204
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
protruding outside the chamber through a rubber
stopper. Pressurized gas (usually N 2 ) is gradually
applied to the chamber until the pressure is sufficient to force water back to the cut surface. The
pressure at which this occurs is an estimate of 'I' for
the tissue being measured. Assuming a rehydrating
plant comes to equilibrium with the soil overnight,
predawn measurements of 'I' also provide an indication of the wettest soil 'I' in which the plant's
roots are actively taking up water. Midday measurements of 'I' provide an indication of the water
stress a plant experiences (Jackson et a1. 1994).
The plant pressure chamber provides a simple,
inexpensive method for estimating plant 'I' and soil
'I' within the active rooting zone of the plant. Measurements should be obtained within minutes after
cutting the plant and the tissue should be placed in
a sealed plastic bag to minimize evaporation until
measurement. The point of excision should not be
re-cut at any time during the procedure and it
should protrude minimally outside the chamber. In
some cases, it may be important to standardize the
age/position of selected leaves where large phenological or physiological gradients occur. Some disadvantages of the pressure chamber are that it is
labor intensive and difficult to automate. Because
compressed gas is used, safety glasses should be
worn and the plant sample should be viewed from
the side rather than from above. For a more comprehensive treatment of the theory and assumptions
behind the pressure chamber see Koide et a1. (1989)
and Boyer (1995).
Filter Paper Method
The filter paper method is a simple, low-cost alternative for determining soil '1'. The technique was
developed by hydrologists and soil scientists, and
its use has been largely restricted to these fields
(Gardner 1937; Fawcett and Collis-George 1967;
Hamblin 1981; Greacen et a1. 1989). The method
uses a known relationship between 'I'm and the
gravimetric water content of a homogeneous porous material such as filter paper to estimate soil 'I'
(Gardner 1937). Filter paper disks are placed in
contact with field soil, extracted core samples, or
loose soil samples and left undisturbed until equilibrium is established between the paper and soil.
The filter paper disk is either sandwiched between
two intact core sections bound together or placed
within loose soil samples lightly tamped down to
establish soil-paper contact. While field equilibration must occur under prevailing conditions, laboratory samples are placed in a sealed container in
an isothermal environment and left for periods between 1 hour (saturated soils) to 6 days (dry soils)
to establish equilibrium between soil and paper.
After equilibration, the filter paper is removed,
weighed immediately, and the gravimetric water
content calculated using the oven-dry weight. Soil
water potential is determined either from published
calibrations for the selected filter paper or from local calibrations. While local calibration is required
at high soil water potentials (soil 'I' > - 0.1 MPa),
reliable measurements are possible using published
data for the drier conditions often observed in the
field (e.g., Whatman's No. 42 paper, Greacen et a1.
1986; Greacen et a1. 1989). The major sources of
error are poor contact between paper and soil, temperature gradients/fiuctuations during equilibration,
and insufficient time for equilibration (e.g., Campbell and Gee 1986). When these errors are avoided,
the approach may offer a low-cost alternative for
estimating soil 'P over a wide range of conditions.
Time Domain Reflectometry
Time domain refiectometry (TDR) measures soil 9v
and is based on the relationship between the apparent dielectric constant (Ka) of a soil and the
soil's free water content (Davis and Chudobiak
1975). The unitless dielectric constant, also known
as relative permittivity, is the ratio of electric field
strength in a vacuum to that in a polarized substance (the dielectric)-the more polarized the dielectric, the greater the reduction in field strength
(Parker 1983). As the dielectric constant for water
(80) is much greater than that for air (1) or dry soil
(3 to 7), the dielectric value for field soil is determined primarily by soil moisture (Cassel et a1.
1994). Dalton and van Genuchten (1986) discuss
the physical and mathematical principles of dielectric constants in detail.
Time domain refiectometry estimates Ka by measuring the time needed for an electromagnetic pulse
emitted by a TDR instrument to travel along a
probe in soil and refiect back to the soil surface
from the probe end (Fig. 13.3). Ka is related to the
propagation velocity (V) of an electromagnetic
wave:
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
protruding outside the chamber through a rubber
stopper. Pressurized gas (usually N 2 ) is gradually
applied to the chamber until the pressure is sufficient to force water back to the cut surface. The
pressure at which this occurs is an estimate of 'I' for
the tissue being measured. Assuming a rehydrating
plant comes to equilibrium with the soil overnight,
predawn measurements of 'I' also provide an indication of the wettest soil 'I' in which the plant's
roots are actively taking up water. Midday measurements of 'I' provide an indication of the water
stress a plant experiences (Jackson et a1. 1994).
The plant pressure chamber provides a simple,
inexpensive method for estimating plant 'I' and soil
'I' within the active rooting zone of the plant. Measurements should be obtained within minutes after
cutting the plant and the tissue should be placed in
a sealed plastic bag to minimize evaporation until
measurement. The point of excision should not be
re-cut at any time during the procedure and it
should protrude minimally outside the chamber. In
some cases, it may be important to standardize the
age/position of selected leaves where large phenological or physiological gradients occur. Some disadvantages of the pressure chamber are that it is
labor intensive and difficult to automate. Because
compressed gas is used, safety glasses should be
worn and the plant sample should be viewed from
the side rather than from above. For a more comprehensive treatment of the theory and assumptions
behind the pressure chamber see Koide et a1. (1989)
and Boyer (1995).
Filter Paper Method
The filter paper method is a simple, low-cost alternative for determining soil '1'. The technique was
developed by hydrologists and soil scientists, and
its use has been largely restricted to these fields
(Gardner 1937; Fawcett and Collis-George 1967;
Hamblin 1981; Greacen et a1. 1989). The method
uses a known relationship between 'I'm and the
gravimetric water content of a homogeneous porous material such as filter paper to estimate soil 'I'
(Gardner 1937). Filter paper disks are placed in
contact with field soil, extracted core samples, or
loose soil samples and left undisturbed until equilibrium is established between the paper and soil.
The filter paper disk is either sandwiched between
two intact core sections bound together or placed
within loose soil samples lightly tamped down to
establish soil-paper contact. While field equilibration must occur under prevailing conditions, laboratory samples are placed in a sealed container in
an isothermal environment and left for periods between 1 hour (saturated soils) to 6 days (dry soils)
to establish equilibrium between soil and paper.
After equilibration, the filter paper is removed,
weighed immediately, and the gravimetric water
content calculated using the oven-dry weight. Soil
water potential is determined either from published
calibrations for the selected filter paper or from local calibrations. While local calibration is required
at high soil water potentials (soil 'I' > - 0.1 MPa),
reliable measurements are possible using published
data for the drier conditions often observed in the
field (e.g., Whatman's No. 42 paper, Greacen et a1.
1986; Greacen et a1. 1989). The major sources of
error are poor contact between paper and soil, temperature gradients/fiuctuations during equilibration,
and insufficient time for equilibration (e.g., Campbell and Gee 1986). When these errors are avoided,
the approach may offer a low-cost alternative for
estimating soil 'P over a wide range of conditions.
Time Domain Reflectometry
Time domain refiectometry (TDR) measures soil 9v
and is based on the relationship between the apparent dielectric constant (Ka) of a soil and the
soil's free water content (Davis and Chudobiak
1975). The unitless dielectric constant, also known
as relative permittivity, is the ratio of electric field
strength in a vacuum to that in a polarized substance (the dielectric)-the more polarized the dielectric, the greater the reduction in field strength
(Parker 1983). As the dielectric constant for water
(80) is much greater than that for air (1) or dry soil
(3 to 7), the dielectric value for field soil is determined primarily by soil moisture (Cassel et a1.
1994). Dalton and van Genuchten (1986) discuss
the physical and mathematical principles of dielectric constants in detail.
Time domain refiectometry estimates Ka by measuring the time needed for an electromagnetic pulse
emitted by a TDR instrument to travel along a
probe in soil and refiect back to the soil surface
from the probe end (Fig. 13.3). Ka is related to the
propagation velocity (V) of an electromagnetic
wave: