Hydroponic is the method of growing plants without the use of soil, as an agent of growing the root system. Plants grow in water where the necessary nutrients are added. The roots of the plants grow either in other "substrate" materials (rock wool, perlite, coconut, expanded clay and peat) through which water constantly passes, or in water in which all the necessary nutrients have been dissolved (nutrient solution). The type of nutrient solution used depends on the type of system. The systems are divided into open and closed. In open growing systems the substrate is constantly supplied with new nutrients, while the old ones are removed through the drainage system. In closed or recycled systems, nutrients are not removed by drainage but are collected and returned to the plants. This is very useful when no substrate is used or when the substrate retains relatively little moisture (expanded clay and perlite). In hydroponic systems it is very important that the nutrients are in the right proportions of all the necessary elements that the plant needs.