What Kind of Bonsai Soil Should I Use?

Much like asking Chicagolanders “who makes the best pizza?” or asking Washingtonians “What’s the best way to get from Falls Church to Old Town at rush hour?” bonsai lovers seem to enjoy discussing (debating) what components of their growing mix they pot their trees in.

My first year, or two was spent listening to various bonsai comrades and trying to find various components they recommended, repeatedly taking notes and scratching out what someone told me just months before.

I’ve decided to take the debate amongst the advanced and the experts to mean that if people who have been doing this for 20 or 30 years sit around and debate it, and you can’t figure out what to use, buy, or make then don’t feel bad, it’s not obvious.

So, what do we use?  If we back up, one thing that’s pretty universally agreed upon, is to use well draining, well-aerated soil. A second common statement is don’t use potting soil.

 

What is well aerated soil?  And how do I know if I have it?

The point of soil in nature is two-fold, to give the roots something to anchor onto in order to support the tree and hold it up, and to give the tree access to nutrients.

If you think of growing plants as a spectrum, one side would be in the ground, and the other side would be in a bonsai pot. Think of it this way, when a tree is naturally in the earth and it rains, the water wets all of the soil and drains down deep into the earth. (Unless it just sprinkles and then only the top gets damp or wet.) Most of the time most areas do drain (except, say, swamp or marsh like conditions, which is another thing). When it’s dry and doesn’t rain, a plant in the earth can go longer than a pot without water because it can pull on reserves. Its roots pull down deeper in the earth to get water. Bonsai need more vigilant care than regular potted plants because they’re in smaller pots. If trees dry out there’s nowhere for the roots to go to reach out and get moisture.

Now well-drained soil or well-aerated soil is related here, because roots of plant that sit in water can drown or rot because roots also need oxygen from the soil around it. (Roots sitting in water are sometimes called wet-feet). That’s why it’s important to have pots with holes. But having a hole in the bottom of the pot (or more than one hole in the bottom of the pot) is only one part of the equation. The other part is what runs through it.

 

EXAMPLE

Visualize this, take two clear plastic cups and poke holes in the bottom. Fill one full of pebbles the size of standard marbles. Fill the other with dirt from your yard. Fill the cups ¾ the way full. Shake them a little to let them settle.

Now, pour one cup of water in each cup (at the same time). Which cup will water run out the bottom of first? The pebbles, of course. After 60 seconds where will the water be? Probably the cup with the pebbles will have had most of the water run out. The cup with the plain earth in it might still have the entire cup of water still in it. If you touch the pebbles though they will still be damp.

 

The principle here is that these substances all retain moisture, but different amounts. Clay retains moisture. If you’ve seen a clay flower pot or one of those “thirsty coasters” you know that after they get wet, you can see the clay change color and get darker, you know it’s wet when you touch it.

You want particles that are all about the same size to keep the soil draining well. Just as if you put sand in a cup full of pebbles, the sand would fill in the spaces between the pebbles making the mix denser, so too will different sized particles make the mix denser. Small sized particles fill in gaps, increasing the density.

As well composite materials need to be large enough to allow air or oxygen to get to the roots. Too dense a soil chokes the plant.

With bonsai potting soil, because your tree is in a really small pot without reserves you want the roots of your tree to grow their best. So you want them to be able to get air and to get water, but not sit in it. Thus, they need well-aerated soil.

Can’t I just use dirt and be done with all this? Nope. Turns out soil from the earth is pretty dense and water retentive. It will likely kill your bonsai in the long run. Might just kill your potted plants for that matter. Plain old potting soil is next in density. Soils labeled bonsai mix are generally less dense, better draining, well-aerated soils or mixes. Some bonsai soils don’t even have any dirt in them at all, so they just call them “mix.” (I use the terms interchangeably so I don’t necessarily mean dirt if I say the word soil.)

In short, you need fast-draining, well aerated, moisture retaining soil mix. You want space between particles for the roots to get oxygen and water so that they can grow, but also have the substance itself retain moisture for the roots to drink.

 

TEST

Perhaps you’ve heard this before, and think your mix is good but aren’t sure. To test if bonsai in a pot with at least one hole is well draining, first water it thoroughly until the mix and the pot have absorbed their fill. Wait for excess water to drain. Then, simply put the tree on a surface you can see the water come out of, and pour water in. The excess water should run through within seconds.

 

What is bonsai soil made of?

When you get into the question of what to pot in the most fundamental concept is organic vs. inorganic components. Basically, organic components such as moss or bark are decomposing plant matter. Inorganic components, such as clay or lava rock, aren’t.

There is no right answer as to what ratio of organic vs. inorganic components to pot in. Different people prefer different balances and different components to make up those mixes. There is no right answer, just personal preference. Most mixers will have some of each, but it is now generally the case that a larger percentage is allocated to inorganic substances. Also consider that the more organic components in the mix the more water will be retained. Each bonsai tree must be watered according to its soil mix.

Bonsai soil components are like small pebbles rather than dirt, to allow for the aeration and drainage we discussed above. That’s why you may see bonsai soil or growing mixes to look like a bunch of really small pebble bits rather than like dirt. Know though when mixing your own that it’s also true that the size of the soil may correlate to the size of the bonsai. Getting into bonsai, larger trees can take three or four people to lift and be quite large, and there are also small miniature bonsai you can put in your palm. Smaller components are more appropriate for smaller trees and larger components for larger trees. Most bonsai specimens hover around the middle though.

SOME POPULAR COMPONENTS ARE

ORGANIC – organic materials retain water, but also can help hold together loose inorganic components. Organic materials can add help bind and bulk up a mix. Remember that organic components will eventually break down in time and make the mix more compact. The higher the percentage of organic components in the soil mix, the more you have to watch for the soil getting too hard.

Bark – frequently pine bark, but other types can be used.
Peat – peat is partially decayed plant matter. This happens naturally in boggy wet conditions. Peat moss is commonly used. Peat moss adds acidity as well to your soil mix.
Leaf-litter or compost – partially decayed leaves, park, and dead plant matter, or other organic matter.
Potting soil – here’s where you scratch your head and say, but they said to not plant in potting soil. Ah, but some people do use potting soil as part of their mix. Some disagree and won’t use it. Regardless of if you use it or not, know that it retains the most water and adjust your watering of any trees you plant it in accordingly.

INORGANIC – do not retain much water. Excess water simply immediately drains away. Think of the pebbles in the cup analogy. After they get wet, the rest of the water just runs off of them. If you tend to overwater your bonsai trees than this is the stuff for you.

Akadama is baked clay made in Japan especially for use on bonsai. Akadama clay is perhaps one of the most popular soil choices and when discussing clay bonsai mix components it’s sure to be listed first. Be forewarned though it is inorganic, Akadama breaks down into yellowish muddy goo within a year or two, requiring you to repot.
Turface is a fired clay product. Fired clays are similar to Akadama, but do not break down as quickly. Turface was designed for use under golf courses and baseball fields to help aerate the grass.
• Several other fired clays are produced in the US, ask at your local store or bonsai club and you may find other fired clay soil mix products. Some bonsai lovers have even use kitty litter. We are talking about actual, literal, kitty litter. This is not a nickname for another product.
Diatomaceous earth – this material was designed for use in auto shops or similar applications to soak up soil spills for easier clean up. For bonsai it also soaks up water without breaking down.
Lava rock – the same as the larger pieces used as mulch for flowerbeds, but smaller. Lava rock is indeed rock from hardened molten lava from volcanic sites.
Pumice – the same as the stones for your feet in the shower, but smaller. Pumice is a particular kind of lava rock that cooled capturing bubbles within it. It’s volcanic glass.
Perlite – perlite is kind of twice baked. Whereas pumice was bubbly lava that cooled fast. Perlite it isn’t until after the lava has all cooled that we humans come back and harvest it and heat it. At a certain temperature the volcanic glass cracks and forms white round balls. You may have seen these before used in potting soil mixes as little white foam looking balls. They’re there to improve oxygen circulation and lighten the potting mix.
• Grit is the generic term for really small bits of stone or sand. You’ll hear people say, “I added grit to the mix.” Grit helps drain the soil. Understand that some mixes will already contain sand or other forms of grit. If you’ve heard other practitioners who say “I throw a handful of sand in my mix.” Before you do it yourself, check what’s in your mix and know that you may be double dipping, so to speak.

 

A couple notes:

• Mixes can be “light” or “heavy.” Mixes made with clay, potting soil, or granite are heavy. Perlite, pumice, and lava rock are light. Granite is sometimes used but I wouldn’t recommend it. It doesn’t retain water well and it’s very heavy.

• If you are purchasing potting mix, think twice before buying the cheapest bag on the shelf. It may retain more moisture and be more compact than it’s counterparts. Be especially sure to sift out smallest bits and discard them.

• Though akadama is the substance of choice for Japanese masters and all kinds of clay are used and listed as inorganic because they don’t decompose; clay actually does break down. When it breaks down it becomes a thick congealed mass that your roots won’t be able to grow through and the tree will need repotting. Figure use of clays will require repotting every couple years or so and take that into account.

Where do you get these components? You can find them through a bonsai shop online, Amazon carries some of them, or a local bonsai nursery. Not all garden centers will carry these components. Beware sending your partner to pick up pine bark or lava rock for bonsai at Home Depot and them coming home with a 20-pound bag of chunks the size of golf balls, for use mulching. At a typical Home Depot, Lowes, Wal-Mart, or similar garden center I’d guess more than half the time they won’t carry these components. If in a pinch, and you want to pick up something right away some bonsai lovers use orchid soil because it’s usually composed of similar components: bark, charcoal and perlite.

When you mix, sift your components to ensure they’re roughly the same size and to filter out dust or small particles. You can do this with a basic soil sifter set. The multiple sets of screens can be used and to separate different sized particles into several different sized soil mixes. Even if you purchase store bought soil mix in a bag, sieving the mix isn’t a bad idea to remove smaller components broken in transit or accumulated dust or other foreign particles. Rinsing isn’t a bad idea either to remote dust.

Keep in mind also that different trees have different water requirements. A pomegranate doesn’t mind draught like conditions, but a cypress loves water. Tailoring the soil mix to the tree in question is advised. Lets say you choose to put 25% organic materials in your general all-purpose bonsai mix. You might want to put 10% more in your mix for your cypress. You might want 10% less for the pomegranate.

What kinds of bonsai have different moisture requirements or preferences?

• Tropical bonsai – like water as you might guess, plants from tropical humid areas with higher moisture levels like higher amounts of moisture. Some popular tropical trees include ficus, Fukien tea, banyan fig, and jade

• Fruit and flowering bonsai – need plenty of water in their growing period. Some popular flowering and fruiting trees include bougainvillea and crepe myrtle, which are popular for their plentiful and beautiful flowers, lemon, cherry and orange are popular fruiting options (though lemon and orange are tropical and cherries are not).

• Coniferous bonsai like it dry, and would like less of the organic mix. Some popular conifer bonsai include juniper and pines.

If the word “coniferous” is triggering a memory deep in your brain, that you learned this a long time ago in school but can’t remember, precisely what a conifer is… Coniferous trees grow pinecones. Generally speaking your conifers have needles and are evergreen or don’t loose those needles in the winter. This is opposed to deciduous trees, which generally have what we typical see a “leaf” to be. A pine tree is a conifer, an apple tree is deciduous.

Though it’s not a type of tree, I’d also again note to consider your location as well. If you live in humid Florida you may choose different potting mix than someone in dry Arizona.

Remember too that moss and some types of components are hard to thoroughly wet. One watering danger is to water, feel the top of the soil mix and think it’s wet, but just an inch below the surface is dry. That’s why when I get orchids I’ve gotten the instruction for some of them to submerge it for several minutes before letting it drain. Doing this makes sure the whole soil mix gets thoroughly wet.

 

WHAT ABOUT pH?

If you’ve heard your plant thrives in acidic soil, or are growing nothing but azaleas then pH is indeed important. Peat moss is more acidic. So if you do azaleas, peat moss might be your choice for organic medium in your soil mix.

But generally speaking the soils you mix will begin with a neutral or slightly acidic pH to begin with. Though you certainly could start with a small quantity of store bought medium (such as lime, calcium carbonate, or sulfur) or home made medium (such as egg shells or coffee grounds) to mix in, I’d recommend tailoring the pH slowly over time. If you’ve had poor luck with other plants and a more alkaline pH, know that hard water slowly over time pushes up the pH and treatment to make soil more acidic may not be as beneficial at planting as later down the line. Remember too to test the soil periodically if you’re going to adjust the pH. Treating soil without knowing your starting point may at best be unnecessary and at worst have the opposite of the intended effect and hurt the tree.

 

 

APPLICATION

This scientific discussion of the components is well and good but it doesn’t answer the real question for me and probably for most people. That’s where I see the websites stop. The real question is, which is, what do I buy, and where?

It also leaves off an important factor, cost. Lets get real. Spending $15 on a two-quart bag of soil mix is expensive. It adds up. If you’ve started to have something more than a mall tree you may have gotten into the topic of harvesting, or buying from big box stores and turning nursery stock into bonsai. In either case you may be considering training pots vs. show pots. That is, if you’re planning on filling even a small training pot that can take that whole two-quart mix. Forget something larger.

If you have just a few bonsai and they’re in bonsai pots or smallish training pots, buying pre-made bag soil might work for you. I’d consider this route if you have maybe five trees or less. But how many really depends on you, your budget, your space, and whatever you want to do.

Pre-mixed soil can also be purchased for tree types. You can buy an all purpose mix, tropical mix, conifer mix, or deciduous mix.

Lets say you have a dozen trees and they’re all tropical. Just buy a big bag of tropical soil. You know what is best for your space and your needs.

After that many bonsai lovers just figure out how to mix their own. Lets suppose you have that dozen trees but they’re not all tropical. You’ve got some tropical, and some junipers, and some deciduous. You’re then faced with the unpleasant option of what to buy, mix, and store without breaking the bank. One option might be to consider buying, say, three types of components. You could buy the all-purpose mix with an organic and inorganic to mix in, or just three sizable bags of an organic and two inorganics such as clay and lava rock (or something similar). You make a basic mix of components and before repotting any tree read the tag or do some research on the water needs of the tree. Add an additional handful of each inorganic for the pomegranate or an additional handful of the organic for the cypress. Take notes and vary slowly.

Last let me say when you decide, find what works for you. Listen to what other people use and consider it but don’t be swayed. You’re always going to find someone who grew multiple beautiful bonsai in potting soil. Or soil from the garden. Or whatever. And it works great for them. Much like the health of human beings, the health of trees sometimes defies logic. Sometimes people do things to a tree where it probably shouldn’t survive, and it does. Sometimes we do everything right and it doesn’t make it. There are no guarantees. But by understanding what’s going on as I’ve outlined above and making reasonable choices based on the science you’re improving your odds.

The last tip I heard that I enjoyed was to pat yourself on the back. Making your own potting mix for bonsai is a sign of passage, showing your commitment to the hobby as you commit to the health of your bonsai trees. Control and awareness over these components will serve you well in coming years and you will hone your mix in time with successes and losses. I’ve also been told that “expert” in bonsai might as well mean “has killed at least 1,000 trees.” It was said in jest but the point is that everyone has losses. They’re a necessary part of learning.

And congratulations on taking your hobby to the next level.

 

 

In this post I’ve referenced some of the following excellent articles:
• “How to Mix Your Own Bonsai Soil” by dengarden
• “Bonsai Soil” by BonsaiMary.com
• “An Introduction to Bonsai Soils” by bonsai4me
• “Adjusting Acidity and Alkalinity” by bonsaidilettante