18
Salinity Index
The TDR 150 measures the bulk EC of soil that may or
may not be saturated. There are two competing mecha-
nisms at place. As the soil dries, the remaining solution in
the pore space becomes more concentrated which increas-
es its EC. However, reduced water in the pores leads to a
longer and more tortuous path between the sensor elec-
trodes, which decreases EC. The second mechanism dom-
inates. This means that bulk EC will decrease as soil
moisture decreases. EC measurements at different sam-
pling times are comparable when the moisture content for
each measurement is the same. This is easily accom-
plished if readings are always taken when the site is at
field capacity. Field capacity is defined as the condition
that exists when a saturated soil is allowed to drain to the
point where the pull of gravity can no longer remove any
additional water.
The TDR 150 also gives the option to report EC in the
form of the Salinity Index. The salinity index is defined as
the ratio of the bulk EC to the volumetric 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 reported as 1.14 (0.25 ÷ 0.22 = 1.14). Therefore,
the Salinity Index combines VWC and EC (corrected for
temperature) into a parameter that will be less dependent
on the sub
-
saturated water content.
