Maximum Yield Australia/New Zealand July/August 2020 | Page 36

Raft or float/pond systems are typically run as fully closed.
NFT is an example of a
commonly used closed system.
Nutrient drainage collection
channels can be used on both
open and closed systems.
Ebb and flow irrigation systems are also usually closed systems
which recirculate the nutrient solution.
Closed Systems
Closed systems may seem preferable when it comes
to hydroponics as they make efficient use of water and
fertilisers and, in theory at least, the same solution can
be recirculated for an extensive length of time. In reality,
however, closed systems can be complicated when it comes
to solution management as the ratio of individual elements
can change over time without the grower being aware of
problems developing. For example, if the nutrient product or
formulation used is not precisely matched to plant uptake of
each element, accumulation and depletion can occur over
time in the nutrient solution. This most commonly occurs
with macro elements such as nitrogen or potassium and it
is possible for a rapidly growing crop to completely strip
these elements from a recirculating nutrient solution if
monitoring is not regularly carried out. Because it is difficult
to predict the exact ratio of nutrients taken up by a crop (and
as this varies between growing environments, systems,
and even growth stage), changes in the composition of a
recirculating nutrient solution occur over time, even when
fresh stock solution concentrates are added to maintain the
EC level. Commercial growers with large closed systems
that recirculate the nutrient solution manage this issue
by having regular solution analysis carried out by an
agricultural lab, and some may even carry out testing onsite.
This gives valuable information about which elements are
accumulating and which may be depleting over time, so that
correct adjustments can be made as required. For smaller
growers where regular solution analysis is not cost effective,
a simpler method is to partially or completely replace the
recirculating solution in closed systems on a regular basis
before any imbalances begin affecting plant growth.
“
CLOSED SYSTEMS
MAY SEEM PREFERABLE
when it comes to hydroponics
as they make efficient use of
water and fertilisers.”
A second issue with closed recirculating systems is when
the water source or a growing substrate contains unwanted
minerals that may accumulate in the nutrient solution over
time. A common example is where sodium and chloride are
present in a water supply — these unwanted elements can
accumulate rapidly in recirculating systems, particularly
where the volume of solution in the system is low and where
water use is high, thus introducing more and more of the
unwanted elements with each water top up. Other examples
include water sources which may be hard and high in
calcium and magnesium or have levels of trace elements
that can accumulate in closed systems over time creating
imbalances and even toxicities.
Ideally, closed systems are the most efficient when a highquality,
low-mineral water source is used, and this, combined
with a carefully balanced nutrient product or formulation,
gives the longest usable life of a nutrient solution. Reverse
osmosis water has no unwanted elements present, thus the
nutrient solution can be fine-tuned to the crop being grown
and the requirement for solution dumping and replacement
can be minimised. If well-managed, closed systems with
high-quality water and a well-managed nutrient formulation
can run the same nutrient for a considerable length of time;
many growers only replace this once per crop.
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