by George Buehler

We like our bonsai trees with healthy limbs and leaves. In order to have this, we have to have healthy roots. Therefore, soil is probably the most important aspect of bonsai horticulture and is unfortunately often the least understood aspect of growing bonsai. The other two primary areas, watering and feeding (some might include pest and disease control), are indeed important and generally get more attention than soil does. Without appropriate watering and feeding, a tree will not be healthy, but good watering and feeding will be less effective if a poor soil is used. On the other hand, a good soil can help make up for poor watering practice and helps to make feeding more effective.

Soil for bonsai is more than just "dirt". It not only provides the support matrix but also stores all the ingredients the tree needs for survival. It drives just about everything you do and don't do in caring for your bonsai.

Keep in mind that soil science is a vast topic and a detailed discussion of soil and growth of trees in pots could take a year to discuss thoroughly. And of course if you talk to 10 different bonsai experts, you will get 10 different opinions on the best mix for bonsai soil. Unfortunately there are no hard and fast rules about what exact components should be included in a good bonsai mix, but only about the particle size of the components selected and even that has variations. Regardless of what is included in a soil mix, it must always comply with two basic requirements.

First and most important, the mixture should drain water quickly. Your worst enemy is not drought, but root decay caused by over watering or poor drainage. As well as being free draining, the soil must also be able to hold sufficient water to satisfy the tree's needs until the next watering.

Second, it should be essentially pH neutral... that is, neither wildly acidic nor basic. A pH value somewhere in the 5.5 to 7.0 range is just right; although some tree species (i.e. evergreens) like the pH range slightly lower.

There is a third requirement which is probably more important than pH and that is the soil particles must be sharp to encourage feeder root division. When I say sharp, this is not knife sharp but jagged enough that when a root comes in contact with a particle, it will cause it to split into two or more parts rather than just having the root grow over or under the particle.

The term "soil" is really somewhat of a misnomer. The components normally used to create a good potting mixture are, in fact, soil less. They are designed to provide an ideal environment for root growth. In creating this "perfect" environment we also create a few problems for ourselves. First, it should be obvious to even a rank beginner that the rocky concoction described below will dry out a lot quicker than the sticky black potting soil which most people have been using for years for garden plants. Secondly, most of the components in the soil mix are really one kind of "rock" or another, which means there are virtually no nutrients in a properly prepared bonsai potting mix. Both of these aspects are unfortunate, but necessary if our soil mixture is to have the success we hope for.

Before we continue, let's look at some of the misleading words in order to understand what the books and magazines are actually referring to.

• "Soil": bonsai soil in no way resembles potting soil. Or garden soil. Or any other "soil" you can buy in a bag. Once we cover the ingredients you'll understand why.
• "Sand": when you see this in the magazines or books it is usually in reference to "coarse sand"- the last thing this refers to is "sand" like at the beach or children's play sand. It's not builders' sand either - although a number of references I found used this terminology. All this is much too fine. Particles should be a few millimeters in size (approximately 1/16 inch), which is more like finely crushed rock.
• "Clay": again this is not exactly what it refers to. By "clay" they refer to bigger gravel or man made materials that have been high fired to produce a porous material, typically 1/8-1/4" in size. There are several products available that approximate this i.e. Turface, Perlite, Haydite, and "Grower's Grit" (sometimes called chicken grit). Hydroponic soil and aquarium gravel could also be used in a pinch. However, these options can be expensive if you are going to make any sizeable quantity. One can also find crushed brick in this size also. The key to this "clay" is that it does not break down. Kitty liter or "Oil-Dri® has also been suggested. If kitty litter is used, and it should be used only as a last resort, since it typically contains "anti clumping agents as well as perfumes that could harm your trees. Material labeled as Oil-Dri® comes in several formulas per the manufacturer. Only the chemical free types should be used and once again only as a last resort. Perlite is good, but not as the major component since it is so light it won't hold the tree and can float out when you water.
• "Organic": while most of us would think compost, this tends to break down too fast and holds too much moisture - which means the roots of your bonsai will rot. Organic usually means pine or fir bark. The desirable size is about 1/8" or so. You can shred your own to get the particles in this size. "Professional" compost or mulch mixes tend to be smaller in size. However, you can often get enough in the appropriate size range to be worthwhile if you are willing to put up with the mess during sizing.

The advantage of manufactured particles such as Haydite, Turface, Perlite, etc. compared to plain gravel is enhanced retention of moisture and fertilizer. These products are typically high fired particles, i.e., clay, which creates pores and channels that allow free draining but hold a limited amount of water. [Figure 1]

SIFTING

To get the particles you want and get rid of the ones you don't want you will need to sift the components. Sifting is passing the components through a series of screens with holes the size of the particle you want. Those retained are at least as big as what you want. Bonsai soil screens can be obtained from your local bonsai supplier and are generally available in sets containing three different mesh size screens (typically 1/8", 3/16" and ¼"). You can also make your own using hardware cloth materials available at local hardware stores or home improvement centers. If you are going to mix large quantities of soil components, using bonsai screens will cause lots of frustrations due to their size (about 10" in diameter). Making your own larger screens is probably indicated. A simple design using 2x4's and hardware cloth is presented in Figure 2.

Particle sizes used in mixes are typically matched to the size tree. Each soil component gets sifted and then mixed in various ratios depending on the tree - a nice mindless activity for winter months when there isn't much else to do. All components in a bonsai soil should be screened at least mildly to remove the dust. For a normal sized bonsai container (about 11" wide by 1 or 2 " deep) the components should be screened to provide particles which are about a quarter inch or less in diameter. Mixes for smaller bonsai (shohin and mame class plants) should be screened to produce particles which are between 1/8" and 3/16". The ideal situation is to have the various components all in the same size range (i.e. 3/16" to 1/4" or 1/8" to 3/16")

SOME ADVICE FOR BEGINNERS WHO WANT TO MAKE THEIR OWN SOIL

1. Avoid very small bonsai containers. The soil construction and watering practice is less critical in larger containers.
2. Seek the counsel of an experienced bonsai grower in the area. This first-hand advice is worth more than all the words we could write here. However, bear in mind that experienced growers have their own preferences for soil which is generally based on first hand trial and error.
3. If you are going to mix your own soil, read, read and read! Don't accept the advice of only one author. Compare the advice of several authors (See below in basic compositions section for some of the differences).
4. Until you get more experience, it is probably better to buy commercially prepared soil. It may not be an ideal mix for each type of tree; it is in general acceptable for a broad spectrum of tree types. There are several commercial suppliers of good general mixes that I have used for years.

DETERMINING FACTORS FOR SOIL TYPES

1. Where you live
   Here in the Ohio Valley, we need a mix that holds water but is fast draining. This means adding more organic material and/or some type of water retaining clay.
2. What size bonsai containers you are planting in.
   If your trees are very small, you will need to pay a lot of attention to drainage or you will likely have some difficulties. Some beginners lose trees because the bonsai containers are so small that maintaining proper hydration is a balancing act that an experienced grower would find challenging.
3. What kind of exposure your trees have
   If you were fortunate enough to live in a mild, very temperate climate like the Pacific Northwest, you might be able to grow many different common bonsai in the same bonsai soil with almost the same watering regime and enjoy good success. However, that is not the case for the Ohio Valley. Since we have hot dry weather one week and cool wet weather the next, the bonsai soil needs to be able to handle the changes.
4. How frequently you are able to monitor your trees
   If you can check your trees once or twice daily, you may be able to succeed at using a soil with less than perfect drainage but this can still be a problem for the roots of your trees.
5. What species of trees you want to grow

Some trees like wisteria, willow, hornbeam and birch will use more water than others, such as pine, and cedar. Most deciduous trees require more water than needle evergreens.

BONSAI SOIL COMPONENTS

As a general rule a bonsai mixture contains about 75% inert aggregate (inorganic) and 25% organic materials. What aggregate and what organic materials, as previously stated, is a matter of some debate between bonsai people. We will look at several typical components.

Inorganic

Akadama -Akadama and Kanuma are particles of pelletized clay which the Japanese use extensively for potting their bonsai. It has a tendency to hold moisture better than some other type clays. Remember that if you use Akadama, it will probably break down within a couple years (or sooner) and will have to be replaced. Kanuma is also a pelletized clay but is slightly more acidic than akadama and is therefore used by some bonsai artists for azaleas. In our area this material can be very expensive if your collection is large.

Turface - Looks a lot like Akadama, but is not. This product is used for the aeration of grass on golf courses and baseball diamonds. Turface is usually only available in fifty pound bags, from turf supply companies and generally not available at places like Wal-Mart or Home Depot. Essentially it is clay that has been heated in a fire until it becomes hard and will not decompose (turn into mud) with prolonged exposure to water. If you were to take a terra cotta pot and crush it up, you would have essentially the same thing. Each particle is full of tiny holes which absorb water and release it back to the plant slowly and its pH is relatively neutral. Proper sifting of a 50 pound bag will net you about 25 to 30 pounds of usable material for bonsai. Use the rest to aerate your garden.

Haydite - Haydite is the rock equivalent of Turface. It goes by a variety of different names (Haydite, PermaTill, Staylite, etc.) and comes in different colors (brown or gray mostly) depending upon what locality you obtain it from. Historically, it is the primary ingredient used in the manufacture of concrete blocks to make them lighter. Only in recent years has its value as a soil amendment been discovered. Haydite is actually expanded pieces of shale or slate. The term "expanded" means it has been heated to over 2000 degrees which causes this already porous rock to become even more porous. Like the Turface it is full of tiny holes which absorb water and release it back to the plant. Some research even indicates haydite releases water more readily than does the Turface. Depending upon where it comes from, haydite can be slightly pH acidic, which can be easily corrected by adding a little bone meal to your soil mix.

Chicken Grit - You can purchase it by the bag from your local feed or farm supply store already separated into the correct grades. Chicken grit is composed of fragments of crushed granite which farmers feed to their chickens to help them grind up corn. The particle size in any given bag will vary depending upon whether it is intended for baby chicks or full grown turkeys. Typically that intended for chicks or regular chickens is about the proper size. Unlike the Turface or haydite, crushed granite absorbs no water. It is, however, completely inert, neutral in pH and has sharp edges on each particle which cause fine feeder roots to split and divide when they hit them. There is a word of caution for using chicken grit. Make sure the chicken grit you are buying is granite. Certain brands are composed totally out of crushed sea shells, which would be highly pH basic and disastrous in a bonsai mixture. There is also some chicken grit which is pink granite with white flecks. It works fine mechanically, but would not be aesthetically pleasing in a pot.

Construction Gravel - It is basically river rock and is the ingredient usually mixed with cement to make concrete. You can find it from places like Home Depot or Lowes. If you purchase it by the bag make sure it is all rock and not a rock/cement mix. What I have seen at local home centers tend to be a little to large for most bonsai applications except drainage in the bottom of large pots. It does not hold water like Turface or Haydite. Of course it generally is smooth and does not promote the splitting of feeder roots. This therefore should be used only in the last resort.

Lava Rock - this is a highly porous material readily found in home centers, sold as mulching materials. Unfortunately, what I have seen in most home building centers is too large to be used in bonsai pots. Of course it could be broken up but this would be a tremendous task. Lava rock comes in several colors, both natural and synthetic. This is somewhat unsightly but can be somewhat hidden by sprinkling some crushed clay over the top of the soil. The GLBS sold some sized Colorado lava rock in 20 pound bags recently and may get some more in the near future.

Organics

There are some bonsai growers who believe a good bonsai soil mix need not contain any organic ingredients whatsoever. They maintain that drainage and aeration are the two single most important aspects of a good bonsai mix and that you, as the grower, have the responsibility for making up for this by routinely putting in all the chemical components your bonsai may require. Lee Squires tried this organic free mix on several of his pines and found that it did not work well. He lost one 30 year old mugo and the other pines are not doing well. He believes that the pines need some natural acid organic component in the soil like pine bark and if the pine bark breaks down to "mush" there is still have the baked clay and grit which will not break down and the pot will still have the drainage needed. I don't believe I could convince Lee to try this soil mix on any of his deciduous trees.

The preference among most bonsai growers is to include something in the finished soil mixture which, at least, looks a little like "dirt". The organic component in an average soil mix is usually about 25 percent of the total. This organic will decompose gradually and release nutrient for the trees to use. In addition, it will retain a bit more moisture than the inert components and will also absorb fertilizer for the plant's use. The exact nature of the organic component used is largely up to the grower. Pine bark and sphagnum peat moss are generally the most prevalent because they are inexpensive and easily obtainable from local garden centers. One fifty pound bag yields about 25 pounds of usable material after it has been passed through screens to remove the large pieces and the dust. Some people use oak leaf mulch, old compost, or decomposed sawdust. The list can get quite long. Remember that the organic component is simply a vehicle for dispersing nutrient and moisture and that it should be pH neutral to slightly acidic. You should also avoid using anything that might be too "hot" (too much nutrient) for tender young roots, such as cow manure or fresh compost.

BASIC SOIL COMPOSITIONS

As mentioned earlier, there isn't a shortage of compositions published in the literature. Some of the compositions listed are rather confusing (at least to me!). Naka in his Bonsai Techniques I (Bonsai Institute of California, Whittier CA, 1999, pg 88-95) talks about a homemade soil made from garden soil, organic fertilizer, sand (later defined as decomposed granite) and compost that has been aged for several months. However, I could not find any further clarification to the garden soil. In his Techniques I he list 45 different tree species and the soil mix each requires for both young plants and older more mature trees. He also breaks it down to the size of the tree. I also couldn't find what I would call a basic soil mix in his book. Based on the quality of trees he had, his soil tables would be an excellent starting spot. But be prepared to mix 4 or 5 components for each tree type.

Yoshimura and Halford in their The Art of Bonsai (Tuttle Publishing Co., Inc., Singapore, 2002, pg 48-50) break down the soil requirements into two main categories - those being trained and after being trained. Within each main category they divide the pot into three sections - bottom, main, top. The soil in each of these sections is a different composition, from two components to five components. They also break out evergreens, deciduous flowering, etc. I think if I had to follow their layering scheme with the various components, none of my trees would get transplanted.

Peter Chan in his Bonsai Masterclass (Sterling Publishing Co. Inc., New York, 1987, pg 20) lists bonsai soils as presented in table 1:

Although he doesn't define loam, indications are that he is referring to a processed organics such as pine bark, etc. and not garden soil.

Tomlinson in Bonsai (Readers Digest Publishing, Pleasantville, NY, 1995, pg 177) list three types of soils:

1. Basic soil mix - One part loam, two parts sphagnum peat moss, two parts granite grit.
2. Free-draining soil mix - One part loam, one part sphagnum peat moss, three parts granite grit.
3. Lime-free (ericaceous) soil mix - One part loam, three parts sphagnum peat moss, one part granite grit

THE LOCAL MIX

After seeing the above lists of various types of components and mixes, I thought I would go to one of our resident experts, Lee Squires, and determine what he uses. Lee responded to my question as follows:

"I use a basic mix and then add other components to suit the individual tree's needs.

BASIC MIX :

• 50% Fafard 52 mix - 60% processed pine bark / 40% combination of Canadian peat, perlite, and vermiculite
• 50% baked calcined clay - new brand : SoilMaster Red - this does not break down , adds porosity and air spaces to root environment, helps form dense root systems, aids in migration of water, fertilizer and oxygen throughout pot soil profile, and drains quickly. I always screen this through window screen to remove small dust particles which will clog soil.

This mix works well on deciduous trees and azaleas. To evergreens I add lava rock in various proportions.

Pines - 50% basic mix / 50% lava rock

Junipers, false cypress - 75% basic mix / 25% lava rock

For other trees: research their soil needs and adjust the mix accordingly. I don't recommend a deciduous mix and an evergreen mix. There is too much variation on what plants need to restrict yourself to only two mixes. Also, experience tells you a lot about what to add. All trees will survive in the basic mix for a few years but you eventually have to modify it. For smaller trees you can try to use smaller particles in the basic mix. I do not sift for smaller trees. They all need good drainage with some larger particles."

Fafard #52 contains approximately 60% aged pine bark (a blend of fine and coarse), Canadian sphagnum peat, perlite and vermiculite. The mix also has dolomitic limestone (to adjust pH to 5.5-6.5), gypsum, a water-soluble nutrient charge, blue-chip (a slow release source of nitrogen) and a wetting agent.

Soilmaster® Red is a soil conditioner used to manage the moisture level of sports fields and to prevent compaction of the soil. I could not find the exact composition of this material except that it is an expanded clay.

Lee also gave me the following information:

• When the lava rock was not available from the club, he used chicken grit (crushed granite) or kenlite baked shale.
• Lee does not screen the Fafard due to its dustiness. The SoilMaster Red is screened
• Both the SoilMaster Red and Fafard are available locally from Bunton Seed Co.
• DO NOT substitute "kitty litter" or Oil-Dri® for SoilMaster Red. Kitty litter sometimes has chemical additives and deodorants and may be the cause of some unexplained deaths of some trees in the past. These chemicals are extremely toxic to pines. Oil-Dri® turns to mush quickly when exposed to the repeated wetting and drying our bonsai ae exposed to. Also some Oil-Dri® contains chemicals used for oil adsorption. SoilMaster Red has NO additives and is meant to be used by the agronomy and horticulture professions.
• On the soil / pot surface sprinkle some baked clay. This hides the perlite and lava and gives everything a uniform appearance. Also, it will hold some moisture and help formation of moss.

FINAL RECOMMENDATIONS

Whether you know it or not, your soil mix and your watering technique are more than casually related. You can't vary one without altering the other. When I have had trouble with a new tree, it's usually because it's not in "my" soil type and I've been over or under watering. Once a tree has been repotted, watering becomes much simpler.

Treat soil mix formulas with extreme suspicion; especially from persons in other countries and climates. I like a coarse soil without any fine particles, but what works for me and my lifestyle might not be best for you. Start with a basic formula like that used by Lee Squires. As you get more experience you can alter it to what you think best. Ingredients used in magazines are often unavailable locally, so find out what the people in your area use.

Finally, I like being able to alter my mixtures and ingredients each year. Every spring I re-evaluate the past year and refine how I do bonsai. (Not always without some mistakes). This year's soil mixtures are a little different from last year's; and next year's will probably be different again.

For Shohin you need to be extremely careful about removing the dust as it will settle in the bottom potentially causing root rot problems.

Naka says that a soil should be mixed according to what the tree needs. For new people this may not be readily apparent. But generally for speedy growth use coarse soil, where rapid growth is not necessary use a slightly finer mix.