Agriculture Reference
In-Depth Information
Table 2.2
Effect of cultivation at different soil-water states on components of percentage
porosity in a Vertic Tropaquept clay soil
Sampling state
Total porosity
>
50
µ
m0
.
5-50
µ
m
<
0
.
5
µ
m
Before cultivation
66
26
6
31
After moist
cultivation
61
5
15
41
After saturated
cultivation
62
3
16
43
1
1
1
1
SE
Source
: Reprinted from Painuli
et al
. (1988) with permission from Elsevier Science.
under laboratory conditions. Puddling decreases percolation rates by up to three
orders of magnitude. Among the soils in the table, the sandy Entisol and clayey
Andept were difficult to puddle and consequently had greater percolation rates.
However, the low percolation rates in the other soils are comparable to rates in
most rice soils under field conditions. Puddling generally leads to an increase
in total porosity because the destruction of aggregates decreases intra-aggregate
pores but increases inter-aggregate and inter-domain pores, as shown in Table 2.2.
The percolation rate also depends on the depth of water standing on the soil
surface. Consider the submerged soil shown in Figure 2.1. The soil overlies a
compacted traffic layer caused by repeated working for wetland rice cultivation,
beneath which the soil may be saturated or unsaturated depending on its properties
and the depth of the ground water. The flux of water through the soil is related
to the gradient in water potential by Darcy's law:
v
=−
K
d
ψ
d
z
(
2
.
8
)
where
v
is the flux in direction
z, K
the hydraulic conductivity and
ψ
the water
potential. If
v
and
K
are constant in a given layer in Figure 2.1, as they generally
ψ
0
floodwater
z
1
puddled layer
z
2
z
3
compacted layer
Figure 2.1
Changes in water potential with depth in a puddled flooded soil
