Resistivity Methods - Agricultural Geophysics - page 88

of soil solution (decreased θ), resulting in the soil solution dissolved ions becoming more concen-
trated within the soil solution that remains. This increased ion concentration decreases ρ W because
there are now a greater number of electric current charge carriers per unit amount of soil solution.
Interestingly, the likely overall result for this soil drying scenario (decreased θ and ρ W ) is a rise in
ρ, the reason for which will be discussed in more depth later in the section. Although there is an
understanding that ρ W and θ are often interrelated, in order to focus the discussion within the next
three paragraphs strictly on ρ W , especially regarding potential effects on overall soil resistivity, ρ, it
is assumed, for the sake of argument, θ remains constant.
The ρ W in Equation (5.3) is itself a function not only of the dissolved ion concentrations as previ-
ously implied, but also the dissolved ion mobilities. Dissolved ion mobility is, in turn, governed by
the ion type and temperature conditions. Equation (5.3) indicates that ρ W and ρ are directly related
when there is no change in θ. Consider a case where there is an increase in the total dissolved ion
concentration (decreased ρ W ), while θ remains constant. This case obviously results in a greater
number of charge carriers per unit volume of soil, thereby enhancing the capacity of the sandy or
silty soil to transmit electrolytic current, in turn leading to a decrease in ρ. One possible agricultural
situation involving a scenario in which ρ W and ρ are reduced, while the beginning versus ending θ
conditions are the same, is a fertigation event where a soil initially at field capacity and having a
dilute soil solution is intensely flushed with a more concentrated solution containing nutrients (NO 3 − ,
PO 4 3− , and K + ), followed by the soil then being allowed to drain back to field capacity. (Field capacity
for a particular soil corresponds to the remaining θ value that occurs when all the possible gravity
drainable water has been leached from the initially saturated to near-saturated soil.)
The capacity of a sandy or silty soil to deliver electrolytic current depends not only on the total
amount of ions present but also on the mobility of the various ions within the soil solution. Accord-
ingly, the distribution of the types of dissolved ions within the soil solution potentially has a strong
impact on ρ W , and likewise ρ, given constant θ. The reason for this impact is that the different dis-
solved ions typically found in the soil solution have different mobilities. As an example, at 25°C,
given a constant electric potential difference driving electrolytic current in an aqueous solution, the
mobility of SO 4 2− is approximately twice the mobility of HCO 3 − (Keller and Frischknecht, 1966).
Therefore, assuming all other aspects are equal, a soil solution with SO 4 2− as the dominant anion
will have a lower ρ W value than soil solution with HCO 3 − as the dominant anion.
Temperature conditions additionally influence ion mobility and, therefore, ρ W . Ion mobility in an
aqueous solution is inversely dependent on the viscosity of the solution, which is inversely dependent
on solution temperature. For instance, as temperature decreases, soil solution viscosity increases,
ion mobility is reduced, and ρ W rises (soil solution electrical conductivity is lowered). An equation
commonly used to adjust ρ W due to changes in temperature is

ρ

ρ = + − ( )

WC
zT

−° 25

(5.4)

WT −

1 25

4

where ρ W - T is the soil solution resistivity at a temperature of T (°C), ρ W - 2 5 ° C is the soil solution resis-
tivity at a reference temperature of 25°C, and z 4 is a temperature coefficient with a value of 0.022/°C
(McNeill, 1980). Equation (5.4) reveals that a decrease in temperature from 35°C to 15°C would
increase ρ W by 56 percent. Based on the direct relationship between ρ W and ρ in Equation (5.3), this
temperature change would also cause the overall sandy or silty soil resistivity to increase by 56 per-
cent (given negligible evapotranspiration losses and θ remains constant). The important implication
of Equation (5.4) is that seasonal and even daily temperature fluctuations can have a significant
effect on near-surface soil resistivity values. (Daily temperature fluctuations in near-surface soil of
10°C are not uncommon.) As a special case, extremely cold conditions, in which the soil solution
freezes, will cause the overall sandy or silty soil resistivity to become extremely high (conductivity
falls to zero), due to the difficulty in transmitting electric current through ice.