Water quality

Water used in horticulture is taken from different sources and has different dissolved impurities. Soft water has very few impurities, whereas hard water contains large quantities of calcium and/or magnesium salts which raise the pH of the growing medium, especially where impurities accumulate (see liming). Even small quantities of micro-elements, such as boron or zinc, have to be allowed for when making up nutrient solutions which are to be re-circulated (see hydroponics). Water taken from boreholes in coastal areas can have high concentrations of salt that can lead to salt concentration problems. The quantity of dissolved salt in water can be measured by its conductivity; the higher the salt concentration the greater its electrical conductivity. Providing the levels of useful salts are not too high, the water can be used, but the additional nutrient levels (fertilizers) must be suitably adjusted (see conductivity).

In the re-circulation systems that are again becoming more prevalent in protected culture, the salts not used by plants can become concentrated in the water. These dissolved salts can interfere with the uptake of useful salts such as potassium, making it difficult to create a balanced feed within the safe conductivity limits and to reduce the plant growth rates as they become too concentrated. Water drawn from rivers, lakes or even on-site reservoirs may contain algal, bacterial or fungal pollution, which can lead to blocked irrigation lines or plant disease (see hygienic growing).
Rainwater is increasingly being used as a major source of water. It is usually of high quality, i.e. low conductivity, but there can be contamination related to the location or the method of collection or storage, e.g. high levels of zinc when collected through galvanized gullies. Good quality rainwater can be used to dilute otherwise unsuitable water to bring it into use. Alternatively, poor quality water can be treated using reverse osmosis; water under pressure is forced through a membrane which holds back most of the dissolved salts. Alternatively, deionization can be used, this involves passing the water over resins to remove the unwanted salts. In both cases, an environmentally sound method for disposal of the concentrated solution produced remains a problem. High energy distillation and electrodialysis methods are generally too expensive for cleaning water for growing. To avoid disease problems, water supplies can be sterilized. On a commercial scale this is usually done by heat sterilization. Ultra-violet light or ozone treatments are usually more expensive and the use of hydrogen peroxide tends to be less effective.