I come to you with the goal of making a one-stop shop for bonsai soil information. While I myself am not a soil expert, I do consider myself resourceful in finding the information I need.
Within this post will be sections about particle size, soil mixture recommendations, different substrates and their ability in different categories, as many comments on substrates I can find from reliable sources, and a few reliable places to find the substrates. I welcome additions to the lists from any and all locations.
I know there is no standard particle size that everyone agrees on, but here is what I've carved out due to a bit of research. Going a tad bit bigger or smaller doesn't kill the tree, but aim for these relative sizes.
Large Anything 30" and up 1/8" - 5/16" (3 mm - 8 mm)
Medium 10" to 30", two hand trees 1/8" - 1/4" (3 mm - 6 mm)
Small Can go up to about 10" to 12" 1/16" - 1/8" (1.5 mm - 3 mm)
If you don't like my sizing guides here are a few other posts and details you can check out:
I know there is no standard soil or mixture that people agree on, but there are a few agreed upon principles when mixing soil. The main things you need to consider are having adequate aeration, water retention, and ability to hold nutrients or CEC.
I feel the most common and most agreed upon mixture is 1/3 pumice, 1/3 akadama, 1/3 lava and a 1/2 cup of charcoal per 5 gallons of mix. Here are the different mixture suggestions I came upon with my research:
1/3 akadama, 1/3 pumice, 1/3 lava
1/2 akadama, 1/4 pumice, 1/4 lava - Deciduous trees
1/2 akadama, 1/4 bark, 1/4 lava
2/3 akadama, 1/3 lava, a bit of organic something - Conifers
1/2 pumice, 1/2 lava with slow release fertilizer
2/3 pumice, 1/3 lava
100% pumice for collected trees
*akadama, pumice, lava, and bark can be replaced with Turface, DE, DG, or other substrates that provide similar water retention, aeration, and CEC.
These are things that we need to take into consideration regardless of the mix we choose:
Take into consideration the species of the tree when choosing a soil mixture. Some like to dry out completely, some need to be acidic, some need to remain moist but not wet. This can make different mixes and some substrates better or sub-optimal.
Age, size, and trees in refinement can require different repotting intervals which can alter an optimal mixture
Deep pots have a tendency to dry out quicker
It is common to put a layer of larger substrate on the bottom to retain good drainage
Shallow pots have a higher water table than deep pots.
Location and weather need to be considered when choosing a mix and substrates.
High heat and wind can dry out a tree quick through increased evaporation
Frequent rain can cause problems if you don't have a soil equipped for extra water.
Extreme cold can cause several freeze-thaw cycles and can break down certain substrates quickly
There are two videos from the Appalachian Bonsai that test and rank substrates in water retention and freeze-thaw cycles. I'll refer to those rankings below. The details of the test and the list is in the details in the video details. I need to add, CEC stands for Cation Exchange Capacity and measure how well a substrate can hold nutrients (fertilizer). The higher the number the better it is as holding nutrients.
What it does like no other: Roots can grow through the akadama. This means that a pot with 100% akadama leaves the roots with 100% of the pot to grow in. Other mediums restrict the percentage of the pot that the roots can use to grow.
Complaints: Lack of availability or the cost. People also frequently order akadama without knowing the "hardness" of it and that can lead to it breaking down too early and can block drainage as it turns to mush. Many claim that lack of knowledge on how to properly water and fertilize leads to a dislike of the substrate.
Water Retention: Akadama holds water well. Ranked at #2 in water retention
Aeration: Great until it starts to break down which can vary depending on brand and hardness.
CEC: 21/100g #5
Durability: Can not be reused and breakdown over time. It does not hold up well against freeze-thaw cycles and that can cause problems. Even though it breaks down you can still have roots that grow through it, you just need to be careful in watering and understanding your tree.
Freeze-Thaw Cycles: Ranked 10th out of 10.
Availability: Difficult to come by in US. It is expensive to get it due to the limited availability. There are definitely places to get it, just how much you pay for it depends on who is getting and selling it.
Professional Bonsai Artists Opinions on Akadama: If you're arguing for or against akadama please do read this post from Bonsai Empire on differing opinions on akadama from professional bonsai artists so you can understand what you're talking about. These are my favorite snippets for the lazy:
"I do not use Akadama. Have tried it and it doesn't do its job here. (Northern Europe)" - Morten Albek
"In Indonesia, we only use volcanic lava soil from Indonesia for all of our Bonsai. for all stages." - Robert Steven
"Products like Turface are soil amendments that the manufacturers recommend using at no more than 10 - 15 percent of the total volume. I follow that recommendation. I have noticed that turface gives good results for a year, possibly two years when used for newly collected plants, but during year three there is a deterioration in vigor at the time when one would normally expect an improvement." - Collin Lewis
"Yes, I use akadama for all my trees. I use less akadama on the tree in training (25% or less)." - Boonyarat Manakitivipart (Bonsai Boon)
One additional comment from Adam's soil blog post comment section: "On Peter Tea’s last visit back from Japan we were talking about soil mixes and he said, “do you know why they use akadama in Japan? It’s because that’s what the sell at the hardware store.” That’s not to say it doesn’t work great for our purposes. His point was, it’s worth looking into other things to use, that work just as good, that are more affordable, and don’t need to be imported from the other side of the globe." - Anonymous Adam
Worldwide Map of Bonsai Nurseries
This is going to be your best bet in buying Akadama. Clubs, bonsai nurseries, or wholesale bonsai suppliers are going to be the main sources of akadama.
SoCal Bonsai Supply - https://www.facebook.com/permalink.php?id=1026762080767062&story_fbid=1039194236190513
West American Import Export - David Nguy - 4207 Walnut Ave, Chino 91710 (714) 914-7001
San Diego Bonsai Supplies (Tim Hua) - lava (scoria), pumice, and akadama supplies.
Grove Way Nursery - 239 Grove Way, Hayward, CA 94541 Phone: (510) 537-1157
What it does like no other: It does not break down and can be used over and over again. This makes it a fantastic component for trees that are not repotted often as it will continuously provide the drainage and aeration that other components might lose over time.
Complaints: Only complaints that I have ever heard about lava is availability. Not being able to buy it in smaller particle size makes this a frustrating search for "cheaper" lava. Bonsai retailers typically have this available, but you do have to pay for it.
Water Retention: Ranks 8th out of 10, which is not particularly fantastic.
Aeration: Fantastic aeration that doesn't change with time. Might be the best out there other than just rocks that hold no water at all. (But then again these are lightweight!!!)
CEC: 10/100g Be nice, it's just a rock. #8
Durability: I mean, it's a rock, it's durable...
Freeze-Thaw Cycles: Ranked 5th out of 10.
Availability: Available most places, but not in 1/8" particle sizes. To get that size you typically have to get truckloads delivered or go through a bonsai supply store. I've tried to help by listing places below. Some might only have 1/4", but that can fly depending on how particular you are.
Tuscon, AZ - http://www.acmesand.com/soil-amendments/black-lava-sand/
Irwindale, CA - http://sunburstrock.com/redcinder.html
SoCal Bonsai Supply - Fantastic prices, especially if you can pick up yourself.
Fillmore, UT - http://gngrock.com/rock-products/scoria-cinder-lava-rock
Springfield/Eugene, OR - http://laneforest.com/bulk-rock-and-gravel/red-cinder-rock/
Clearlake Oaks, CA - http://www.cllava.com/landscape_rock.html
Minot, ND - (possible correct sizing) http://www.gravelproductsinc.com/photoMinot.html
What it does like no other: Great for yamadori collecting. It seems to be one of the best things to put collected trees in, just 100% pure pumice. Others will suggest adding in other substrates with it.
Complaints: I honestly didn't come across a single complaint of pumice. I'm not sure why someone wouldn't like it other than availability. Its cheap and mostly available around the US.
Water Retention: It ranks number 3 out of 10, but I'm not sure about the differences between the Kanuma Pumice used in the ranking trial and Horticultural Pumice.
Aeration: Pumice provides great aeration while retaining moisture, which is one of its many fantastic qualities!
CEC: 15/100g #7
Durability: Over a long period of time I've heard it breaks down, but I've been told that it doesn't degrade even though it is a tad bit soft and can be crushed with your hands. Either way it's good in terms of durability.
Availability: Pumice seems to be pretty available from my understanding. It can be bought as dry stall if not under the name of pumice (working with them to get a supplier list). I found mine at a hydroponics store that sold a large bag (like around 50 lbs.) for like $13.
The best distributor around the US and Canada I found was Featherock and Sunlight Supply ($15.95 for a 47 lb. bag - 1/2 cf! Waaay cheaper if you can find it than bonsai retailers). I think Sunlight Supply had a retailer in almost every state I checked, and you can find Sunlight Supply's retailer finder at https://www.sunlightsupply.com/page/findretailer, while Featherock has distributors in these states:
Arizona, California, Florida, Illinois, Michigan, Missouri, Nevada, Nebraska, New Jersey, Ohio, Oregon, Pennsylvania, Texas, Washington, Wisconsin, Canada
Download Distributors Master List 2017
Eugene/Springfield, OR - http://laneforest.com/bulk-rock-and-gravel/horticultural-pumice/
Olancha, CA (Shipping as well) - http://www.generalpumiceproducts.com/horticulture/
Canby, Oregon - http://www.phillipssoil.com/retail/
Salinas, CA - http://mas-dist.com/pumice-2/
Bellingham, WA - https://natures-footprint.myshopify.com/products/pumice-3-5-gallon?variant=18101887171 ($22 shipping to Southern California for reference)
Amazon - https://www.amazon.com/Natures-Footprint-Pumice-amendment-5-Gallon/dp/B008YSVPHW
UK Aylesford - http://www.techfil.co.uk/full-product-range/pumice/pumice-aggregates/
UK Staffordshire - http://appliedminerals.co.uk/products/am-pumice/
Sparks, NV/Rhode Island/Puerto Rico - http://reade-px.rtrk.com/products/pumice-powder-amorphous-aluminum-silicate
Baton Rouge, Louisiana -