Introduction to Soil Chemistry

94 electrical measurements
Figure 5.1. Zinc and copper strips inserted into wet soil and producing a voltage as shown on the
voltmeter.
are those used for the determination of pH, salt content, and soil water
content. Of these three, pH is the most common measurement, and frequently
the first measurement made prior to all other determinations. Although pH
can be determined by many methods, for soil the most common is to use a pH
meter and electrode. Conductivity or resistance is used to measure soil salt
content, while several different electrical characteristics of soil are used in
determining its water content. In addition to their inherent importance, the
pH, salt, and water content are important in determining how other analysis
must be carried out or in determining the effects or interferences that these
parameters have on other analytical methods, particularly spectroscopy and
chromatography.
5.1. THE BASIC ELECTROCHEMICAL CELL
The terms cathode and anode often cause confusion and to a certain
extent the characteristics of an electrode depend on whether they are
being viewed from the outside in or the inside out. One way to keep
the terms straight is that anode begins with a vowel, as does electron;
that cathode begins with a consonant, as does cation; or that both
cathode and cation begin with a c.