Agriculture Reference
In-Depth Information
Stumm and Morgan (1996) discuss the problems for simple aquatic systems
and van Breemen (1969), Ponnamperuma (1972), McBride (1994) and Patrick
et al
. (1996) discuss the additional problems for soil systems. I here give the
main points.
Measurements of
E
H
are usually made with a platinum electrode placed in the
soil solution together with a reference half cell electrode of known potential. The
platinum electrode transfers electrons to and from the soil solution without react-
ing with it. Reducing half reactions in the soil tend to transfer electrons to the
platinum electrode and oxidizing half reactions to remove them. At equilibrium
no electrons flow and the electric potential difference between the half cell com-
prising the platinum electrode and the soil solution and the half cell comprising
the reference electrode is recorded.
The first problem to mention is that thermodynamically meaningful measure-
ments of
E
H
must be made on soil solution extracts and not directly on the soil
itself. Although
E
H
values measured in soil following reduction may show the
expected qualitative trends and expected differences between soils, they are not
satisfactory for quantitative interpretation. Hence duplicate measurements of
E
H
in soil can vary by as much as 100 mV and values are generally far too low
in terms of the Fe
2
+
and Mn
2
+
concentrations measured (IRRI, 1964). This is
firstly because the measurement indicates the potential in the immediate vicinity
of the electrode and not that of the whole soil, and there may be large microscale
variations in
E
H
especially near the surfaces of bacterial cells. Secondly they
are subject to liquid junction potential errors. It is therefore necessary to make
measurements in solution withdrawn from the soil ensuring the minimum of gas
exchange during sampling. It is particularly important that no O
2
is allowed to
enter the solution and that no CO
2
is lost. This may be achieved by withdrawing
the solution through porous tubing into previously evacuated tubes.
Apart from these sampling errors there are a number of intrinsic errors in the
measurement of
E
H
in soil solutions. The measurement depends on there being
no net flow of current through the circuit made by the platinum electrode and
reference electrode. However the current in one direction, called the exchange
current,
i
0
, is not zero. Its value for each half-reaction varies with the electrode
potential and with the concentrations of the oxidant and reductant. Figure 4.8
shows this schematically for the Fe
2
+
-Fe
3
+
couple. As can be seen from the
figure, an infinitesimal shift in the electrode potential from its equilibrium value
will make the half reactions proceed in one direction or the other and a net current
will flow through the circuit. The equilibrium potential of the system can be found
from the potential at which no net current flows. How precisely and reproducibly
this measurement indicates the equilibrium potential depends on how steeply the
net current deviates from zero near the equilibrium potential. The greater the
exchange current,
i
0
, the more steeply the net current varies with the potential.
This in turn depends on the redox couple operating and its concentration.
Modern instruments will give reliable measurements for
i
0
values greater than
about 0
.
1
µ
A. Figure 4.8 shows that for the Fe
2
+
-Fe
3
+
couple,
i
0
≈
100
µ
A
