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Notice that at C s = 0, ie when distilled water was used to moisten the soil, the
salinity index possesses a certain baseline magnitude, X si , which is assumed to
result from initial salinity, C si , of water, coming from residual soluble salts
present in the soil. Even if distilled water, having practically zero conductivity, is
used as the input solution, X s remains greater than zero. This seems to be because
the matrix of any soil possesses some soluble components, such as residual salts,
carbonates, silica, etc., which can contribute to the initial conductivity of the soil
water extract (ie distilled water as the input solution). Therefore electrical
conductivity of soil water defined as the soil salinity, EC w , is always greater than
zero.
3.3.3. Summary
Direct calculation of bulk electrical conductivity of the soil from attenuation
of the electromagnetic pulse leads to significant error.
The TDR attenuation-based method for the determination of the soil bulk
electrical conductivity can be applied after fitting an appropriate correction to the
EC a-4p (EC a-TDR ) relationship according to the polynomial from Fig. 5.
Salinity index, defined as the derivative of bulk electrical conductivity to bulk
electrical permittivity, which can be determined simultaneously by TDR
measurements, can be applied as a moisture-independent electrical variable to
express relative changes in the soil salinity without entailing any calibration.
3.4. Choice of frequency of the electric field in dielectric measurement of
soil water content in saline soils
Water concentration in soil (soil water content) and soil temperature are the
quantities of strong temporal and spatial variability, while the concentration of
solid phase (soil density) as well as the density of its elements (particle density) is
practically stable.
Changes of electrical capacity of a capacitor with soil as the dielectric are
attributed to the soil water content changes because the dielectric constant of
water (equal to 81 at 18 ºC) dominates the dielectric constant of soil solid phase
(equal to 4 ÷ 5) and air (equal to 1). Considering above, the electric measurement
of soil water content is done on the base of the correlated dielectric constant
(Gardner et al. [32], Malicki [51], Malicki and Skierucha [59], White et al. [101]).
Such a solution to the problem is not ideal. The presence of ions in the “soil
water” causes the conduction electric current. Therefore soil has dual nature.
Simultaneously it possesses the properties of an insulator characterized by the
dielectric constant of water, soil solid phase elements and air, as well as a
conductor characterized by the electrical conductivity of an electrolyte (it is
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