Agriculture Reference
In-Depth Information
The management of open irrigation
systems (free drainage) depends on the
performance of the irrigation system, espe-
cially the controller. If the controller is a
simple programmer, which does not process
received information, the management will
be limited to pre-fixed doses and frequency,
or to start and stop the irrigation system as a
function of the signals received from the
different sensors. If the controller is compu-
terized, the management is more dynamic,
as it retrieves real-time information from
different sensors, processes the data with
pre-established models and adopts manage-
ment decisions.
The irrigation dose (volume applied at
each irrigation episode) will be determined
by the substrate's water retention capacity
(see Chapter 10).
The oxygenation of the roots is impor-
tant in soilless crops for root respiration,
especially as the temperature rises, due to
the decrease in available oxygen when tem-
perature increases.
p h y t o m o n i t o r i n g
.
A phytomonitoring device
is a data acquisition system supplied with a
number of climate sensors (solar radiation,
temperature and air humidity), plant sen-
sors (sap flux, stem diameter, fruit diameter
and leaf temperature) and soil sensors
(water content and temperature), together
with
analysis
and
data
interpretation
software.
The high cost of such systems has lim-
ited their use to research labs and high value
crops in high-tech greenhouses.
in soilless crops
Introduction
As seen previously, the irrigation strategy
when growing in soil is usually based on
irrigating when the 'allowable soil water
depletion' is reached in the soil or when the
water tension in the soil reaches a pre-fixed
threshold. In soilless crops, the limited stor-
age capacity of water in the substrates and
the difficulty to rehydrate them, after they
dry out, necessitates a much higher irriga-
tion frequency than required in soil-grown
crops. For rockwool, a number of irrigations
between one and four per MJ m
−2
of global
radiation (Jolliet, 1999) has been recom-
mended, although other authors suggest a
threshold of 2 MJ m
−2
per irrigation episode
(Urban, 1997b), which is approximately
equivalent to applying 0.5 l m
−2
in each irri-
gation, under Mediterranean conditions.
A high irrigation frequency and surplus
water supplies limit salinity problems, as in
soilless cultivation the plants suffer saline
stress first and then water stress. This is
because there is an increase in salinity in
the vicinity of the roots if the nutrient solu-
tion film surrounding the roots is not
refreshed frequently (Pardossi, 2003). This
situation can be avoided if the water circu-
lation is permanent, such as in the NFT
system.
Covering the substrate with a plastic
film limits water consumption exclusively
to transpiration needs, plus any drainage
losses.
Methods for irrigation scheduling
in substrates
In practice, the most usual methods for irri-
gation scheduling in substrates are outlined
below (Medrano
et al
., 2003).
Measurement of the potential
(or ten-
sion) of the water in the substrate. The avail-
ability of water decreases enormously as the
tension rises from 0 to 3 cb (kPa). Therefore,
the advisable tension threshold is 1.5 cb
(Raviv
et al
., 1993). The use of tensiometers
with pressure transducers has proved effec-
tive into these measurement ranges (Terés,
2000), although they are more expensive
than conventional tensiometers, which are
not effective in substrates due to the meas-
uring range.
The use of
solution level sensors
placed
in a tray which supports several substrate
units has greatly expanded in commercial
greenhouses. Also known as a
demand irri-
gation tray
, it works by activating the irriga-
tion when the solution level decreases
below a pre-fixed threshold.
The
weighing lysimeter
, which regis-
ters the weight losses due to transpiration,
