Agriculture Reference
In-Depth Information
Reduced soil
Oxidized soil
SO
=
Fe
3+
Mn
4+
NO
-
O
2
4
3
-300
-200
-100
0
100
200
300
400
500
600
Redox potential, E
h
(millivolts) (corrected to pH 7)
Figure 5.9
Threshold redox potentials at which the oxidized forms shown become unstable (White
1997). Reproduced with permission of Blackwell Science Ltd.
The gas N
2
O is an intermediate product, and at pH
5, may be compara-
ble in concentration to N
2
. But at pH
6 and temperature
25°C, most of the
N
2
O is reduced to N
2
, especially when the soil approaches complete saturation.
Denitrification is carried out by facultative anaerobic bacteria, such as
Bacillus
and
Pseudomonas
species.
As the NO
3
is depleted, reducing conditions intensify, and other compounds
become susceptible to reduction in the following sequence:
MnO
2
(solid)
4H
2e
Mn
2
2H
2
O
(5.13)
Fe(OH)
3
(solid)
3H
e
Fe
2
3H
2
O
(5.14)
SO
4
2
10H
8e
(5.15)
The intensity of reducing conditions, or the reducing power, is measured by the
redox potential E
h
, which is explained in appendix 6. The approximate
E
h
value at
which the oxidized form of each substance (on the left-hand side of the equations)
becomes unstable is shown in figure 5.9. The soil is
poised
in a particular
E
h
range
until most of the oxidized form has been consumed, and then the soil
E
h
drops
to a lower value as the reducing power intensifies. Reactions such as 5.12 and
5.15, in which a gas of low water solubility is produced, are irreversible. The crit-
ical
E
h
for damage to vine roots from anaerobic conditions is
100mV.
H
2
S (gas)
4H
2
O
Redox Reactions and Nutrient Availability
Denitrification is a loss of available N (as NO
3
). Any N
2
O produced during
denitrification is undesirable because it is a potent “greenhouse gas.” Because hy-
drated iron oxides are plentiful, many waterlogged soils are poised by the Fe
3
-
Fe
2
redox system. The change from the red-brown color of Fe
2
O
3
to the blue-
grey color of Fe
2
forms is typical of a gleyed soil. Both Mn and Fe are more
soluble in the reduced form and may be leached. The dissolution of MnO
2
can
also release coprecipitated Co into the soil solution. Further, the solubility of el-
ements such as Co, Ni, Mo, and Cu (which are not reduced at the
E
h
values at-
5.6.2