Uptake (meq L-1)
Crop
Substrate(max) (meq L-1)
Na+
Cl-
Na+
Cl-
1
1.2
10
15
Cucumber
1.2
2
8
10
Pepper
0.8
1
8
12
Lettuce
1.5
2
8
15
Chrysanthemum
0.3
0.9
6
8
Rose
0.3
1
6
8
Gerbera
0.4
1.5
10
10
Tomato
Table 1: The uptake concentrations of Na+ and Cl- by some of the most common greenhouse crops as well
as the maximum concentrations allowed to maintain optimal growth and crop yield.
High Zn-levels can often be found
in water gathered from galvanised
roof surfaces or fed trough galvanised pipes. Manganese toxicity
problems may develop on lettuce
(open or loose tulip shaped heads)
where seedlings are raised on
sphagnum peat, due to high levels
of Mn in this substrate. Both iron
(Fe) and manganese (Mn) may also
cause production problems by
blocking irrigation drippers.
The pH of the water to be used as
feeding water for the nutrient solutions should also be indicated on
the water analysis. Most soils are
well-buffered which means that
their pH values do not change easi-
ly. However, in soil-less production
systems this is not the case and special precautions are needed to control the pH since the pH will affect
the availability of nutrients in solution. The recommended pH level for
soilless systems is normally between
5.8 and 6.5. Nutrients are absorbed
by plant roots as ions (NO3-, NH4+,
H2PO4-, Ca2+) but at high pH values
many of these ions, including the
micronutrients Fe and Zn, can precipitate as insoluble s