Bonsai Soil

The preparation of a workable potting mixture in which to grow bonsai is certainly not the most exciting or interesting aspect of the art, but it is just as certainly one of the most critical. Because the growing space in a bonsai container is limited, it is important that soil placed into it should perform perfectly. The health and well-being of the tree are dependent on it.

The right recipe for bonsai soil is like the right recipe for spaghetti sauce. Everyone has a slightly different idea of what should go into it, but the basic ingredients generally remain the same. Bonsai people will argue for hours about which ingredients will work the best. The actual fact is that most thoughtfully prepared mixtures perform fine so long as they provide for excellent aeration and drainage. The objective here is not to describe an exact mixture for making bonsai soil, but rather to discuss the principals and elements necessary for an effective potting mix so that readers can construct a workable medium tailored to their own individual needs and growing conditions.

Bonsaists spend a great deal of time sifting various soil components through a series of sieves. The objective is to make all components approximately the same size. In the process not only are larger unusable chunks eliminated, but also fine dust, which would plug up air holes between soil particles and inhibit proper drainage. Screening and mixing soil is mostly common sense and not rocket science. This is an important point to remember, because it is possible to get so involved in the creation of the world’s most perfect soil mixture that one can lose site of the principal objective… perfect drainage and aeration.

The actual components and the amount of each component used in any soil mixture can vary from region to region and garden to garden. Exactly what should be included in the final preparation is an individual matter. Components included in the mixture will be determined by several factors. First, what types of materials are readily and economically available in the grower’s immediate area. Second, what are the demands made by the local growing conditions, i.e., do you live in a desert or a rain forest? Third, How large is the container into which the tree is to be planted, i.e., shallow pots will dry out more quickly than deeper pots. Finally, what is the moisture and pH preference (acid or base) of the particular variety of bonsai being planted into the soil mix.

Any usable soil mixture must always meet two basic requirements if it is to have any hope of success. First, the mixture must drain water quickly. This is generally referred to as “perfect drainage.” Second, it should be essentially pH neutral… that is, neither wildly acidic or basic. A pH value somewhere in the 6.5 to 7.5 range seems best. There are all kinds of pH testing kits available on the market. It is a good idea to get one and use it to test soils. Local agriculture extension agents also offer soil testing services for a modest fee.

“That Gravel You Plant Your Trees In”

The appearance of a correctly prepared bonsai potting mix is so radically different from the heavy black dirt the public usually purchases for their general potting needs that newcomers often describe bonsai potting mixes as “that gravel you plant your trees in.” They are not far wrong in this assessment, but what they fail to realize is that there are some very specific reasons for preparing the soils in the manner we do and some very specific advantages to be gained by doing so.

Why do bonsaists insist on a very loose well draining soil mixture? The answer is simple. When god created little green apple trees… and all other plants for that matter… his plan was to grow them in the ground. It was man who devised the idea of putting them in containers. Horticulturally speaking, our bonsai are being asked to grow roots in what is essentially an unnatural environment… the small confines of a bonsai pot. Even though the container may suit our artistic vision for a bonsai masterpiece, it is an alien environment insofar as the tree’s root system is concerned.

When a loose, well draining soil mixture is used, it creates an environment into which the tree can easily grow new roots. Vigorous root growth translates into vigorous top growth and overall plant health. Because the growing space in a pot is limited, bonsai practitioners attempt to gradually trim away heavier roots, thereby making more space for the growth of fine feeder roots which are better able to nourish the plant. It is the development of these fine, hair like, feeder roots that a good bonsai mixture is designed to encourage.

This concept is fairly simple to understand. As proof, consider the kind of potting mixture which plant propagators use for the rooting of cuttings. Normally it is coarse sand or perlite. Both of these substances have a uniform particle size, drain water exceptionally well and have no fine dust which would inhibit air movement through the soil (aeration).

These are exactly the same characteristics on which a workable bonsai mix is based. The actual ingredients from which you assemble your soil mixture are certainly important, but regardless of the components,… if the final mixture does not have good drainage and aeration, it is wrong.

There Is No Soil in Bonsai Soil

The term “soil” is really somewhat of a misnomer. The components normally used to create a good potting mixture are, in fact, soilless. They are designed to provide an ideal environment for root growth. Unfortunately, in creating this “perfect” environment we also create a few problems for ourselves.

Obviously the soil mix described thus far will dry out much quicker than the sticky black potting soil most people are familiar with. Most of the components used are aggregate (rock of one kind or another). This means there are virtually no nutrients in a properly prepared bonsai potting mix and that the container will hold onto only enough moisture for its immediate needs. Both of these aspects are unfortunate, but necessary if any soil mixture is to be successful.

Watering and fertilizing then become critical elements for the development of healthy bonsai. Since the mix used will hold neither nutrient or moisture, a failure to establish regular, effective programs for both will quickly cause trees to weaken and die. Likewise, an effective fertilization and watering regime will cause a tree planted in a good bonsai mix to respond with a growth and vigor that could not be achieved using any other kind of potting preparation. Water and fertilization are subjects for another article. Suffice to say that both must be done on a very regular basis and never, ever, neglected.

Bonsai Soil Composition

A good general bonsai mixture should be composed of about 75 per cent inert aggregate and 25 per cent organic materials. What aggregate and what organic material becomes a matter of personal choice and often considerable debate between bonsai people. The best suggestion is to use materials that are available in the local area. This will allow the grower to keep costs down and hassle to a minimum.

Inorganically Speaking

Aggregate is the largest and most critical component and will comprise an average of 65 to 85 percent of the total soil mass. Aggregate is the best term to describe these substances, although, rock, gravel and drainage material will also work. The aggregate portion of the mix may be composed of just a single component or a combination of components. The only requirement is that the aggregates used have a uniform particle size and a neutral pH value. It is not necessary to ship such materials across the country or around the world in order to obtain a workable potting mix. In all probability, they will be easily available locally.

As one travels and meets bonsai growers in other locations they quickly discover that soil components change by region based upon what is available in a particular location. People in Florida use a lot of sand in their soil mixtures. That’s because they have got a lot of it. Colorado bonsaists can obtain all the decomposed granite they want from the sides of their mountains. In Hawaii the primary inert component in a soil mixture is… you guessed it… crushed lava rock. The inert ingredient(s) can vary greatly…. but generally fall into three categories: hardened clays, expanded aggregates and non-porous aggregates.

Hardened Clays

Akadama

Akadama and Kanuma are particles of pelletized clay which the Japanese use extensively for potting their bonsai. They are virtually unobtainable in the west except from a retail bonsai dealer. There are several different grades and sizes of these clays.. Some have been fired until they are rock hard like the turface described below. Other grades are simply small pellets of clay that will break down when they become wet. In either case the particles will absorb water and nutrient and release it back to the plant gradually. Purists in the art of bonsai will tell you that growing trees in kanuma and akadama is the only way to go and that if you are not using this product, you are not really doing right by your trees.

While certain types of trees like to have a little clay included in the soil mixture (notably wisteria and azalea), The rush to include akadama and kanuma in western potting mixtures is somewhat of a fad. Its popularity is based on the belief among many western practitioners that if it comes from Japan it has got to be better than anything available in the west, i.e., Japanese bonsai are very beautiful. Therefore Japanese soil ingredients must be the answer to growing beautiful trees.

The inclusion of minor amounts of Japanese clays in a working bonsai mixture may improve and will certainly not impair the mixture’s usefulness, but its overall benefit is a matter of debate. In any case, the importation of pelletized clay from half way around the world as the primary ingredient for a soil mix will quickly prove to be a very costly business. If you have a lot of trees, you may want to consider less expensive domestic materials.

Turface

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 and lawn maintenance companies and generally not available at local garden stores. Essentially it is clay that has been heated in a fire until it becomes hard and will not break down and 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. Its pH is relatively neutral. Proper sifting of a 50 pound bag will net you about 25 pounds of usable material for bonsai. You can use the rest to aerate your garden.

Although “turface” is the term generally used to describe fired clay, it is, in fact, a brand name. Other manufactures market similar products under the names such as Soilmaster or Terragreen. There may be others. Note, however, certain brands of cat litter and oil absorbent products on the market contain fired clay as their prime ingredient. You should be extremely cautious about using such products because they often contain chemical additives which would be detrimental when used in a soil mix.

Expanded Aggregates

Haydite

Haydite is another brand name and is the rock equivalent of turface. Similar products may be found marketed under names such as permatil and staylite. They come in different colors (brown or gray usually) depending upon where it was made and what kind of stone was used. Historically, expanded rock is the primary ingredient used in the manufacture of concrete blocks to make them lighter. Only in recent years has this product’s value as a soil amendment been discovered. Haydite, which is brown in color is expanded shale. Permatil is grey and made from slate. The term “expanded” means it has been heated to over 2000 degrees which causes these two types of 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 and is less inclined to accumulate salts from watering. Depending upon where it comes from, expanded rock may be slightly pH acidic. This can be easily corrected by adding a little horticultural lime to the soil mix.

Lava Rock

Yet another expanded stone product is lava rock. This is usually available in garden centers and is red in color. Bonsai people are fond of using lava rock as a finishing dressing on the surface of the pot, but it can also make an excellent primary aggregate component for any mix. Like expanded shale and slate, lava rock is full of tiny holes which absorb water and then release it back to the plant slowly. Unlike expanded shale and slate, it was the volcano gods who did the expanding and not man.

Non-Porous Aggregates

Poultry Grit

We’ve used this substance for years and swear by it. You can purchase it by the bag from your local feed or farm supply store already separated into the correct grades. Poultry grit is composed of fragments of crushed granite which farmers feed to their chickens to help them grind up corn. (As it turns out, chickens have no teeth). The particle size in any given bag will vary depending upon whether it is intended for baby chicks or full grown turkeys. Unlike clay or expanded rock, crushed granite is dense and solid. It absorbs no water, is 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. A word of caution. Make sure the chicken grit you are buying is granite. We have found certain brands that are composed totally of crushed sea shells, which might be fine for the chickens, but would be highly pH basic and disastrous in a bonsai soil. We have also seen chicken grit manufactured from pink granite with white flecks. It works fine mechanically, but like perlite, is not aesthetically pleasing on the surface of a pot. If you can’t find poultry grit, check with your local stone quarry. You may be able to buy the same product, but will have to sift it through several screens to obtain the necessary particle sizes.

Construction Gravel

This is the substance most often included in a good bonsai soil mix. It is basically river rock and is the ingredient usually mixed with cement to make concrete. You can find it “down by the riverside;” or at your local construction site; or at the local concrete manufacturer; or in bags from building supply centers. (If you purchase it by the bag make sure it is all rock and not a rock/cement mix which would only compound drainage problems the first time you watered.) Seriously,… river rock, a.k.a. – construction gravel, is one of the best products you can include in your soil mixture. It is non absorbent, dense, inert, pH neutral and readily available in most areas. Don’t get it confused with the children’s play sand and the blasting sand also available in home centers.

Organically Speaking

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 supplying all the nutrients and moisture your bonsai may require.

This is absolutely true, but it sounds a little too much like hydroponics for most people. Although there is some merit to the argument, 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 20 to 30 percent of the total volume. This organic will decompose gradually and in so doing, release nutrient for the tree’s use. In addition, it will retain a bit more moisture than the inert components and will also absorb more fertilizer.

The exact nature of the organic component used is largely up to the grower. One of the most popular is pine bark because it is inexpensive and easily obtainable in fifty pound bags from local garden centers. One bag yields about 25 pounds of usable material after it has been passed through three sets of screens. Some people use oak leaf mulch, some garden soil, old compost, or even 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. 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.

Tailor Mix To Your Needs

All components in a bonsai soil MUST be screened. You will need a variety of screens ranging from one half inch mesh through one sixteenth. You can purchase ready made bonsai sieve sets from local bonsai suppliers or make your own by visiting the hardware cloth section of the local building supply store. For a normal sized bonsai container (about 11” wide by 1 or 2 “ deep) the components described above 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 one eighth and one sixteenth inch. In both cases, fine dust particles should be removed.

A little common sense is the best approach when deciding what soil mixture is right for you. A recipe which is composed of three quarters aggregate and one quarter organic materials will produce a mix which is good for all evergreens and most types of deciduous bonsai. However, quite often varying the components and ratios in a mix to accommodate particular varieties of plants can be a good idea. Likewise, if the plant prefers a damp soil mixture (larches and bald cypress) more organic material, which holds more water, may be called for. If the tree prefers a dry soil, (pine, ficus, juniper) more aggregate may be advisable.

Take a look at the growing conditions in your back yard. If you have a shaded location you may have problems with pots staying continually wet. In such a case it might be wise to increase the aggregate content of your soil mix and thus cause it to dry out more quickly. On the other hand, if your yard is exceptionally sunny, you might want to add more organic ingredients which will cause the soil to retain more moisture.

Adding Something Extra

Above and beyond the materials already discussed, there are numerous other additions which some growers may or may not choose to include in their mix. Activated Charcoal is one addition because of its ability to purify and neutralize any impurities which may get into the soil. For growers who are working with acid loving plants such as azaleas (pH of around 5.5) the inclusion of peat moss in a potting mixture is a good idea. Some azalea growers actually include the peat moss as the primary organic ingredient in the base mix. Better yet, the use of Kanuma and pumice 50/50 is ideal for azaleas. For plants that like alkaline soils, the addition of one or two handfuls of horticultural lime is a useful addition. Many growers like to add a couple of scoops of bone meal (a mild, organic fertilizer) to a base mix. As noted in the beginning, the right mixture for a working bonsai soil is largely a matter of personal preference, but remember when adding ingredients not to impair efficient drainage.

A Couple Of Tips

Moss – Decorative ground covers such a moss can add a great deal to a bonsai’s appearance. They can also be dangerous. Dry moss will actually shed water away from the plant. If you use moss on your bonsai make sure that the moss does not cover the entire surface of the pot and that you are always able to inspect the moisture condition of the soil. There is a Japanese rule which says moss may be permitted to touch only three sides of the container. If followed it means that you will always be able to inspect the condition of your soil easily.

Old Soils – Try to insure that as much of the old soil as possible is removed when transplanting. Incompatible soil mixtures can cause problems in maintaining adequate moisture levels and fool you into thinking the soil is completely saturated when it is not. The success of a good soil mix can be severely impaired if it is not uniform throughout the pot. If the old soil mass holds onto more moisture than the new soil mix it may remain too wet and a condition of root rot will set in. Likewise, if the center of the root ball is hard pack clay it may resist absorption of water and roots will not grow. Such problems are usually only encountered with plants collected in the wild. Trees that have been grown in a proper bonsai soil mixture for a number of years normally “release” most of their old soil particles upon transplant making the job of soil replacement much easier. This is another advantage of growing trees in a properly prepared bonsai mixture.

Soil Composition

Protocol What follows is the recipe for the soil mixture used in a hot sunny garden in North Carolina. These ingredients in the ratios indicated make an excellent general purpose potting mixture. They are, by no means, the only mixture or combinations possible. You may wish to amend these ingredients or alter the ratios to suit your own specific growing conditions. Remember that the best general combinations of components will be 75 percent inert aggregate material and 25 percent organic material. Consider the climate and growing conditions in your own back yard and create a soil mixture which will fit you and your bonsai’s specific needs.

Standard Bonsai Mix – Components should be passed through two sets of screens to produce particles of about 1/4 to 1/8 inch in diameter. Eliminate dust.

Haydite – 1 part

Granite or River Rock – 1 part

Turface – 1 part

Pine Bark – 1 part

Shohin & Mame Bonsai Mix – Components should be passed through three sets of screens to produce particles of 1/8 inch to 1/16 inch. Eliminate dust.

Haydite – 2 parts