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The soil of a Bonsai has to be very porous to allow for better gas exchange and water flow which helps the root system grow to support the huge foliage mass you see on Bonsais. This is actually the factor that makes the growing of Bonsai trees in shallow pots possible. It is the topsoil or first few inches that are the most important to the functions of any plant or bonsai tree, even huge oak trees get the majority of their nutrients from the top portion of the soil around them. Most people falsely think that the transpiration process happens only in the leaves but if this were true then plants that were completely cut back would not be able to grow back from the roots.