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Bonsai soil can take many forms and can be comprised of numerous ingredients. Tree type, weather conditions, watering schedule, fertilizer type and many other factors can influence how well your tree grows in various soil mixtures. It is recommended that you talk to local bonsai enthusiasts for recommendations on soil mixtures for your trees and location.
The 3 main purposes of bonsai soil are:
- Water Retention - Soil must be able to retain some amount of moisture. If your soil mix is too porous, 100% gravel for instance, you would need to water your tree numerous times per day.
- Root Growth / Fertilization - Soil must be porous with numerous microscopic air pockets to allow good root growth. Typical potting soil is not good for bonsai, as it compacts too tightly, does not have good circulation air and retains too much water. Also, nce potting soil dries out completely, it can be difficult to fully saturate the soil mass without numerous waterings. The organic components of soil provide a tiny amount of fertilization and a place for helpful microbes to live.
- Ballast / Weight - Heavier soil components are often chosen to act as a ballast to keep your tree upright. A tree in a small pot may topple over with a small breeze. Using heavier components will help keep your tree locked in place. (Trees should always be wired into the pot as well.)
The 3 main components of a typical bonsai soil are:
- Organic Component - This component typically retains moisture, supplies small amounts of fertilizer and provides an environment for microbial and fungal growth, which are important for healthy plant growth. Typical organic components are pine/sequoia bark (often sold as orchid soil), sphagnum or peat moss, coffee grounds and other various plant tissues.
- Semi-Porous Inorganic Component - The purpose of this component is to retain some moisture while also allowing air to remain near the roots. Most tree roots cannot absorb water in liquid form; they require water to be in a vapor form before it is absorbed. Semi-porous stone provides the tiny air pockets required to generate water vapor. The two best known examples of semi-porous components are lava rock, Haydite (expanded shale), Hydrocks and Turface (calcined clay product used in the field turf industry). Akadama is a Japanese soil component that is widely used, but is rare and expensive in the United States. Some people use certain types of kitty litter or oil absorber. These products usually aren't high fired, so they can turn to mush after a few months of exposure to water.
- Non-Porous Inorganic Component - This component is main used as extra filler and ballweight for the pot. The hard and/or sharp edges promote good root growth and the irregular shapes help create air pockets in the soil. The density of this component helps provide a heavy and base to help hold the roots in place and prevent wind from knocking over the pot. A layer of this component is often placed at the bottom of the pot to aid in drainage of water. The validity of this use is debated. Typical examples: river sand(sharp is best), granite grit (chicken or turkey grit), or sharp gravel. Be warned, chicken/turkey grit can be sold with feed and vitamins added. If you choose this component, make sure it does not contain these additives.
A very important step in creating a bonsai soil is the act of screening out the proper size of material. Remember, roots grow best in a soil media with plenty of tiny air pockets. We must remove the smallest dust particles that would otherwise fill in these tiny air pockets. A simple way to do this is by shaking your soil components over a window screen for a few seconds and keeping all of the particles that remain on top of the screen. Special bonsai soil screens are readily available that make this task much easier and they are relatively inexpensive.
These bonsai soil screen sets typically come with 3 screen sizes. You generally want to use the particles that are larger than the smallest screen holes and smaller than the largest screen holes. Small components like river sand may fall through the smallest screen size. To remove the finest particles from tiny aggregates, it can be dropped from a few feet into a bucket on a windy day. Organic components like peat/sphagnum moss and pine bark may require some ripping or shredding to get to the ideal particle size. Pine bark is often sold as sequoia bark for orchid soil. The ideal particle size you are looking for is between 1/16" and 3/8".
Once you have all of your components independently sifted and the finest and largest particles removed, you can combine them all together and mix thoroughly. A wheel barrow is often used for large batches. At this point, many bonsai growers will also mix in a slow release particle fertilizer. This ensures your trees are always getting some nutrients in case you miss some fertilizing at a later date.
A common mixture that generally works for most trees is 1/3 Organic, 1/3 Semi-Porous, 1/3 Non-Porous, by volume.
A typical, easily obtainable example of this would be 1/3 pine/sequoia bark, 1/3 Turface, 1/3 granite grit. (all screened to remove fines)
After you have your main mixture, you can make other additions as you see fit. Many growers will use a mix like this for all of their trees; tropical, deciduous, confierous, etc. Others will tweak the recipe for specific types. Pines generally like a little less organic material. Some people will mix slow release fertilizer pellets directly into their soil mix. This ensures your tree is always getting some fertilizer. Even if you do this, you should still fertilize regularly with other types of fertilizer such as soluble fertilizers or topical poo balls.
Older bonsai references will tell you to put large particles in the bottom of the pot first, then fill the rest with normal sized particles. This is supposed to help drainage. This has been debated for a long time, but it is generally accepted to be a myth now a days. Most people use the same size particles through the entire soil mass. The exception to this is when you want to grow moss of the soil surface. Moss will not latch on to large particles easily, so you may need to put a highly organic, fine mixture on top so the moss can grab on. Moss typically needs more water than the top layer of coarse soil can retain.
We hope this article helps beginners understand the differences between potting soil and bonsai soil mixes. It has been said that you can grow bonsai in ANY kind of soil, as long as you give it the right attention. Different soil mixes work differently depending on where you live, how you water and how often you water, etc.