Agriculture Reference
In-Depth Information
Box 6.1
Terms Describing the Energy Status of Soil Water
, as defined in appendix 7, is expressed as energy per
mole (Joules (J)/mol). To be compatible with older pressure measurements,
Soil water potential
is
converted to energy per unit volume (J/m
3
) by dividing by
V
w
, where
V
w
is the
volume (m
3
) per mole of water. The units of J/m
3
are equivalent to newtons/m
2
or
pascals (Pa), which are units of
pressure
. In saturated nonsaline soil,
is zero.
can be thought of as a negative pressure. A negative pressure implies a suction
force acting on water to draw it into dry soil. Thus, soil water potentials are often
referred to as
suctions
, when the negative sign is dropped. The term suction is
meaningless when
As water is withdrawn from a wet soil,
becomes negative. In this case,
0 (for example, in saturated soil below a watertable). In
this situation, the soil water pressure is positive. The normal range for
is
in soil is
from 0 to
1500 kPa.
To describe water movement in soil, hydrologists and soil scientists usually
express the water potential in terms of
hydraulic head
, or simply
head h
. Then
is
expressed as energy per unit weight of water (J/kg). Head
h
is defined by the
equation
h
(B6.1.1)
g
the acceleration due to
gravity (9.8 m/s
2
). If the soil water potential is 10
5
Pa (
the density of water (1000 kg/m
3
) and
g
where
1 bar, the normal
pressure of the atmosphere), the equivalent head is given by
100,000
10.2 m
h
(B6.1.2)
9.8
Head units are the same as units of length (m). Hydraulic head expressed in
cm normally ranges from 0 to
1000
15,000 cm. To make these numbers less
unwieldy, the
pF scale
of soil water potential was introduced. The pF value is
defined as log
10
h
(in cm). The pF scale is not used much at present.
Comparative values for the energy status of soil water in potential, head, and
pF units at several states of wetness are given in table B6.1.1.
Table B6.1.1
Soil Water Potential, Hydraulic Head, and Soil pF at Different Wetness States
Water Potential
Head
h
State of Soil Wetness
(kPa)
(m)
pF Value
—
a
Saturated, nonsaline soil
0
0
Water held after free drainage for
10
1.02
2.0
48 hours (field capacity)
Approximate water content at
1,500
0
153
4.2
which plants wilt (wilting point)
Soil at equilibrium with a
22,000
2244
5.4
relative vapor pressure of 0.85
(approaching air-dryness)
a
Undefined
