BONSAI SOIL INFORMATION
Soil Types
Akadama Soil

Soil from loam strata of Kanto region in Japan. Ibaragi prefecture is well known for Akadama soil. Red brown in color due to the oxidized iron content. Neutral to slightly acidic. Soil is dug up and dried and broken up into walnut to corn kernel sizes. Soil aggregates have small spaces which allow for both good water retention and drainage. Relatively not broken up easily (Will normally last 1 to 2 years). For bonsai, Hard type is preferred over the reformed products. Used as a basic bonsai soil component for all tree species in Japan.

Kanuma Soil
Eroded volcanic pumice from Tochigi (Kanuma region and Ibaragi coast area), Japan. Pale yellow brown in color when dry which turn to yellow brown in color when wet. Slightly acidic. Very porous with both a good water retention quality and good drainage. Due to the high porous and soft quality, Kanuma soil is best suited for plants with fine hairy roots such as Azaleas. Easily broken up when wet. Since the soil tends easily to turn acidic, it is not good for plants that do not like acidic condition. Used as a basic bonsai soil component for Azaleas.
Kiryu Sand
Sand/Pebbles from eroded volcanic pumice from Kiryu area of Gunma, Japan Weathering and decomposition have not progressed as much as Kanuma soil. It is much harder than Kanuma soil and has sharper edges. Gray color and slightly acidic. High water retention with porous nature. Good aeration and drainage quality. High iron content. Used by mixing with Akadama soil to increase the aeration and drainage of a pot soil.
Hyuga Soil
Volcanic pebbles from Hyuga area in Miyazaki, Japan. Hyuga soil looks like Kanuma soil but is a pumice. Good aeration and drainage quality. Porous and high water retention quality. Used by mixing with Akadama soil to increase the drainage.
Fuji Sand
Volcanic cinders/soil from Mt. Fuji area. Have sharp edges. High in iron content. High water retention. Used by mixing with Akadama soil to prevent the hardening of pot soil.
Pumice
Produced in the same manner as other volcanic soils. However, pumice is from relatively newer volcanic materials without much decomposition. Very porous and float in water. Good water retaining quality. Good aeration and drainage.
River Sands
Decomposed granite pebbles washed by water action of a river. Good drainage and aeration. Water retention varies by the particle size—i.e., larger the size, poorer the water retention quality.
Mixing A Potting Soil

Mixing a potting soil for bonsai depends on many factors such as climate, location, tree species, tree/pot sizes, stages of bonsai, watering and fertilizer application practices, and other variables such as personal experiences and preferences. So, it is not possible to give a general composition of bonsai potting soil.  Examples of bonsai soil mixes used by Japanese bonsai professionals in various parts of Japan are provided as a guide.
(Data is from Kindai Bonsai June 2008.)

Japanese example of soil mixes grouped by geographical regions

Japanese example of soil mixes grouped by bonsai tree species