Coarse soil used at the bottom of the pot for good drainage. Soil for bonsai and different sand types are separated according to purpose of use. The coarsest grain among them measuring between 7 to 10 millimeters is known as gorotsuchi. Types of gorotsuchi include akadama, Kanuma and Fuji. They are laid lightly at the bottom of the pot and then soil for horticulture is placed on top. Generally, soil is matched to the tree. For instance, soil with a sand pedigree is used for bonsai requiring a large amount of sand, and akadama is used for deciduous trees. Gorotsuchi plays an important role in discharging surplus water and enabling air to enter easily through the opening at the bottom of the pot after water runs out.
translated by Kyodo News