Agriculture Reference
In-Depth Information
Table 4.2
Instruments for Measuring Soil Suction
Type of
instrument
Basic construction
Suctionrange
Installation and comments
Standard
tensiometer
Cylindrical porous
ceramic cup sealed
to a water-filled
PVC tube
connected to a
vacuum gauge
(aboveground).
0-85kPa(or
centibars,cb)
Can be installed at different
depths; reading gives the
combined gravitational and
matric components of soil
suction, so must be corrected
for depth to give the matric
suction.
a
Purge air bubbles
before reading.
“Loggable”
tensiometer
Can be a standard
tensiometer or a
tensiometer with
a semi-conductor
sensor installed in
the cup; suction
is measured
by a pressure
transducer
connected to a
data logger.
+100 to −160
kPa
As for a standard tensiometer;
cups can be made out of
different materials to vary
the operational range; can
readpositivepressures(below
awatertable);needtobe
purged and refilled
Gypsum block
(“Gblock”)
Cylinder or
rectangular
block of porous
material with
embedded gypsum
containing
two electrodes
to which an
alternating current
is applied; can be
logged.
10-200 kPa
for“Lite”
blocks;
60-600 kPa
for “Heavy”
blocks
Electrical resistance of the
block changes with its water
content, in-built calibration
of suction-to-water content
relationship; not affected
bysalinityupto5dS/m.
Correct for depth to obtain
matricsuction.“Lite”blocks
for sandy soils and “Heavy”
blocks for clays.
a
Forgravitationalsuctionatzeroatthesoilsurface,matricsuctionatdepthisequaltothetensiometer
orgypsumblockreadingless1kPaforevery10cmofdepth(seeigure4.10);positivevaluesindicatea
positive pressure.
water,
RAW
),whereasthatheldbetween60and400kPaislesseasilyextracted
(calleddeicitavailablewater,or
DAW
).
A suction of 400 kPa is called the stress limit, indicating that although vines
can extract water held at greater suctions, the rate of supply is not fast enough for
them to function optimally. The stress limit changes with soil texture: it is100
kPa in sandy soils, 200 kPa in loams/clay loams, and 400 kPa in clay soils. These
stress limits apply to mature vines. Vines less than three years old should not be
subjectedtosuctionsgreaterthan60kPainthemid-rootzone.
