t wrong per se, but it lacks specificity.

Rather, my new questions were: How foolish was I to pot in the fines? Should I really throw all that material out? And I am interested in tiny bonsai, what size should I use for that?

 

The Answer

Like many things in bonsai, there is no answer. If you can find a rule, someone else will disagree with it. As such, below is a list of a few bonsai blogs and sites and their opinions on particle size. As I can’t find a system I’m getting ballsy and stepping out here onto new ground. I’m suggesting, in general, 1/16 to 2/16 for small bonsai, 2/16 to 3/16 for average bonsai, and 3/16 + for large bonsai. Using as my base the size chart I lazily pulled from Bonsai Outlet, I recommend the following. All sizes are given in inches unless otherwise stated.

 

Triangle Society (A Bonsai club from the Raleigh / Durham area of North Carolina)

  • Small:   1/16 – 3/16
  • Large: 3/16 – 4/16

 

Bonsai Jack (Manufacturer of bonsai soil mixes both as components and pre-mixed)

  • Small:   1/16 – 2/16
  • Regular: 2/16 – 6-16 target ideal is about ¼ inch or 4/16
  • Discard higher than 6/16

 

The Bonsai Workshop by Herb L. Gustafson

  • Soil particles no smaller than 3/32
  • Soil particles no larger than 1/8
  • (I.e. toss smaller than 1.5/16 and larger than 2/16)

 

Virginia Bonsai Society

  • No smaller than 3/32

 

Japanese classification Required hands to move bonsai Approx. size Particle size range Target particle size
Imperial Eight-handed 60-80 inches Large 3/16 to 6/16
Hachi-uye Six-handed 40-60 inches Large 3/16 to 4/16
Dai or Omono Four-handed 30-48 inches Medium/Large 2/16 to 3/16
Chiu or Chumono Two-handed 16-36 inches Medium/Regular 2/16 to 3/16
Katade-mochi One-handed 10-18 inches Medium/Regular 2/16 to 3/16
Komono One-handed 6-10 inches Small 1/16 to 2/16
Mame One-handed 5-8 inches Small 1/16 to 2/16
Shito or Keshitsubo Fingertip under 2 inches Small 1/16 to 2/16
* Komono, Mame, and Shito are all called Shohin

 

Ta-da! My sieves are 0.25, 0.20 and 0.10 inches. If we do the math, 1/16 is 0.06 and 2/16 is 0.125. Which means that:

  1. I was not crazy to think what was sieved out looked nice and want to pot something in it.  If you’re interested in very small bonsai, you want some of this soil mix.
  2. I was not crazy to toss everything that didn’t make it through the finest grade. If very small bonsai isn’t your bag, you want to toss these bits.
  3. If I’m getting into really small bonsai, I need to get another filter.

 

The larger the particle size the better the drainage, but the smaller the particle size the finer the roots it will develop.

Should you keep any of these small particles?  It depends on what you’re trying to do. Again, it depends on the size of your tree and where that particular tree is in its lifecycle. There is also balance to consider. Make the particles too large and water will simply drain through the pot without being retained. Make the particles too fine and you end up with dust, which just makes gooey slurry mud when you add water and will kill your tree.

 

Application

What I needed was a smaller sieve size. The three sizes I’ve written are the sizes I had in my standard sieve set. Looking for a finer sieve, I considered my kitchen sieve, but first cast my eye on my soil scoops. They have a sieve built into them. Using the not very scientific method of holding the soil scoop up to the sieve, it was clear the soil scoop mesh was a bit finer.

Success! Using the soil scoop (pictured) sieved the bits again. The stuff on the left was tossed. The stuff on the right is very roughly about 1/16th particle size.

I sieved the smallest size from the sieves through the soil scoop and got much more satisfactory results. Perhaps 1/2 to 2/3 of the content was retained, the rest that fell through I discarded. The remaining 1/2 to 2/3 I used to repot a smaller bonsai.

 

Some things to keep in mind

Storing sized particles you aren’t using in gallon and half gallon milk jugs isn’t a bad idea, but remember that if you pre-mix and are using a combination organic and inorganic mix, the inorganics will suck the moisture out of the organics.  You may be shortening your shelf life by mixing them.

The above is for bonsai soil mix. Pre-bonsai are a different bag. You will likely want to use different soil mix on pre-bonsai and newly harvested specimens than if you have an established mature bonsai.

Most importantly, this chart is a ROUGH, quick and dirty rule of thumb.  It’s a stab at quantifying the irritating tendency to say “use small particles for small bonsai.”  Above all things, use your sense of logic and reason, understand the chart is a suggested estimate.  At the end of the day you know your trees best.