A
feature common to most systems is that plants are fed from above the roots
(i.e. referred to as “top-feed”). In flood & drain however, the
nutrient is fed from beneath (Fig
5.2). This process floods the root zone to a pre-determined
maximum level after which the nutrient is allowed to drain back into the
reservoir. This process is controlled by a pump, timer and a
specialized ‘inlet’ and ‘overflow’ device:
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‘Inlet’: This also serves as the drain
outlet. It is positioned as close as possible to the base of the flood &
drain tray. Once pumping stops, the nutrient is able to fully drain to the
reservoir via the inlet and pump. To allow reverse flow the pump
must be of a ‘centrifugal’ type. Complete drainage is necessary to
prevent water-logging.
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‘Overflow’: This is positioned at the
maximum desired flood height. This means pumping can only flood to
the height of the overflow. To maximize feeding and flushing of the root
zone this valve should be set as high as possible. However, if a
medium is used avoid wetting the upper surface otherwise disease, pest and
algal problems can occur.
Suitable media for flood & drain include
expanded clay, perlite or even none at all. The medium’s water holding
capacity determines the feeding frequency.
Advantages to Flood & Drain
systems
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The flooding action helps minimize local salt build-up in the root zone
and achievement of satisfactory root zone pH and conductivity.
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Flooding purges stale air from the root zone. Drainage draws fresh air
back into the medium.
+
Flooding promotes the penetration of sterilizing agents (etc) throughout
the root zone.
Disadvantages to Flood & Drain systems
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Unable to easily inspect roots if medium is used.
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If no medium is used, pump failure can result in plant death within a few
hours.
