Nobody gets excited about bonsai soil. It is a small miracle that readers have even come to this article. Soil just doesn’t attract the same interest in the bonsai beginner as do wiring or carving. Eventually, though, when beginners have seen the effect of a proper soil mix, an odd obsession with soil will begin to develop.
The soil used in bonsai is much more coarse than for other potted plants. It must allow water to flow freely between the particles. Fresh water forces the toxic carbon dioxide, released by the roots, out of the pot. As the water freely flows out of the pot, beneficial oxygen is drawn in from the soil surface. This regular exchange of water and oxygen is critical to good bonsai health.
Another benefit of using a coarse soil mix in our hot Manitoba summer is that it will dry more quickly and allow for daily watering during the summer months with minimal danger of root rot.
The soil in a bonsai pot must perform several functions. It must:
- anchor the tree
- retain moisture
- supply food
- supply air
- shelter the roots
In Manitoba, to accomplish these requirements, we use a mixture (more accurately called a growing medium, but that requires too much typing, so we continue to call it soil) made up of Sea Soil™ and pumice.
For many years, the classic mix of 1/3 crushed quartzite, 1/3 Turface, and 1/3 bark chips was recommended by many and used by most club members. This recipe has now been improved to yield better results.
Pumice, a natural, porous, volcanic rock from British Columbia, has replaced Turface, a fired clay that absorbs water, slowly releasing it as water vapour. Pumice has two advantages. It absorbs and releases nutrients when the plant is fertilized and watered. A plant potted in Turface can contain dry pockets or water-logged pockets, too little or too much water. Pumice’s wicking action helps distribute moisture more evenly to all parts of the soil mass. In wetter regions like Canada’s West Coast, many use 100% pumice for their bonsai.
Sea Soil™, an odourless, organic compost made of fish and tree bark, has replaced Bark chips. Unlike freshly chipped tree bark, this product is already broken down and its nutrients are readily available. For those who want a more forgiving mix, this optional additive will provide some nutrients and more moisture retention.
Beginners must not be tempted to use the dense, black loamy earth in which the trees in the yard are growing. For bonsai, this soil quickly become compacted, starving the roots of oxygen and making it difficult to water the plant properly. A porous soil mix allows for daily watering in the heat of summer and allows ample exchange of vital gases.
Before using any soil ingredient, you must sift out the very fine and the very coarse particles. Anything so fine that it falls through a sifting screen of 1/16 inch, (window screen size mesh), must be discarded as it will compact and prevent water from draining quickly. Any particles larger than 1/4 inch, that is, particles that will not pass through a 1/4 inch screen, must be discarded. Use whatever is left for your bonsai.
Experienced bonsai artists sometimes adjust the ratio of pumice and Sea Soil™ to better suit individual trees with more specific requirements. Various bonsai books may recommend various other components or somewhat different proportions because their authors live in different climates or have different materials available to them. The above recipe works well in Manitoba!
At club meetings, beginners may hear members discussing other mixes, such as akadama, shale, lava rock or river sand. The seasoned bonsai grower can adapt the watering and feeding program to suit the species, the potting mix and the amount of rainfall in southern Manitoba. To learn more about bonsai soil mixes, here are two articles to help you with your research: