Agriculture Reference
In-Depth Information
Box 6.3
Measurement of Soil Water Potential in the Field
Tensiometers and resistance blocks measure the combined effect of
m and
g . Tensiometers operate in the range 0 to
85 kPa, and resistance blocks from
600 kPa. A tensiometer consists of a porous ceramic cup (with an air-
entry value of at least 85 kPa) sealed to a length of polyvinyl chloride (PVC)
tubing. The tube and cup are filled with water so that when the tensiometer is
installed, the water in the tensiometer comes to equilibrium with water in the
surrounding soil. The suction on the soil water (less the osmotic effect of any
solutes that diffuse freely through the cup walls) is transmitted through the water
column, to be read directly with a pressure gauge or transducer sealed to the top
(fig. B6.3.1, left). The units of measurement are kPa or the equivalent centibars.
Thus, the tensiometer potential
10 to
t measures the soil water potential
minus any
osmotic component. This is satisfactory for all but very saline soils.
Resistance blocks consist of two electrodes embedded in a porous material.
Traditionally, this porous material was gypsum. But now synthetic materials more
durable than gypsum are used. An example is the “Watermark” sensor that
functions in the
200 kPa. These are also called “Lite” sensors
for use in “light soils” (fig. B6.3.1, right). The more traditional sensor is a “heavy”
sensor for use in “heavy soils.” Water moves in or out of the block in response to a
change in
range
10 to
in the surrounding soil, until equilibrium is established. The change
in water content causes a change in electrical resistance of the block. The
relationship between electrical resistance and water potential (
) for each block
must be determined for individual blocks—this is called calibration. Ions diffuse
into and out of a block, which can affect its electrical resistance. Lite and heavy
sensors are not affected by salinity up to 1 decisiemen (dS)/m and 6 dS/m,
respectively (Goodwin 1995), so gypsum blocks can be used in quite saline soil.
The measurement of soil salinity is discussed in chapter 7.
Figure B6.3.1 A tensiometer with vacuum
gauge and a set of “Lite”
gypsum blocks. Photograph by
the author.
 
 
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