Potting soil mixes for bonsai is an evolving topic and there are many considerations. We all know about soil, right? Ever since we were little kids and played in it and got in trouble if we tracked it in the house or had too much on our clothes or our bodies. So what’s the big deal? Plants grow in the stuff, so put some in a pot and plant your tree in it, it will grow. Trouble is folks, it ain’t that simple. To gain a little understanding of what it is all about, I’ve searched the literature (more like a little term paper and not a doctoral dissertation).
Three areas to consider:
1. The physics – the structure of the soil mix and its capability and capacity to let water and air move through the mix. The roots of most plants don’t take in water. They take in ‘water vapor’, a mix of water and air. Both are needed in the soil. As water is pulled through the soil by gravity, it pulls air down into the soil cavities which mixes with the residual moisture to create the needed water vapor.
The second physical function of the soil is to provide a stable base or foundation to support the plant and protect the roots.
2. The chemistry – the chemical makeup of the soil and its capability and capacity of providing the nutrients the plant needs to support and maintain vigorous, healthy life. Soils vary in their makeup to be more or less acidic, alkaline or neutral. A broad mix of elements and compounds are required nutrition for the plants. This is much more complex than the nitrogen, phosphorous, potassium (N-P-K) that we usually see, perhaps with a little iron added. Those elements must be in a form usable and available to the trees.
3. The biology. Soils, in general, are composed of mineral and organic components in varying proportions. Clay soils are composed of fine mineral particles, often 
with some fine organic residue mixed in. Sand is composed of larger mineral particles, while loam has a larger portion of organic particles and a smaller portion of minerals, either fine (clay) or sand. In addition, varying largely by area, the soils also contain very large amounts of other life forms, bacteria, fungi, yeasts, as well as insects, their eggs and larvae.
Clay soils are very valuable for gardening or farming but become problematic in containers. Some forms, such as acadama or kanuma work very well in pots. Most garden clay soils don’t work well in containers for woody plants. They are too dense to serve well as bonsai soil.
A sub-topic of the soil biology is the newly evolving science of the soil ‘microbiome’. There is an enormous community of living microbes living in the soil and the plants that serve valuable and needed support functions for our trees. This adds to the complexity of the issues involved in the culture and care of bonsai trees. One of those microbiomes most of us are familiar with is a family of fungi known collectively (commercially if not scientifically) as mycorrhizae.
A few extracts from the Bonsai literature provide some context for the reader and help build a sense of the changing ideas about what is best for bonsai potting soil. Note that historically the Japanese used garden soil, particularly sub-soils (to avoid the insects that often live in the topsoil). Also note that they grew trees in garden soils for several hundred years before the advent and general availability of specialty soils like akadama, lava rock, pumice and perlite. Another forty or fifty years of experience will allow the development of a more scientific basis for best managing bonsai soils. There are a number of corollary issues besides root development, moisture retention and basic nutrients to be studied and given consideration in the development and care of our trees.
Tatsuo Ishimoto, The Art of Growing Miniature Trees, Plants and Landscapes, 1956
“Your soil mix should match as closely as possible the original soil provided to your plant by nature.”
Lynn Perry, Bonsai: Trees and Shrubs, A Guide to the Methods of Kyuzo Murata, 1964
Ms Perry writes a great deal about the soils used in Japan and their history. She also discusses ph levels. Her general formulas are various proportions of sharp coarse sand, and subsoil and/or topsoil. Noting “In cases where the individual prefers to incorporate black soil into the potting mixture, it is recommended that a greater portion of subsoil than topsoil be used.” She notes that at one time in the Tokyo region only sand was used as a potting soil. This necessitated a greatly enhanced regimen of watering and fertilization.
John Naka – copyrights 1973-1996, Vol 1. pp 90-95 Six pages of various formulations for trees of different species, ages and sizes. He shows only 3 ingredients, soil, mulch and sand in 3 different sizes: large between 1/8” and ¼” screens, medium between 1/16” and 1/8” screens and small between 1/40” and 1/16” screens, with dust to be discarded.
Vol II p. 56 In the description of the potting of a Coastal Redwood –
“Soil mixture was 1 part soil, 2 parts mulch (peat or other organic material and 2 parts sand.”
The Japanese Art of Miniature Trees and Landscapes, Yuji Yoshimura, 1957.
Yoshimura devotes a couple of pages to soils and gives mix formulations for 1 – different size pots (smaller for small pots and larger for large pots). And 2 – for different types of trees in different stages of development (in training, mature or training completed). He lists as ingredients for the several formulations sand or fine gravel, black loam sieved, black loam powdered, clay or reddish loam (35-50% clay), light clay with sand (Kanuma), peat, Leaf mould or humus and sphagnum moss.
Yoshimura also recommends spreading out the soil in the sun for drying for about a week. Also, all soils should be sieved and separated as to texture/size. Powders or dust should be discarded unless specifically called for as they tend to clog the roots and contribute to root rot, a major cause of the loss of potted trees. They may be suitable for bamboo (a grass), and willows (water loving) and some herbaceous accent plants, grasses and herbs. He also calls for some powdered black loam with broad leave evergreens and deciduous, and specifically maples and zelkovas.
The Masters’ Book of Bonsai, Compiled by the Directors of The Japan Bonsai Association. First U.S. printing 1983
“Soil: The consistency, quality and care of soil for bonsai determines the health, vitality, and appearance of your dwarf trees. Ideally, soil for bonsai should be lumpy, should drain easily, and should usually be free of manure or artificial fertilizer.
“Red loam: Brownish-red soil with hard, solid lumps; free of manure. Used as the basic soil for bonsai. Screened into three grades: large, medium, and small lumps.
(Note: This appears to be a general description of a soil that we know as ‘Akadama’, and is the first reference I know of to this type of soil rather than just garden soil or sub-soil)
Sand-and-clay mixture (Kiryu-tsuchi): Resembles red loam, but grayish in color and sandy in texture with hard lumps. Usually mixed with red loam to produce soil for evergreen bonsai. Screened into three grades: large, medium, and small lumps.
Black loam: Dark brown soil with hard, solid lumps; some manure content. Mixed with red loam to produce soil for various types of bonsai and, when finer lumps are used, for decoration soil. Screened into three grades: large, medium, and small lumps.
Sandy light clay (Kanuma-tsuchi): Yellowish-white soil – turning yellowish-brown when damp – capable of retaining large amounts of moisture. When screened and mixed with 30% sphagnum or “mountain” moss, it is used for planting azaleas, and when mixed with red loam for cultivating cuttings.”
(Note: Satsuki azalea bonsai master Suisho Nakayama in his talk and demonstration at the BABA Exhibit Saturday (3-9-2014) evening mentioned that his azaleas were planted in Kanuma.)
“Screening: Prior to screening, all the soil types described above should be dried. This is usually done by spreading the soil on straw mats and exposing it to the sun and wind for about ten days. After screening, lumps that are very large or very fine should be discarded, leaving only the three grades of large, medium and small. For large grade, a mesh of ½” should be used; for medium grade, 3/8”; and for small grade, ¼”
“It is always best to use soil of the same type found in the natural environment in which a particular species grows. Since it is often impossible to create such soil artificially, however, red loam is usually used as a substitute in bonsai cultivation. Red loam can be used either as it is or with a mixture of pumice, small pebbles or river sand to improve its drainage capacity, or of sphagnum moss to improve moisture retention.”