The quality of the primary water supply is very important when operating within a closed hydroponic system. Rain water is pure and so is naturally the best primary water source for use in hydroponic cultivation. Rain water is usually collected from the roof of the greenhouse and stored on the property. It is advisable to discard water collected from the first rain showers after a prolonged period of drought, since this may contain a lot of dirt and in coastal areas also sodium chloride. If discarding is not possible then this water can be added to the dirty drain water basin, so it will go through the cycle of purification and disinfection before being used as irrigation water. When rain water is scarce, other sources of water are required. However water quality does vary from region to region and within regions from location to location. Table 4 provides an overview of minimal standards for “good quality” primary water. If water does not meet the criteria of at least class 3 it is not suitable for use in greenhouse hydroponic horticulture. If you are in any doubt or require further advice in respect to the quality of your primary water supply, we recommend that you contact Groen Agro Control or a reputable laboratory in your region.