Agriculture Reference
In-Depth Information
Free Energy Change and the Definition of Soil Water Potential
Appendix 7
Suppose a small quantity of water is added to a dry, nonsaline soil at a constant
temperature. This water is distributed so that the force of attraction between it
and the soil is as large as possible, and the free energy of the water is reduced to
a minimum. More water added to the soil is held by progressively weaker forces.
Finally, with the last drop of water necessary to saturate the soil, the free energy
of the soil water approaches that of pure water (no solutes) at the same tempera-
ture and pressure. The reference water must be “free”—that is, not affected by
any forces other than gravity. The energy status of soil water is then defined in
terms of the difference in free energy between 1 mole of water in the soil and 1
mole of pure, free water at a standard temperature and pressure, and a fixed, ref-
erence height.
The free energy per mole of a substance defines its
chemical potential
. So for
water in soil, we define the
soil water potential
by the equation
w
(soil)
w
o
(standard state)
RT
ln
e
/
e
o
(A7.1)
the ratio of va-
por pressure of water in the soil to vapor pressure of pure water at a standard tem-
perature (298 K) and pressure (1 atmosphere);
R
the Universal Gas Constant
(8.314 Joule/Kelvin/mol); and
T
the absolute temperature (K). The reference
height used for measuring soil water potentials is usually the soil surface. Note
that for water at the standard temperature and pressure, and reference height, the
maximum value of
w
chemical potential (free energy/mol) of water;
e/e
o
where
is obtained at
e/e
o
1 for which
0.
Measuring Saturated and Unsaturated Hydraulic
Conductivity of Soil
Appendix 8
The
ring infiltrometer
is a metal cylinder 30-60 cm wide that is driven a short dis-
tance into the soil. Water is ponded in the ring and when steady state infiltration
is achieved, the infiltration rate
IR
is measured as
Volume of water
IR
(A8.1)
Area of ring
Time
Water movement into the soil, especially if the soil is initially dry, will be
both lateral and vertical, under the influence of
g
gradients. The infil-
trometer will therefore overestimate the vertical
IR
(gravity effect only), which
equals
K
s
, unless an outer “buffer ring” filled with water is used, as in the
double-
ring infiltrometer.
In this case, the infiltration rate is measured from the inner ring
only.
The
disc permeameter
or tension infiltrometer operates on a similar principle
to the ring infiltrometer, but it is more versatile because water can be applied at
pressure heads (
p
) from 0 to
120 mm. Water infiltrates from a circular disc
(radius
r
o
) covered by a porous membrane of air entry pressure
2 kPa. A thin
layer of fine sand is spread on the soil surface under the disc to ensure good
m
and
