Agriculture Reference
In-Depth Information
O
2
partial pressure (kPa)
4
8
12
16
20
0
Winter
(wet)
Summer
Summer
(dry)
Winter
Sandy
loam
soil
Silty clay
soil
100
Depth profiles of O
2
partial pressure in contrasting soils during a dry summer and wet
winter. Solid symbols, winter; open symbols, summer (redrawn from White 1997).
Figure 3.6
Box 3.7
Factors Controlling the Rate of Gas Diffusion in Soil
Diffusion is a molecular process whereby individual ions or molecules move in
response to gradients in their concentrations between one point and another. Gases
in the air diffuse relatively quickly; for example, the diffusion coefficients of O
2
and CO
2
in bulk air are about 0.2 cm
2
/sec at normal temperatures. But the
diffusion coefficients of these gases dissolved in water are 10,000 times smaller.
This difference has a marked effect on the rate of gas diffusion through gas-filled
pore space, compared to water-filled pore space in soil. The rate of gas diffusion or
diffusive flux J
g
is the volume of gas diffusing across a unit area perpendicular to
the direction of movement in a unit of time. (Because 1 mole of a gas at 0°C and
1 atmosphere's pressure occupies 22.4 L, gas quantities and concentrations are
usually expressed in volume units). The equation for
J
g
is
dC
J
g
D
(B3.7.1)
dx
The term
dC/dx
is a differential calculus expression for the change in gas
concentration
C
per unit distance
x
in the direction of diffusion.
D
is the gas
diffusion coefficient, and when
C
is expressed in units of mL/mL and
x
is in cm,
D
is in cm
2
/sec. However, even in the air-filled pores,
D
values for O
2
and CO
2
in
the soil are always less than their values (
D
o
) in bulk air, because of the restricted
volume of the pore space and the tortuosity of the diffusion pathway. The effective
diffusion coefficient in soil,
D
e
, is given by the equation
D
e
f
D
0
(B3.7.2)
(continued)
