Introduction to Soil Chemistry

106 electrical measurements
along with elemental iron. 3 Zinc, copper, and especially manganese can apparently
exist in a mixture of oxidation states simultaneously in soil. Add to this
a multitude of organic species that are also capable of oxidation–reduction
reactions, and the result is truly a complex voltammetric system [12,13].
5.5. ELECTRICAL CONDUCTIVITY
The electrical conductivity (EC) of soil or water depends on the amount of
salts present. As shown in Figure 5.1, salts are always present in soil. In humid
regions they are at low concentration and do not affect plant growth, while in
semiarid and arid regions and near salt lakes or oceans they may be at high
concentration and have detrimental effects on plant growth. In all these cases
the electrical conductivity is simply measured by determining the amount of
electricity passing through a cell of known dimensions and configuration when
it contains a salt solution. In the case of soil, the electrical conductivity of a
soil paste or solution extracted from soil is related to the soil’s salt content.
5.5.1. Whole-Soil Paste
Direct determination of the conductivity of a soil is carried out by making a
paste and placing it in a special, standard, cell containing two electrodes. The
cell is made of a circular nonconductor with flat-strip electrodes on opposite
sides of the cell. A soil paste in water is prepared and added to the cell; the
top is leveled and the electrodes attached to a meter that measures its resistance,
which is then related back to the soil’s salt content.
5.5.2. Water and Soil Extracts
Electrical conductivity is usually determined on solutions of salt in water. A
soil sample is mixed with water until a paste, which is allowed to stand
overnight, is obtained. After standing the paste is filtered, and the EC of the
solution obtained is determined.A conductivity cell for water, shown in Figure
5.5 (D), a sample is simply placed in the cell, or the cell is inserted into the
water and a measurement made. Standardization is carried out by preparing
standard solutions of salt, usually sodium chloride (NaCl) in distilled water
and the electrical conductivity determined.
The basic unit used to represent in electrical conductivity is siemens (S).
For direct current it is the reciprocal of the resistance, and for alternating
current it is the reciprocal of the impedance (both in ohms). For soils dS/m
are the units used where 0.1S/m = 1dS/m. 4
3
The occurrence of elemental iron in soil is not common but is possible where there is contamination,
for example, from an accident.
4
From Encyclopaedia Britanica Deluxe Edition 2004 CD-ROM.