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Irrigation Principles The
first very important piece of knowledge that a grower must understand is
that it is light intensity that drives plant transpiration.
The light energy impinges on the plants leaves and warms them.
The leaves respond by opening their stomata exposing water to the
atmosphere and this evaporates and cools the plant.
If the sun is too intense and the plant cannot provide enough water
to the leaves then wilt will occur. The
simple way to protect the plant from this is to provide shade.
Note that air temperature has a minor influence as compared with
light intensity and a plant can survive at much higher temperatures if
some shading is provided. The
other important point that this raises is that it is the light intensity
that tells us how much irrigation is required.
By measuring light and integrating it (summing it over time) we can
accurately control the frequency of irrigations required to keep
sufficient water in the root-zone. This
is particularly important for greenhouse crops that grow in relatively
small volumes of media which consequently have small buffering capacities.
From
the light received we can accurately calculate the amount of water that
the plant will drink per hour. However,
to provide the plant with that total amount of water we still need to
figure out whether to apply the water in larger busts with long intervals
in between or else to apply the water in many irrigations, each of small
quantity. It turns out that
this decision has far reaching effects. Water
application Application
rate – dripper selection
The
few, large irrigations result in a lower EC (because of the greater
run-off and consequent better flushing action) and the lower average
moisture content will lead to a more generative growth habit where the
plant puts more energy into growing flowers and fruit than in growing
stems and leaves whilst the many, small irrigations will have a higher
root-zone EC (which may need to be compensated for) with higher moisture
content and a more vegetative growth habit. For
this reason, we tend to adjust the size of the irrigations to help us
achieve other targets and in particular for keeping the crop in balance
between too vegetative and too generative.
For instance, in the high-light intensity of summer when the plants
will naturally tend toward generative habit we can elect to have many
small irrigations at a slightly lower irrigation EC to help bring the
plant into better balance. Also,
when wetting up in the morning small irrigations will be best but once
wetted up and we want to encourage good flushing action to prevent EC
build up in the pot we could switch to fewer, larger irrigations –
although, in summer other considerations may indicate maintaining a small
irrigation quantity through the whole day.
It
is normal to try to achieve a run-off of around 30% from about 11:00am on
for vegetables and 40% to 50% for flowers. Once
we have decided on the amount of water to be applied in each watering we
can decide on the frequency at which it should be applied.
Remember that the total amount needed by the plant is dictated by
solar energy and so a solar integrator is used.
However, we need to set this appropriately.
In the mid-part of the day in summer we receive about 1000Watts of
energy per square metre (3.6 MJoules/hour/m2, 360
Joules/hour/cm2 or 1800umol/sec/m2).
The plants in the greenhouse might receive about 70% of this or
less if shading is used and, in turn, will use about 70% of the light they
receive for transpiration. Working
backwards from that we can derive the energy absorbed by the crop which
will accurately predict how much water they need – add to this the
amount of run-off needed and you can predict the irrigation requirement.
This works out to about 1.1 ltr/m2 in full summer sunlight. (varies
a little by crop from 0.8ltr/m2 to 1.2ltr/m2)
This assumes that the crop intercepts all of the suns rays and for
a vine crop this will generally be true except when the crop is very
young. At a planting density
of 2.2 to 2.5 tomatoes per m2
this works out to a little less than 500ml/plant/hr.
If irrigation quantity (round size) is set to 100ml then we need 5
irrigations per hour and the solar integrator would need to be set to
0.8MJoule, 80Joule/cm2 or if a PAR sensor is used about 1.4mols.
If the round size is changed to 50ml then these figures would need
to be halved. Round sizes for
vine crops grown in slabs are frequently in the range 20ml to 200ml and
most commonly around 80-120ml. By
careful monitoring of root zone conditions (input, output and media
moisture) and observing the crop habit, these settings can be adjusted by
trial and error to get the best from your irrigation system. See
also Steering tomatoes by
irrigation manipulation Jeff
Broad
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