Introduction to Soil Chemistry

108 electrical measurements
cools when a current is passed through it, causing water to condense on the
thermocouple. When the electricity is turned off, the condensate evaporates at
a rate inversely proportional to the relative humidity in the soil.A voltage generated
by the cooling junction is measured and related to the soil moisture
content.This moisture content is related to both the matrix and osmotic potentials
of the soil being investigated.
There are many additional methods and variations on the methods discussed
above (see Section 4.13). Most are designed for determination of soil
water content in the field and are rarely used in laboratory analysis of soil
components such as available plant nutrients or contaminants [18–20].
5.9. CONCLUSIONS
Soil has electrical characteristics associated with its components, salts, ions in
solution, and the diffuse double layer. All, singly or in combination, can affect
electrical measurements in soil. Electrodes inserted into soil are used to
measure various soil characteristics, most often soil pH, salt, and water content.
Fouling of electrodes by salts, organic matter, or inorganic components, including
clay, is an important potential source of error in any soil measurement
involving electrodes. Because of the potential errors, electrodes must be standardized
frequently during procedures that involve multiple measurements
over an extended period of time. Analytical procedures for the determination
of soil characteristics using electrodes have been developed and are used in
conjunction with other soil procedures and measurements. Development of a
new method or procedure will require detailed investigation of the relationship
of this new method to previously developed methods and to associated
or dependent procedures or measurements.
PROBLEMS
5.1. Diagram a basic electrochemical cell. Diagram a similar cell using soil
instead of water as the supporting medium.
5.2. What do the terms Eh and Eh0 stand for? What types of electrodes are
used for the determination of Eh in soil?
5.3. Describe the basic design of a pH electrode. What “kinds” of protons in
soil can a pH electrode can measure?
5.4. Diagram an ISE electrode. What characteristics of ISE electrodes make
them difficult to use for direct soil measurements?
5.5. In terms of Eh–pH, what limits the range of Eh–pH values in soil?
5.6. Explain, giving examples, why voltammetry is seldom useful in direct soil
measurement of components present.