Bonsai Soils

Selecting the right soil is very important while growing a Bonsai Plant.  The soil provides the nutrients to the growing plant.  It also serves to retain water so that the plant can absorb.  Each Bonsai species has specific requirements and hence requires the proper soil.
The soil in a Bonsai plants should
  1. retain water so that the plant may absorb
  2. provide nutrients for the plant to absorb
  3. Provide aeration for the roots to absorb oxygen
The Bonsai soil should not get compacted.  This can prevent the free flow of water and air within the soil and can damage the plant.  The soil may be turned periodically to prevent compaction. 
The soils used for Bonsai Plants are classified into two main categories. 

Organic soils and Inorganic Soils. 
Organic soil contain components  formed by the decomposition of natural plant matter.  Examples are leaf litter, peat and bark

Inorganic Soils are materials which are materials such as fired clays, volcanic lava. These soils do not contain any organic matter. 

Organic soil materials have the advantage of containing the nutrients required for plants.  However, there are significant disadvantages of using organic soils.  Excess water retention can damage the roots.  Organic Soils tend to break down and compact.  This can prevent water flow from reaching specific regions of the roots. 

Inorganic soils do not “break down”.  They retain their original granular structure.  This ensures that the water drains and no excess water remains.  Aeration is better.

Organic soils specially composed for different bonsai species are available at most hobby stores.