The suitability of water for growing crops in greenhouses is unpredictable, independent of whether it is taken from domestic mains supply, wells or tanks or other sources. (Additionally, water is becoming an increasingly scarce commodity with agriculture competeing with domestic supplies. This is likely to downgrade the quality of water available for agricultural and horticultural use - ENB). Laboratory testing of the water by growers is extremely important so as to know the quality and composition of the water. The level of soluble salts  and the pH need to be known before nutrients are added for fertirigation. In addition, water taken from wells may already contain high levels of Ca, Mg, and to a lesser extent, Fe, Zn and Mn and pure rain water can be quite acid.  Only the highest (purest) quality of water can be used in nutrient film techniques or closed recycling systems, while a slightly lower grade is acceptable for  ordinary irrigation of horticultural substrates. A third grade is usually acceptable on substrates/soils that are readily leached. Greenhouse Canada  www.greenhousecanada.com E-mail: greenhouse@annexweb.com