Figure 3 and 4: A typical pump room where the feeding water will be monitored for pH and EC, adjusted
and nutrients added before being delivered to the plants in the greenhouse.
A full chemical analysis of the water
is however always recommended
since the EC alone will not give an
indication of specific ions in the
water. If the moderate to high EC
reading of the feeding water is the
result of high concentrations of essential nutrients such as calcium
(Ca2+), magnesium (Mg2+) and sulphate (SO42-) the potential quality is
essentially quite good. If however
the dissolved ions are mostly sodium (Na+) and chloride (Cl-) it might
not be suitable for use in a hydroponic system, especially not in a
systems where the nutrient solution
will be re-used. Most crop plants
are salt excluders and take up only
small quantities of Na+ and Cl-.
High levels of Na+ can reduce the
uptake of other cations, especially
Ca2+ while high Cl- levels will inhibit
the uptake of anions, such as nitrate (NO3-). Crops also differ in
their tolerance to root-zone salinity
and norm values have been developed for maximum Na+ and Cl- levels allowed in soilless systems.
Some of these are indicated in Table 1. If there is no alternative to
using high EC water or feeding water with a high quantity of Na+ and
Cl-, the best management practise is
to ensure that the leaching fraction
remains above the crop demand to
limit the build-up of salts in the
rootzone.
Micronutrients are usually present
at very low concentrations and even
Figure 5: Strawberries in hydroponics are sensitive
to levels of boron(B) deemed adequate for other
crops such as tomatoes.
high concentrations will not be detected by looking at the EC reading
alone. In some cases, low EC feeding water may contain micronutrients at phytotoxic levels. Micronutrient-sensitivity also differs between crops. For example, strawberries need relatively low B levels
due to physiological problems with
B at >0.32 mg L-1. Tomatoes can
however tolerate B at levels up to
1.1 mg L-1, almost three times higher than its recommended rate. The
recommend Zn concentration for
substrate-grown tomatoes is 0.33
mg L-1 but at only twice this concentration, toxicity can be expected.
Figure 6: Typical leaf burn symptoms on cucumber plants in a soilless systems associated with
a high EC in the rootzone.