Agriculture Reference
In-Depth Information
weeks or months following submergence. The large increases in CO
2
pressure
as dissolved Mn(II) and Fe(II) accumulate would suggest Mn and Fe carbon-
ates should be precipitated. However it is unlikely that simple Mn and Fe
carbonates are formed because Mn
2
+
and Fe
2
+
ions have similar radii (0.083
and 0.078 nm, respectively) and can readily substitute for each other in crystal
lattices. Rhodocrosite
(
MnCO
3
)
and siderite
(
FeCO
3
)
are end members of a con-
tinuous series of solid solutions of Fe(II) - Mn(II) carbonates (Deer
et al
., 1992).
Iron - manganese minerals also readily incorporate Mg
2
+
(radius 0.072 nm) and
to a lesser extent Ca
2
+
(0.1 nm) and other divalent cations. It is therefore likely
that various solid solutions are formed.
There is evidence that mixed Fe(II) - Fe(III) hydroxides are formed. These can
be produced easily
in vitro
by partial oxidation of pure Fe(II) hydroxy salts
and they have some of the observed properties of the solid phase Fe(II) found
in reduced soils, including the grayish-green colours characteristic of reducing
conditions in soils. This material is 'green rust' and has the general formula
Fe(II)
6
Fe(III)
2
(
OH
)
18
with Al
3
+
partly substituted for Fe
3
+
and Cl
−
,SO
4
2
−
and
CO
3
2
−
substituted for OH
−
.
Once precipitation begins, a quasi-steady state will eventually be attained in
which the soil pe and pH are poised by the redox and precipitation equilibria
operating. In the transition to the steady state, protons will be provided by dis-
sociation of acids in the soil solution — e.g. H
2
CO
3
derived from CO
2
-and by
reactions with the soil exchange complex. The course of reduction and the even-
tual steady state will depend on these reactions and it is therefore necessary to
allow for them in predicting what the steady state conditions will be.
In the following section I describe a simple model for calculating the changes
in pe, pH and concentrations of inorganic reductants during soil reduction, allow-
ing for the effects of pH buffering and cation exchange, and the characteristics
of the mineral phases formed. The approach is based on that of van Breemen
(1988) for partial redox equilibrium in soil without pH buffering and cation
exchange.
4.2.3 CALCULATED CHANGES IN pe, pH AND Fe DURING
SOIL REDUCTION
Consider an idealized soil containing ferric hydroxide and readily decomposable
organic matter. The following conditions hold:
•
the soil is initially saturated with the atmospheric partial pressure of O
2
but
otherwise closed to exchange of O
2
;
•
the partial pressure of CO
2
is constant;
the soil exchange complex is initially saturated with divalent cations M
2
+
,i.e.
H
+
is treated as non-exchangeable and there are no other monovalent cations;
•
•
the soil reaches a steady state following reduction in which the soil solution
is in equilibrium with Fe(OH)
3
and Fe
3
(
OH
)
8
.
