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
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.