Once you have chosen your containers or made them from available materials, the next job is to fill them with aggregate or growing medium. Quite a number of different aggregates are satisfactory for hydroponics. Very often, however, it may be most economical to use a growing medium which is in plentiful supply locally and will cost little or nothing. For example, sand, gravel, or small pebbles can frequently be gathered near the home, without expense, while cinders from coal fires only require washing to make them suitable for soilless gardening. Here is a list of common aggregates employed by hydroponicists.