ELECTRICAL MEASUREMENTS - Introduction to Soil Chemistry - page 107

Soils with pH < 8.5 and an electrical conductivity of < 4 dS/m are considered

normal. Soils in the same pH range but with EC > 4 are said to be saline . Soils

high in sodium but with EC < 4 and are called sodic soils , while those with EC

> 4 and high sodium are saline-sodic soils. Sodium is represented as either the

ratio of exchangeable sodium to total exchangeable cations, termed the
exchangeable sodium percentage or the sodium absorption ratio , which is
the ratio of sodium ion to the square root of the calcium and magnesium ions
[2,14-17].

5.6. TIME-DOMAIN REFLECTOMETRY (TDR)

Time-domain reflectometry involves the use of two or more substantial metal
rods inserted into soil. The rods are parallel and are attached to a signal gen-
erator that sends an electrical input down the rods. The time it takes the signal
to travel down the rods is dependent on the soil's apparent dielectric constant,
which in turn is proportional to the amount of water in the soil. On reaching
the end of the rods, the signal is dissipated and the amount of dissipation is
related to the amount of salt in the soil. This instrument can thus measure both
the water content and the salt content of the soil into which it is inserted.
Depending on the type of data needed and the experiment to be done, rods
can be either moved from place to place or left in place for measurements
made over a period of time [18].

5.7. POROUS BLOCK

The porous block is used to determine the water content in soil by changes in
the resistance between two electrodes encased in a porous material buried in
soil. In a common porous block design, two electrodes are encased in gypsum
connected to wires such that the blocks can be buried to various depths in a
soil profile. The gypsum block is buried in soil, and its moisture content comes
to equilibrium with the soil water content. As the water content of the block
increases, the resistance between the two electrodes decreases and vice versa.
The change in resistance can then be related back to the soil water content.
This basic idea is used with “blocks” constructed of materials other than
gypsum. The gypsum block is designed for fieldwork and finds it greatest use
there (see Figure 4.9) [18].

5.8. OTHER METHODS

In addition to the porous block, there are a number of other types of mea-
surement made in soil involving electrodes that are not in direct contact with
the soil. An example is the thermocouple psychrometer, which involves a
Thomson thermocouple in a ceramic cell buried in soil. The thermocouple