Irrigation—Principles & Practices
Part 1 – 238 | Unit 1.5
facilitate locating the instruments for monitoring.
Once installed it usually takes several readings over
a period of several days to start getting accurate
readings.
Tensiometers have a water reservoir above the
sealed column of water that resupplies the plastic
column, since the plant roots constantly extract very
small quantities of water from the sealed tube. To
refill the sealed tube the irrigator simply unscrews
the cap on the reservoir and this opens the seal
below the reservoir, allowing the excess water in the
reservoir to flow into the lower tube. Once the tube
is filled, a small hand-held suction pump is used to
remove air bubbles from the tube. The lid of the
reservoir is then retightened, sealing the lower tube.
It is important to follow all of the manufacturer’s
recommendations for installation and maintenance,
including the use of an additive to minimize algal
contamination of the water in the tensiometer.
When used properly, tensiometers will provide
accurate “soil/water tension” readings on a range of
crops. These readings provide the irrigation manager
with critical information that can be used to estab-
lish irrigation schedules adequate to maintain soil
moisture at levels conducive to good crop growth
and productivity.
Electrical Resistance Sensing Devices
In many ways electrical resistance sensing devices
(ERSDs) are similar to tensiometers—the main dif-
ference is the method used to measure soil moisture.
ERSDs utilize two “electrodes” cast into a porous
material (often gypsum based). The two electrodes in
the “block” are attached to wires that run from the
ERSD to the surface. These wires are often protected
within a ½-inch PVC tube that is attached to the
ERSD. The ESRDs are buried in the soil at various
depths and locations, similar to tensiometers, and
like the tensiometer, a soil/water slurry is used when
the ERSD is installed to establish good soil contact
with the instrument.
To get a reading from the ERSD the irrigator uses
a small, inexpensive, hand-held electrical resistance
meter that is temporarily connected to the wire
leads from the buried ERSD. The meter allows a
very low electrical current to flow between the two
electrodes in the ERSD and displays an electrical
resistance reading. This reading reflects the amount
of moisture within the porous material, since the
buried ERSD takes on the moisture properties of the
surrounding soil. Due to the electrical conductiv-
ity potential of water, the higher the concentration
of moisture within the porous block the lower the
resistance and, conversely, the lower the concentra-
tion of moisture within the block the higher the
resistance.
At field capacity the block is wet; as the grow-
ing plants start to extract moisture from the soil,
the moisture is also pulled from the ERSD and the
conductivity reading will reflect this change in soil
moisture. Note that high salt concentrations in the
soil solution will affect the accuracy of the read-
ing, since salts increase electrical conductivity. This
potential salt impact needs to be taken into account
when deciding which monitoring tool is best suited
to your farm.
Electrical resistance sensing devices are rela-
tively inexpensive and easy to install and monitor.
Like tensiometers, they are left in the field for the
duration of the cropping cycle and provide criti-
cal irrigation scheduling information that enables
the irrigation manager to make informed decisions
about irrigation frequency and quantity based on
site-specific data.Supplement 3: Soil Moisture Sensing Instruments