Agriculture Reference
In-Depth Information
Slow reduction
Fast oxidation
Amorphous Fe(III) oxides
Crystalline Fe(III) oxides
Fe(II)
Fast reduction
Slow crystallization
Figure 3.8
Accumulation of amorphous Fe(III) compounds under alternating reduction
and oxidation (after Moormann and van Breemen, 1978). Reproduced by permission
of IRRI
ferric oxides. In turn, the amorphous materials are more easily reduced during
soil flooding and over time the iron compounds reach a steady state in which eas-
ily reducible amorphous materials are combined with more recalcitrant minerals.
The proportion of amorphous and crystalline materials will be influenced by the
hydrological regime and climate. Concomitantly there are short- and long-term
changes in soil organic matter.
Changes in Surface Properties Following Submergence
For the most part, the overall composition of the clay fraction in soils is deter-
mined by long-term processes and does not change rapidly with changes in
conditions. However the properties of the clay surface can change rapidly and
the surface is often not in equilibrium with the rest of the solid phase. Thus
alternating reduction and oxidation under variable water regimes cause rapid but
transient changes in surface properties, as well as more persistent changes in
the overall composition of the solid phase. The changes in surface properties
following soil submergence are as follows.
Dissolution of Oxide Coatings
. The net negative charge on the soil solid may
increase following reduction as a result of dissolution of oxide and oxyhydroxide
coatings on clay surfaces. The pzc of oxides and oxyhydroxide are at or above
neutral pH (Table 3.10), and so the coatings on clays are positively charged in
neutral and acid soils and neutralize some of the clay's negative charge. Their
dissolution therefore results in an increase in the net negative charge on the
surface. Hence, for example, an oxide with composition Fe(OH)
2
.
5
0
.
5
+
, is reduced
according to the half reaction
5H
+
+
2e
−
−−−→
[soil—]
−
+
2Fe
2
+
+
5H
2
O
(
3
.
42
)
where e
−
represents an electron transferred in the reduction. If the corresponding
oxidation of soil organic matter is (Chapter 4)
CH
2
O
+
H
2
O
−−−→
CO
2
+
4H
+
+
4e
−
[soil—2Fe
(
OH
)
2
.
5
]
+
(
3
.
43
)
