Uwatsuchi
Uwatsuchi (surface soil) refers to microscopic soil that is spread over uetsuchi soil on the surface of a pot. It is also called keshotsuchi (silver white sand). It prevents the soil from drying up, restrains the surface from becoming rough and is used for bonsai to show beauty and calmness. Remnants of fertilizer and dirt are easy to stick to it and if they remain untouched, uwatsuchi deteriorates and hardens to make it hard for water to flow.
translated by Kyodo News