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Electrical Conductivity
Electrical Conductivity
The FieldScout TDR uses EC readings obtained from the same probes
used to measure VWC. To improve the VWC measurement accuracy, EC
is factored out of the VWC reading. This is a key advantage over its pre-
decessor. The value measured is an average for the entire depth sampled.
EC is expressed in units of mS/cm. The EC measured by an electrode is
defined as the bulk EC.
The salinity level of soil is an important component of irrigation and nu-
trient management. The source of soil salts range from the original parent
material, additions from natural sources, and management activity. High
salt concentration in the soil has a negative effect as plant roots cannot
bring in sufficient soil moisture. However, fertilizer exists as salt ions in
that same soil solution. Low salt level can result in plants not getting the
nutrients needed.
Direct measurement of salt content can only be done by subjecting a field
sample to laboratory analysis. Fortunately, the electrical conductivity
(EC) is a function of the dissolved salts in the soil. This proxy measure-
ment is possible because, as salts dissolve into the soil, they disassociate
into ions which conduct electricity.
Salinity Index
The TDR also has the option to report EC in the form of the Salinity In-
dex. The salinity index is defined as the ratio of the bulk EC to the volu-
metric water content (expressed as a decimal). For example, if the bulk
EC is 0.25 mS/cm and the VWC is 22%, the Salinity Index would be re-
ported as 1.14 (0.25 ÷ 0.22 = 1.14). Therefore, the Salinity Index com-
bines VWC and EC (corrected for temperature) into a parameter that will
be less dependent on the sub
-
saturated water content.
The TDR measures the bulk EC of soil that may or may not be saturated.
As the soil dries, the remaining pore space solution becomes more con-
centrated which increases EC. However, reduced water in the pores leads
to a longer and more tortuous path between the sensor electrodes, which
decreases EC. The second mechanism dominates. Bulk EC will decrease
as soil moisture decreases. EC measurements made at different times are
comparable when the moisture content is the same. This is best observed
if the readings are always taken when the site is at field capacity
-
when a
saturated soil is allowed to drain to the point where the pull of gravity
can no longer remove any additional water.
