Chapter 18 Components and Supplements For Bonsai Soil Mixes
This page describes the components used to make soil mixes. I’ve described how to make different general soil mixes for bonsai in Section 19, and growing mixes for accents and other plants in Section 67.
18.1 What IS Soil?
All soil is a mixture of organic and inorganic materials. The inorganic part comes from weathering of rocks, while decomposing plant and animal remains supply the organic part. Different soils have different ratios of these, as well as different particle sizes. Together, these factors determine how loose and absorbent soil is, and how much oxygen and water are available to plant roots.
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Relative ratios of organic and inorganic materials in natural soils (1), and a soil composition graph showing the relative ratios of inorganic materials (silt, clay, and sand) in different natural soil types (2). Link to original image 1;original image 2.
For example, soil in the mountains of North Carolina is rich in organic matter, with relatively less inorganic matter. This very black soil tends to retain moisture. If the organic particles are large, the soil still drains quickly, but if the particles are small, they form a sticky, gooey muck that lets in very little oxygen, and stays wet a very long time. Near the coast the soil is mostly inorganic sand, with little organic matter. The sand particles are fairly large, so water drains rapidly. Minimal organic matter means the soil holds relatively little water and dries out quickly. Clay soil (like we have in the Piedmont) also is mostly inorganic matter, but the particles are so small that they pack tightly and cannot drain well. Once they dry, the fine grains of inorganic clay become very hard to re–wet.
So how does all this apply to bonsai? Not all trees do well in the same kind of soil. Most prefer soil resembling their native habitat. Azaleas thrive in moist acidic soil that is rich in organic matter (our mountain soil), while jades (which are native to southern Africa) develop root rot in anything but very loose, gritty soil that is mostly inorganic matter.
Much of the stress caused by life in a pot can be alleviated by using soil that matches a treeʼs preferred mix of organic and inorganic matter. Even when I grow trees in the ground or an oversized nursery container to thicken trunks and branches, I try to amend the surrounding soil so it resembles the natural habitat of each species.
18.2 Soil-Less Mixes
Most plants will not thrive in a container that contains just field soil as a potting medium. Nearly all container-grown plants are produced in either a bark- or peat-based organic growing medium.
Bags of commercial soil-less mixes. These mixes are designed for small containers and short-lived perennials, not for growing trees. Link to original image, by Thomas D. ‘Tom’ Landis, USDA Forest Service, Bugwood.org.
Commercial growers in northern tier states and especially in the Pacific Northwest tend to use straight organic material alone as a growing mix. All-organic growing medium is not well-suited for tree production in the Southeast. Bacteria and soil fungi use the organic matter as food so it gradually disappears into the air in the form of carbon dioxide. After a season or two the volume of growing medium can shrink a great deal, and the plant sink down to a correspondingly lower level in the pot.
Soil washout from around the roots of a bald cypress tree purchased at a big-box garden center. In my experience, washout happens more often in these stores because they must handle larger volumes of stock. Original image, by A. Daniel Johnson.
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Soil breakdown has significantly reduced the soil volume in this pot. Breakdown differs from washout in that as soil breaks down, there are many fewer large, solid particles of bark and inorganic matter. When wet, decomposed soil also can be squeezed into a tight ball, much like clay soil. The second photo, looking at the pot laid on its side, shows how much space around the root ball has been created by soil breakdown. Original photos by Dan johnson.
The recommendation for growers in the Southeast is to use a mix of bark and “grit” (sand, ground granite, or ground shale) rather than in pure bark. This works to our advantage because we can use the same soil mixes for both general propagation and bonsai training.
18.3 What Does Bonsai Soil Look Like?
These photos show examples of commercially produced soil, and home-made soil.
A commercial soil mix for bonsai composed of haydite (expanded shale, sold commercially as Permatill), coarse sand, aged pine bark, and arcillite (calcined montmorillonite, sold as Turface). Link to original image.
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Homemade bonsai soils. 1. Inorganic-rich soil for conifers. 2. Bark-rich soil for deciduous trees. Original images by Dan Johnson.
18.4 Bulk Organic Materials
18.4.1 Shredded Bark
Bark is the primary component in most outdoor container nursery mixes. Both softwood (cedar, pine, fir) and hardwood bark are good alternatives to peat moss with much the same properties. Pine bark is preferred over hardwood bark since it resists decomposition and contains fewer leachable organic acids.
Un-sifted pine bark nursery mix. This is the main or only component used for nursery stock production in many northern states. It is readily available in smaller bags as bark soil conditioner, but it is much less expensive to buy it in bulk. In our area, a half yard (~100 gallons) bought in bulk costs about the same as 4-5 bags of soil conditioner, each containing about 2 gallons of bark. Original images by Dan Johnson.
Bark may be fresh, composted, or aged. Growers using fresh bark typically add 1 lb N/yd3 to compensate for potential nitrogen draft. Fresh bark can contain tannins, phenols, resins or terpenes which are toxic to plants unless they are broken down.
Composting bark involves moistening the bark, adding 1 to 2 pounds N/yd3 from either calcium nitrate or ammonium nitrate, forming a pile and then turning the pile every 2 to 4 weeks to ensure proper aeration. Composting bark takes 5 to 7 weeks at a minimum, but 4-6 months is better. The higher temperatures of composting also help reduce insect and pathogen levels. If bark is not well composted, plants may suffer from nitrogen deficiency because the composting bacteria need nitrogen to break down the organic matter.
Aging is cheaper but takes a year or more. Aged bark has less humus and can cause greater nitrogen draft in the container than composted bark.
Pre-bagged pine bark can be purchased as nursery mix or as soil conditioner. Larger bulk quantities of 1/2 yard or more are available from nursery and landscaping supply companies. A good particle size distribution for soil mix will be 70% to 80% (by volume) of particles in the 1/4- to 3/8-inch range with the remaining particles less than 1/4 inch.
18.4.2 Coconut Coir
Coir is a relatively new organic material which has been suggested as a potential substitute for peat moss. The raw material looks like sphagnum peat but is more granular. It is derived from the husk of the coconut fruit and originates from several countries including Sri Lanka, India, Philippines, Mexico and Costa Rica.
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Image 1. A dry brick of coconut coir. After soaking in water, coir blocks expand 10-fold or more. Image 2. Coir after it has expanded. Link to original image 1; Image 2 is an original photo by Dan Johnson.
Coir contains more lignin and less cellulose than peat, so is more resistant to microbial breakdown and, therefore, may shrink less. Coir is easier to re-wet after drying than peat moss.
Because it originates from such diverse geographic locations, it is difficult to characterize specific chemical and physical properties. Typically coir contains significant amounts of phosphorus (6 to 60 ppm) and potassium (170 to 600 ppm) and can hold up to nine times its weight in water. Some coir contains high levels of sodium and chloride. It is a good practice to buy smaller quantities from suppliers that pre-test their coir for excess sodium, or use a pocket meter to check the electrical conductivity of any coir you buy in bulk.
18.4.3 Peat
Peat has been a horticultural mainstay for decades. It is formed from sphagnum moss and incompletely decayed vegetation that accumulates in natural peatlands, bogs and similar wet areas. Flooding or stagnant water displaces oxygen, slowing the rate of decomposition. Water saturated layers of plant remains accumulate over thousands of years, compressing under their own weight. These accumulated layers are cut, dried then milled and sifted.
Peat moss. Original photo by Dan Johnson.
Horticultural peat is falling out of favor for several reasons. First it is a non-renewable resource. Second the particle size is too small for good root development. Peat-based soil mix needs additives to open up air spaces. Third, peat that dries out is extremely hard to re-wet. On the other hand, peat is useful for making soil mixes for acid-loving plants, and essential when making your own muck.
18.4.4 Rice Hulls
Rice hulls are a by-product of rice milling. Although they are extremely light rice hulls are very effective at improving drainage.
Loose rice hulls. They usually are sold in compressed bales. Link to original image.
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Particle size and resistance to decomposition is similar to sawdust, but rice hulls do not rob the soil mix of nitrogen like sawdust. I’ve only seen this product available in bulk from online vendors, so I have no direct experience with it.
18.5 Bulk Inorganic Materials
18.5.1 Masonry Gravel
Masonry gravel is available in bags or in bulk and is cheaper than specialty products like Permatill.
Small sharp gravel, screened to an average 10 mm size. Photograph by Bill Bradley. Link to original image.
18.5.2 Arcillite/Turface
Turface (arcillite) is a manufactured product made by heating montmorillonite clay in a furnace until it fuses into particles that typically are 4-8 mm in size. The reddish “soil” of many baseball infields actually is a thick layer of Turface.
Turface, or arcillite. The particles can be tinted before heating to form a range of color-stable particles. Link to original image.
18.5.3 Expanded Slate//Haydite/Permatill
A soil amendment for opening up clay soils. It serves the same purpose as gravel. Also sold under the name Vole-Block.
Expanded slate. While it can be used in place of gravel, it is 2-3 times more expensive than comparably sized masonry gravel. Original photo by Dan Johnson.
18.5.4 Granite Paver Base; Granite Rock Screenings
Paver base is a mix of crushed granite and small gravel sold in home improvement centers in 25-50 pound bags. It is a good source of both grit and micronutrients. Some brands have too much large stone. Sift them out with a 1/2-inch mesh screen, or shop around for a brand with less stone in the mix.
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Image 1. Bike and foot path made of decomposed granite, showing footprints and bicycle tire tracks. Image 2. Close-up of rock screenings. Link to original image 1; image 2 is an original photo by Dan Johnson.
Granite rock screenings, also called cinders, are basically paver base minus the small stones. Packed cinders are used to make walking trails in parks or for filling in between paver stones. It is available in bulk from landscape suppliers. A half yard, which is enough to fill a compact pickup truck, costs $25-30.
18.5.5 Granite Poultry Grit
Poultry grit is available in several pre-sieved particle sizes: hatcher, developer, grower, and turkey. It can be dusty, so benefits from washing before use.
18.5.6 Sand
Sand is typically uses to improve drainage and act as ballast to decrease container blow-over in outdoor container nurseries. Sand rarely occupies more than 10% of the volume of a mix simply due to the tremendous weight (bulk density of 80 to 100 lbs/ft3).
Image 1. Sharp sand, with a pillbug for scale. Sharp sand is preferred because trees tend to form finer roots in soil containing sharp sand and gravel. Sharp sand also does not pack as tightly, leaving more air space. Image 2. A sand sorting tower. Newly mined sand has a range of particle sizes. After sorting, different size ranges are sold for different purposes. Medium to large, sharp masonry grade sand are the best choice for bonsai soil. Link to original image 1; image 2.
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While sand can have a wide range of particle sizes, growers generally use medium to coarse (0.25 to 2 mm) sharp-edged sand. Masonry sand fits these criteria and is available in bags or in bulk. Since salt damages cement, it is washed out of masonry sand, which is another plus. Additionally, masonry sand tends to have sharper edges on granules, which improves root branching.
Pool filtration sand is another option. It is more expensive than masonry sand, but can be easier to find.
Filtration sand is pre-defined mix containing a range of sand and gravel particles, used in pool filtration and bed filter systems. Link to original image.
Do not use calcareous (limestone) or beach sand. Avoid play sand, as it may contain harmful salt.
18.5.7 Perlite and Bentonite
If you need very moisture retentive soil that still drains quickly, you can add perlite (an expanded volcanic silicate rock) or bargain brand kitty litter (bentonite, a soft absorbent baked clay particle) as part of the inorganic matter. These two materials are not good for routine use because they absorb more water and break down into small dust more easily than other inorganics. However, they can be useful in certain situations.
Horticultural grade perlite. It is not often used in bonsai soil mixes because it is stark white, and it tends to break down more quickly than gravel. However it can be a useful option for very large containers where the weight of harder particles would be a liability. Link to original image.
18.6 Manufactured Soils and Supplements
18.6.1 Cultured Black Topsoil
There are several brands of good quality topsoil on the market that can be useful amendments. Be careful when switching brands, as they are not equivalent. Always inspect a new brand of soil before using it in large quantities.
Commercial black topsoil. The photo shows a national brand that I use regularly. I like it because it is more consistent than local store brands, but it tends to be stickier than I like. I almost always will amend this raw topsoil, even when I am using it for general propagation. Original photo by Dan Johnson.
My first-pass quality test is to see if it can be sifted through 1/2-inch hardware cloth. Soil with too much clay will clump and stick even when dry, and not pass through without being forced. The same screen size shows whether the soil has too many large bark pieces. Soil that passes completely through 1/8” screen (window screen) has particles that are too small. Good quality black topsoil will be mostly particles that pass through the 1/2-inch screen, but not the 1/8-inch screen.
Some people recommend potting soil that has been screened this way. Personally I do not like it because the composition and moisture retention varies so much between different producers. It also contains perlite, which is a very good inorganic component for general growing, but looks out of place in a bonsai pot.
18.6.2 Agricultural/Dolomitic Limestone
Almost all organic materials are too acidic to use without adjusting the pH. Dolomitic limestone is a combination of calcium carbonate and magnesium carbonate. It raises pH of the growing medium but also provides some additional magnesium and calcium for plant uptake. Dolomitic limestone dissolves slowly in the growing medium, stabilizing pH for several months.
There is no easy way to calculate precisely how much limestone is needed to adjust the pH of a soilless mix. It depends on the particle size of the limestone, and the cation exchange capacity and current pH of the mix. The starting pH and CEC in turn depend on your source of organic materials. The only way to be sure how limestone affects pH is through trial and error.
When buying dolomitic limestone, look for 100-mesh/fine grade. The more finely ground 200-mesh/superfine grade changes pH more quickly, but does not keep the soil pH stable for as long.
As a general guide, commercial growers typically use between 2-8 pounds of dolomitic lime/cubic yard of soilless media, and rarely need more than 10 pounds per cubic yard. To work out what to use with your choice and source of organic matter, make a test batch of soil with no limestone added. Check the pH, then add HALF as much limestone as you think is needed, or on the low end of the stated reference range. Recheck the pH, and add more limestone if needed to reach the desired pH. Remember, it is easy to raise soil pH, but much harder to lower it again. Once you have made a few batches of soil, you will have a good idea of what is needed for the materials you use in your mixes.
Instructions for testing soil pH at home are located in Section 8.
18.6.3 MicroMax Nutrients
Micromax granular micronutrients is made to be incorporated into soil-less planting mixes. It provides micronutrients slowly to the roots. Rates range from 1 lb to 1.5 lbs per cu yd. The two main brands, ICL and Scotts, have similar makeups:
| Element & Form | ICL | Scotts |
|---|---|---|
| Calcium carbonate | na | 6% |
| Magnesium carbonate | na | 3% |
| Ferrous sulfate (iron) | 12.0% | 17% |
| Manganese sulfate | 2.5% | 2.5% |
| Zinc sulfate | 1.0% | 1.0% |
| Copper sulfate | 0.5% | 1.0% |
| Sodium borate (boron) | 0.1% | 0.1% |
| Sodium molybdate (molybdenum) | 0.005% | 0.05% |
| Sulfates (sulfur) | 15.0% | 12% |
18.7 Niche Amendments
18.7.1 Zeolites
Some growers add 2-15% clay-type zeolites to their soilless media to increase nutrient retention and improve physical properties.
In agriculture, clinoptilolite (a naturally occurring zeolite) is used as a soil treatment. It is a source of slowly released potassium. If previously loaded with ammonium, the zeolite can serve a similar function in the slow release of nitrogen. Zeolites can also act as water moderators, in which they will absorb up to 55% of their weight in water and slowly release it under the plant’s demand. This property can prevent root rot and moderate drought cycles.
I have never tried using zeolites, but they might be useful. If anyone has experience with them, let me know.
18.7.2 Charcoal/Biochar
Charcoal dust or small pieces can improve the CEC of soil-less mixes. Research trials and home tests have reported that a growing mix with 2-10% (5% typically) biochar performs the best.
Like fresh bark, biochar causes nutrient drawn-down. It needs to be pre-charged with nutrients and micro-fauna. If not charged before adding it to soil, biochar will pull nutrients from soil and hamper microbial development. One charging method is to mix biochar 1:1 with moist compost and let it sit for at least 10 days.
DO NOT use crushed charcoal briquettes in soil. They are not true charcoal, but charcoal powder bound together with chemicals and glue. Look for agricultural grade charcoal.