Agriculture Reference
In-Depth Information
CHAPTER
5
ELECTRICAL MEASUREMENTS
In making measurements using electricity, all its parameters—voltage, amper-
age, resistance, capacitance, frequency, and dielectric characteristics—can be
used singly or in combination to obtain information about various conditions
in the medium through which electrons are moving. All soils contain many
ions of many different sizes and complexities: simpler hydrated K
+
, more
complex NO
-
, organic ions, and charged solids. All contribute to the electrical
characteristics of a soil and its solution.
One easily demonstrated electrical characteristic of moist soil is seen in the
production of electricity when two different metals, namely, copper and zinc,
are inserted into it (assuming that the reaction is spontaneous). This is not
unexpected because any salt containing solution, adsorbed in media such as
paper or cloth, and placed between these same two electrodes, will cause a
spontaneous reaction, which will produce electricity. The source of this flow of
electrons is a chemical, oxidation-reduction reaction, represented as the
following two half-reactions, for copper and zinc:
Oxidation
:
Zn
0
Æ+
Zn
2
+
2
e
-
(5.1)
2
+-
0
Reduction
:
Cu
2
e
Æ
Cu
Production of electricity in this way was the original method used to produce
electricity (i.e., an electric current) for scientific experiments. Thus, if copper
and zinc are inserted into wet soil, electricity will be produced by the same
process, although a mixture of salts will be involved. Figure 5.1 shows copper
and zinc strips inserted into a moist soil and connected to a voltmeter, which
displays the resulting voltage, demonstrating the existence of salts in the soil.
This simple setup is not, however, used to measure soil characteristics such as
salt content or pH.
This electricity generation characteristic, along with the salts, their move-
ment through soil, and the diffuse double layer must be kept in mind when
making any soil measurement using electricity or electrodes.
Although all characteristics of electricity have been used to investigate soil
and its properties, only a limited number are used routinely. The most common
