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
 
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