This method is also known as passive hydroponics or semi-hydroponics. This system has the plants growing in a porous material like special clay granules, vermiculite, fibre glass, coconut husk, perlite or a similar substance that holds water, has lots of air spaces but does not act directly on the plants roots. The water is run through the material to keep it wet and the material keeps the plants roots separated to allow a good contact with the air to help the plant use oxygen and carbon dioxide. this method is easy to set up and control and is suitable for a number of plants like orchids or bromeliads that in nature grow with their roots in warm damp air.