Maximum Yield Australia/New Zealand July/August 2020 | Page 35

Drip-irrigated nutrient delivery systems may be either open
or closed depending on whether the solution is collected and
recirculated or drained to waste. Open systems are those that do
not recirculate or reuse the nutrient solution that drains from the
base of the growing substrate. The waste nutrient solution, once
it has flowed past the roots and out the base of the substrate,
is channeled away to be disposed of. While this may seem
wasteful, drain-to-waste solutions often find a second use as
liquid fertiliser sources applied to other crops outdoors such as
lawns, pasture, tree crops, vegetable gardens, potted plants, and
ornamentals. In some countries, nutrient discharge is prohibited
or carefully controlled so traditional open systems can’t be used,
and any waste nutrient drainage must be collected and reused
or disposed of correctly to avoid environmental issues.
Open Systems
Open systems are often thought of as being wasteful of both
water and fertilisers while creating an environmental risk
through the disposal of waste nutrient solution. Despite this,
open systems are still in widespread use in commercial
horticulture due to the ease of operation and nutritional
management. Open systems typically use drip irrigation.
Once the nutrient is applied at regular intervals, it flows
through the root system, replenishing moisture and nutrients
in the substrate and any excess then flows out the base of
the growing container/bed/slab/bucket. This amount of waste
solution should only be a small percentage of the volume
initially irrigated onto the plant with most growers working on
a 10-30 percent drain basis. The drainage solution is disposed
of and not reused on the hydroponic crop but may be collected
for use on outdoor soil grown plants as a useful fertiliser.
The main advantage of drain-to-waste open systems is that
at each irrigation, the root zone receives fresh, completely
balanced nutrient solution so ratios and nutrient levels are
usually close to optimal. This is particularly useful where
regular nutrient solution analysis is not possible as nutrient
imbalances are far less likely to occur. Commercial growers,
however, will collect samples of the nutrient drainage and
regularly have this analysed to determine if the solution
they are applying at each irrigation is matching the needs of
plant growth and make regular adjustments to their nutrient
formulation to optimise this.
“
A DEGREE OF SOLUTION
MANAGEMENT SKILL,
monitoring, and regular
adjustment are the basis of
both open and closed systems.”
Small-scale recirculating aquaponics is considered to be an open system.
In open systems, nutrient drainage may be
collected from underneath the plants to
test EC and pH.
Drip irrigation is often used
in open systems.
Management of such closed systems is usually carried out
to minimise waste, and this includes running the lowest
volume of drainage possible while at the same time ensuring
enough run-off to keep nutrients in balance in the root zone.
Some growers manage to run drainage volumes as low as five
percent in carefully controlled systems with precise nutrient
formulation and a high-quality water source. Open systems
are particularly useful where the water supply contains certain
unwanted elements such as sodium, as the drainage of the
excess nutrient at each irrigation not only flushes through
these minerals but prevents accumulation in the root zone and
nutrient solution. This allows growers to make considerable
cost savings on having to treat and demineralise certain water
supplies if using an open system.
Open systems may use channels to collect waste nutrient solution for
draining away from the crop.
Maximum Yield 35