Agriculture Reference
In-Depth Information
0.6
24
23
30
a
0.5
0.4
15
6
33
16
0.3
53
19
38
37
5
4
0.2
26
17
b
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Volumetric water content,
q
L
Figure 2.3
Relation between diffusion impedance factor,
f
L
, and moisture content,
θ
L
,
in a range of soils. Numbers shown are % clay contents.
a
Mean of six soils;
b
volcanic
ash soil. (After Tinker and Nye, 2000; Olesen
et al
., 2001). Reproduced by permission of
Oxford University Press
relation between
f
L
and
θ
L
is curvilinear. Figure 2.3 shows relations between
f
L
and
θ
L
in a range of soils given by Nye (1979); Olesen
et al
. (2001) give further
values. The figure shows that, at a given moisture content,
f
L
is smaller in clay
soils than in sandy soils, probably because a greater proportion of the soil water
is in fine pores. But at a given water potential,
f
L
is larger in clayey than sandy
soils because they hold more water (So and Nye, 1989).
Effects of Flooding and Redox Conditions on f
L
. As well as increasing the cross-
sectional area for diffusion through the soil pores, flooding affects
f
L
because
the anoxic conditions that develop result in dissolution and re-precipitation of
the soil solid and changes in its electrical properties. Kirk
et al
. (2003) investi-
gated these effects in four soils with contrasting properties. Figure 2.4 shows the
relation between
f
L
and bulk density in the soils under water-saturated condi-
tions. The relation is linear with similar slopes but different intercepts in the four
soils. As bulk density increases, porosity decreases, and the pathway for diffu-
sion becomes more tortuous. The dotted line in Figure 2.4 shows the theoretical
relation between
f
L
and
ρ
for a mixture of spherical particles of various sizes:
f
L
=
ρ/ρ
P
)
0
.
5
where
ρ
P
is the particle density, taken as 2
.
65 g cm
−
3
(Nye, 1979, Section V.B). The values of
f
L
for the more coarse-textured soil,
Iloilo, come closest to this line, but the values are progressively far from it for
the more clayey soils, and they are not parallel to it in any of the soils. There are
several reasons for this. In soils electrostatic and viscosity interactions between
diffusing solutes and solid surfaces are important and tend to diminish
f
L
at a
θ
L
0
.
5
=
(
1
−
