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Fertigation Fertigation
is the irrigation of plants with nutrient enriched (and possibly pH
corrected) water. A full
fertigation controller is thus a combination of a multi-station
horticultural irrigation controller and a fertiliser injection system.
A
modern fertigation controller like the NutriDose II range will normally
have a variety of trigger sources to start irrigations including
time-of-day programmed irrigations, solar integration triggering possibly
with the ability to modify the integration rate to compensate for
humidity, temperature and wind speed.
The solar integrator (with temperature and humidity modifiers) will
automatically increase the frequency of irrigations in sunny, hot dry
weather and reduce it in dull, cool, damp weather.
A rain override is useful for outdoor crops as it can be set to
zero the integration counter if rain exceeds a specified level. These
controllers may generate a single irrigation trigger and then sequentially
irrigate all stations in turn. The
Autogrow controllers can be set to do this or can be put into
multi-trigger mode where each station has its own trigger point and when
reached only that station will be irrigated.
This allows crops with differing watering requirements to be
serviced by a common controller. In
fact with the NutriDose IIb or NutriDose
IIi it would be possible to
mix inside with outside crops and crops requiring watering every half hour
with those that need watering once per week.
Of-course each station can be set to have a different EC and
may even be fed with a mixture having a different recipe. It
is important that the solar integrator, fertiliser injection and
irrigation functions are all within the same controller as this provides
the highest level of functionality. For
example, if a separate solar integrator was used, the manual triggering of
an irrigation will not be “seen” by the solar integrator and so its
counter will continue counting. The
solar integrator will not have its counter cleared and may trigger a
second irrigation immediately after the manual one.
In the case of an integrated controller this will automatically be
taken care of. Using
an electronic controller employing feedback control allows a number
of different methods to be used to actually get the fertiliser into the
pipe. The first and most
obvious method uses variable speed injection pumps to inject direct into
the line. This becomes very
expensive as soon as larger flow rates are required.
The second method uses venturi eductors to entrain the nutrient and
pH stock solutions into the flow of water through them.
To adjust the rate of nutrient uptake it is common to use simple
on/off solenoid valves that continuously pulse on and off.
By varying the ratio of the on time to the off time good
proportional control can be achieved.
This method is usually called “pulse width modulated” (PWM)
control. To work reliably the
venturis need a significant pressure drop across them and this normally
leads to an auxiliary pump being required to develop this.
This is a simple robust method but does require high pressure
sensors in the line and also the system design must ensure satisfactory
mixing occurs before the concentrated stock solutions are allowed to mix
with each other. If they meet
in concentrated form a chemical reaction can take place which will change
the formulation of the fertiliser. In
addition, the resulting precipitation can cause solids that can later
block drippers and be a general nuisance. The
last method uses an in-line turbulent mixing vessel into which the
fertiliser stock solutions are dosed.
This method provides excellent mixing and has the advantage that
the dosing and sensing is done at ambient pressure.
Typically a drum of one-to-two hundred litres capacity is used and
is filled at the same rate as it is being emptied.
The
NutriDose IIi controller can
be used in all three of the above methods, has a built-in solar integrator
counters, irrigation control and interface to a PC to provide a user
friendly interface, remote alarms and data and event logging.
It can be expanded up to a maximum of thirty irrigation stations
which may be a mixture of inside and outdoor growing areas. The
NutriDose IIb controller performs a similar function but is a batching
controller. Instead of adding the fertiliser on-the-fly during an
irrigation, a batching controller mixes a complete batch of fertilised
water into a batching tank which is then used for irrigation.
This is a simpler process and tends to be a little lower in cost
and is also easier to set up to get really high accuracy.
In other respects it is the same as the NutriDose IIi
and can be used in multiple batch – multiple trigger, single
batch – single trigger, single batch – multiple trigger and multiple
batch – single trigger modes. The
down side of batched irrigation is that time is used in mixing the batches
and in a large installation or one where very frequent irrigations are
required, it is possible to run out of time and triggers may start to
queue up and causing excessive delays in the irrigation of some stations. |