Agriculture Reference
In-Depth Information
40
30
FeEDTA stable
20
FeEDTA
unstable
10
Fe(OH)
3
Figure A5.1
Stability of FeEDTA with pH change in soil
(White 1997). Reproduced with permission of
0
2
4
6
8 0 2 4
pH
Blackwell Science Ltd.
Measuring a Soil's Redox Potential
Appendix 6
The redox potential
E
h
is measured as the electrical potential difference between
a platinum (Pt) electrode and a reference electrode (usually a calomel electrode)
placed in the soil. The expression for
E
h
(in millivolts) is
59
(Ox)
E
h
E
7
0
log
10
(A6.1)
n
(Red)
In this equation,
n
is the number of electrons transferred per mole of sub-
stance reduced, and (Ox) and (Red) are the activities of the oxidized and reduced
forms of the substance, respectively. Since H
ion activity affects the redox equi-
librium (see equations 5.12-5.15), the reference potential
E
7
0
is defined at pH 7.
Clearly, systems in an oxidized state have a high value of
E
h
, whereas reduced sys-
tems have a low value (sometimes even negative; see fig. 5.9).
The range of
E
h
for aerobic soils is 300-800 mV. Under anaerobic condi-
tions, the critical
E
h
below which damage to vine roots occurs is 100mV. This
might occur regularly in poorly drained subsoils in wet winters or transiently in
duplex soils in winter because of a perched watertable developing at the top of
the B horizon. The former condition can be alleviated by permanent subsoil
drainage, the latter by deep ripping the soil at the time of vineyard establishment
(chapter 7). Reproducible and meaningful
E
h
measurements can be made only on
waterlogged soils in situ or on soil samples that have been taken in such a way
that O
2
is excluded up to, and during, the time of measurement. Measurements
on soil samples that have been dried, sieved, and shaken in water are worthless
for identifying the redox status of the soil in its field state.
