MATERIALS AND METHODS
The crops were grown in gutters provided with poly urethane slabs for the tomato
crop and with glass wool fibre slabs for the cucumber crops. The water was provided by
drip irrigation. The primary water used in the experiments for preparation of the nutrient
solution was partly rain water and partly water desalinized by reverse osmosis. The
concentrations of Fe and Mn in these types of water were very low and could be ignored
with respect to the preparation of the targeted concentrations of the nutrient solution.
However, the Zn concentrations were significant, especially those of the rain water, due
to the contact with the galvanised parts of the greenhouse construction. In addition, those
of desalinized water were worth mentioning, because notable Zn concentrations were
released from the polyester containers in use. All Zn in the primary water was taken into
account with the preparation of the treatments. The supply of the nutrient solution was
abundant in relation to the water use by the crops, to prevent local accumulations in the
substrate. The nutrient solution drained out was about 75% of the supply, and the drained
out solution was reused in the system.
The experimental design consisted of twelve treatments in three parallels within
randomized blocks. Three factorial experiments were carried out with different Fe, Mn
and Zn concentrations. The treatments are summarized in Table 1 and laid out in a 3×2×2
design for tomato and in a 2×2×3 design for cucumber. Fe was added as Fe-DTPA and
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Mn and Zn were added as sulphates.
The tomato crop was grown from December until September and the cultivar was
‘Astra’. A spring crop cucumber was grown in a second year from December until the
end of May and an autumn crop from the beginning of June until the end of October. The
cultivars were ‘Enigma’ and ‘Jessica’ respectively. The composition of the nutrient
solution used was tuned to the need of the crop according to the standard
recommendations for growers (Sonneveld and Straver, 1994) except for the
concentrations of the micro elements under investigation, which were adjusted to the
concentrations defined in the experimental design. The EC value aimed at in the substrate
solution was 3.0 dS/m and was adjusted according to the growing conditions. In early
spring higher values were maintained to ensure an optimal fruit quality. The pH value was
adjusted between 5.0 and 6.0 by addition of acid or hydroxide.
The concentrations of Fe, Mn and Zn were determined fortnightly in samples
taken from the circulating nutrient solution. The determinations were carried out on the
laboratory by flame atomic absorption. On the basis of these data the additions of Fe, Mn
and Zn were adjusted to the concentrations laid out in the experimental design. Tissue
samples were gathered from young fully grown leaves unless otherwise stated. The
samples were washed with detergent, dried at 80°C and digested according to
Schaumlöffel (1960) to determine the total contents Fe, Mn and Zn and occasionally on
Cu, by flame atomic absorption.
Chlorosis in the fully grown leaves of the crops was visually judged several times
during the growing periods of the crops. The fruits were harvested and the number of
fruits and the total weights were determined. The results were tested by analysis of
variance using GenStat.