Agriculture Reference
In-Depth Information
It will be seen that pe is initially buffered at about 15 until the O
2
is exhausted,
and it then falls rapidly to the point where it is buffered by the Fe(OH)
3
-Fe
2
+
couple. Ferrous ions are now released into the solution and protons removed
from it, and the changes in pe and pH now depend on the buffering of Fe
2
+
and
H
+
by the soil solid. As a result of buffering, the increases in Fe
2
+
and pH as
Fe(OH)
3
is reduced are more gradual.
Comparing Figure 4.7(b) and (c), for which the CEC and [X
−
]
L
are the same
but
b
HS
different, the effect of increasing
b
HS
is to further slow the increase in
pH, and the pH at the steady state when Fe
3
(
OH
)
8
is formed is smaller and the
pe and concentration of Fe
2
+
in solution correspondingly larger. Also a much
larger quantity of CH
2
O is consumed in reaching the steady state. From the
stoichiometry of Reaction (4.30), the amount of exchangeable Fe
2
+
formed in
mmol
c
kg
−
1
is roughly four times the amount of CH
2
O oxidized.
The effect of varying CEC can be seen by comparing Figure 4.7(c) and (d).
With decreasing CEC at constant
b
HS
,[X
−
]
L
and
P
CO
2
, the concentration of
Fe
2
+
at steady state is increased and the pH correspondingly decreased and
pe increased.
In summary, the calculations predict that:
(1) O
2
will disappear rapidly after flooding;
(2) dissolved Fe
2
+
will appear in the soil solution and its concentration increases
to a constant steady-state level;
(3) an initially low pH will increase to between 6.5 and 7 at the steady state;
(4) pe will decrease from about 15 to near 0;
(5) the rates of change in Fe
2
+
, pH and pe and their steady-state values, and the
amounts of organic matter oxidized in reaching the steady state, depend on
pH buffering and cation exchange by the soil.
These predictions can be compared with the results for real soils shown in
Figure 4.7.
In the real soils the ranges of pe and pH are similar and a steady state is attained
in which the concentrations of Fe
2
+
in solution are similar to those predicted with
the model. However the large peak in Fe
2
+
concentrations in some soils before
the steady state is reached is not predicted. The peak occurs because precipitation
of ferrous carbonate is slow and may be inhibited by interfering solutes in the
soil, resulting in supersaturation with respect to the expected solid phases.
Note that although the pe-pH-[Fe
2
+
] relationships shown in Figure 4.7 are
consistent with control by the Fe(OH)
3
-Fe
3
(
OH
)
8
system, in fact various other
reduced Fe solid phases are possible and as discussed above it is difficult to
establish unequivocally which phase controls Fe
2
+
solubility in reduced soils.
4.2.4
MEASUREMENT OF REDOX POTENTIAL IN SOIL
In principle the redox potential provides a simple means of gauging a soil's
redox status. However in practice it is difficult to make reliable measurements.
