The substrate or growing medium is the substance in which plants are sown and grown. In hydroponic cultivation, although it is not a soil as such, it is the place where the root system of the plants will expand. The substrate itself does not contain any nutrients and is called an inert substrate; hence it becomes necessary to irrigate it, either continuously or at intervals with nutrient solution. For many plant varieties the use of a substrate is not necessary, as they can be grown directly in the nutrient solution.