Agriculture Reference
In-Depth Information
(a)
(b)
Pore radius ( m)
µ
Soil core,
Ψ
m
= -10kPa
0.015
0.15
1.5
15
150
0.5
Micropores
Macropores
0.4
Porous
plate
Clay
0.3
100 cm
0.2
Sand
PWP
0.1
FC
0
-10
4
-10
3
-10
2
-10
-1
Ψ
m
(kPa) (log
10
scale)
(a) Soil water under suction on a porous plate attached to a “hanging” water column. (b)
Typical soil water retention curves for a clay soil and a sandy soil, showing the normal
Figure 6.3
m
range and corresponding pore radii.
Water Content versus Water Potential Relationships
6.2
The Soil Water Retention Curve
If a saturated soil core is placed on a porous plate attached to a hanging water col-
umn,
6.2.1
g
created by the difference in height be-
tween the soil and the open arm of the water column (fig. 6.3a). The soil water
now has a greater
falls because of the decrease in
g
than the open arm of the water column; this is compensated
for by a decrease in
m
in the soil. This kind of apparatus can therefore be used
to show how
changes with a decrease in
m
. The practical limit of
m
is about
10 kPa (a head of
100 cm). For greater reductions in
m
, the soil is usually
brought to equilibrium with an applied air pressure that is then equal but oppo-
site to
m
in the soil pores. This is done using a pressure plate apparatus. A graph
of
m
called a
soil water retention
curve, can then be plotted (fig. 6.3b).
The curves represent a sandy loam soil and a clay soil. These curves show that the
sandy loam soil releases a larger proportion of its water at relatively high
against
m
val-
ues, whereas the clay soil releases water more gradually over the full range of
m
.
This is primarily due to the difference in pore-size distribution between the two
types of soil, as discussed in box 6.4.
The pore-size distribution influences not only water retention in soil, but also
the rate at which water moves through soil (section 6.3).
Available Water Capacity
Although the soil water retention curve indicates a continuous relationship be-
tween
6.2.2
and
m
, for practical purposes we recognize two fixed points in the rela-
