Appropriate soil is one of the essential elements to maintaining healthy bonsai. Bonsai soil serves several key purposes: it proves a secure base for the tree to stabilize itself; it holds water for the roots to drink; it holds air for the roots to breath; it holds nutrients for the roots to absorb; and it provides an attractive visual setting for the tree.
One thing in particular distinguishes bonsai soil from typical soils: drainage. Very few trees like to have "wet feet", i.e., soil that is always soggy in the bottom layers. These conditions starve the roots of oxygen and may kill them; damaged or dead roots are then likely to rot away. ("Root rot", a variety of fungal infections that destroy root tissue, is often thought to be caused by too much moisture, but according to bonsai master Walter Pall root rot is the consequence, not the cause, of dead roots.) Beyond good drainage, bonsai growers recommend a wide (sometimes conflicting) array of soil advice, so there is no single "best soil" recipe.
In many cases, "soil" is actually a misnomer, as the potting medium may contain no soil in the traditional sense. For trees growing in nature, the decaying organic matter in soil provides nutrients necessary for growth. In bonsai, limited soil volume means than any nutrients from natural organic matter is quickly depleted, so fertilizer and sometimes other supplements are added to the potting medium to provide nutrition. The fine particles of clay and organic matter in potting soil and most natural types of soil hold too much water and not enough air for all the but the most water-loving bonsai, so bonsai soils usually contain generous portions of course inorganic particles, such as grit and/or porous clay. These inorganic particles allow water to drain freely and create numerous small air pockets in the soil. Baked clay particles (including natural volcanic clays such akadama) also retain water and nutrients. To hold more water, most bonsai soils also contain some kind of organic component; the most common choice is small pieces of conifer bark (e.g., pine bark or fir bark), which provides better drainage and breaks down slower than peat moss or generic potting soil.
Water requirements vary from species to species and climate to climate, so traditional bonsai soils are customized meet the needs of a particular plant and grower. Typically, evergreen conifers (such as pines, junipers, and spruces) are allowed to dry out almost completely between waterings, so more porous soils light on organic components are used. Deciduous trees vary widely in water needs; some (such maples) do well in very fast-draining soil, while others (such as Chinese Elms) can tolerate slightly moister conditions. Many conifers can go completely dry for short periods without any ill effects, while some flowering evergreens (such as boxwoods) will die if allowed to dry completely. In traditional soils with significant fine particles, the organic and inorganic portions of soil are balanced so that a tree goes nearly dry (but not totally dry!) between waterings.
Traditional bonsai soils and watering methods were developed before baked clay products such as arcillite were available. Baked clay particles are course and shed excess water quickly, but are also porous and retain water and nutrients. Soils based on such particles are much more flexible than traditional soils; they can be watered more frequently without concern for overwatering, since they admit plenty of air even when wet, and do not need to be monitored as carefully. Some growers even plant their trees is pure baked clay, although such soils may shift around more (thus disrupting root growth) compared with mixtures with at least some fine, sticky particles.
These are some of the most common components of bonsai soil. Almost all soil components should be sifted to remove dust and very small particles, which can clog a pot and hamper drainage.
- Conifer bark - in particular, "aged pine bark" or "aged fir bark". Conifer bark is often sold as "orchid bark" (since orchids are usually planted in pure bark), as soil conditioner made of "decomposed pine bark" or the like, and as mulch. Most sources of conifer bark must be screened to remove large pieces (larger than about 1/4" or 1/8") and very small pieces (smaller than about 1/16").
- Peat moss - retains a lot of water, so it is used sparingly. Peat moss is slightly acidic; some plants (in particular, azaleas) need acidic conditions to thrive.
- Potting soil - not widely recommended, but it works and is easy to find. May also contain slow-release fertilizer pellets, which can supplement periodic fertilization.
- Leftovers - the decomposed organic components from used soil is often the most convenient source of water-retaining organic material, especially soil from the same tree. There may be a danger of transferring harmful microorganisms (such as root rot). Keeping some soil from a previous potting is also a way to preserve helpful microorganisms (such as the symbiotic Mycorrhizae fungus), which is especially important for pine trees (although not as important as proper soil drainage!).
- Akadama - the traditional soil of Japanese bonsai. A natural volcanic clay surface-mined in Japan, akadama is an expensive (and fast-disappearing) soil component. It breaks down relatively quickly (a year or two) when exposed to freezing and thawing, but many bonsai artists attest to superior root development compared to manufactured substitutes.
- Arcillite - baked montmorillonite clay, sold as Turface, Schultz Aquatic Soil, and other products. Arcillite has an attractive reddish color that darkens to brown when wet. It has high nutrient-retention properties, and is very stable and long-lasting.
- Baked fuller's earth - variable baked clay products, sold as an oil absorbent (e.g., Oil-Dri) and a variety of cat litters (e.g., Johnny Cat), among other products. The most economical, and often easiest to find, porous clay component. Some types of baked clay break down faster than others, and some are less attractive than others.
- Horticultural perlite - A very light-weight, fast-draining substance that holds a small amount of water. Perlite floats in water, breaks apart easily, and can be quite the eyesore on the soil surface. However, it is a helpful component in large pots, which can be extremely heavy when filled with dense soil components.
- Horticultural vermiculite- A light-weight, brown substance than retains more water than perlite.
- Crushed granite - an inert, non-porous particle that promotes drainage, often sold as poultry grit. The sharp edges are thought to stimulate splitting of roots and the growth of many small drinking roots.
- Flint grit - an inert, non-porous particle like crushed granite.
Probably the most common general-purpose soil recipe is:
- 1 part conifer bark
- 1 part porous clay (akadama, arcillite, or baked fuller's earth)
- 1 part non-porous inorganic (stone grit or large-grained sand)
An easy alternative is:
- 2 parts porous clay
- 1 part potting soil
Soil mixtures should be rinsed thoroughly to remove clay dust before use.