thus as the wetting front move down into the soil, the regions between the peds will be the
preferential water pathway. As the wetting font moves through the soil column the soil moisture
measurements may be temporarily biased by the peds. For example, if the soil probe’s
measurement volume is residing entirely in a single ped, the probe would not detect the wetting
front until the water infiltrates the ped. Likewise, if the sensing volume is residing between
several peds, the soil moisture measurements will reflect the movement of water between the
peds. During installation, if a horizon has thick clay films around the peds, the user may want to
use daily averages of soil moisture reading to accommodate soil moisture variations in the peds.
6.4.9 Temperature Affects on the Dielectric Permittivity of Soil
The real dielectric permittivity of water will decrease as the temperature increases. This
phenomenon can be explained by molecular vibration. As the temperature of water increases, the
molecular vibration of water molecules will increase. This increased vibration will inhibit the
molecular orientation associated with the real dielectric permittivity. A quantitative expression
for the temperature affects on the dielectric permittivity can described by the Debye-Langevin
equation (Levine 1993).
Based on the correlations found in the literature, the Hydra Probe provides temperature
corrections for both the real and imaginary dielectric permittivities. The temperature correction
for the imagery dielectric permittivities are used for the temperature corrected soil electrical
conductivity. It follows the trend that the increased molecular vibration due to increased
temperature impedes electrical conductivity. In most applications, the temperature corrected soil
electrical conductivity is used in order to make temperature independent comparisons.
The temperature affects on the real dielectric permittivity of soil is very complex and is still an
area of research. The affects are related to soil mineralogy, ion valence, and the amount of clay.
In general, the dielectric properties of water behaves in sand the way in would as a liquid. In
other words, because temperature/dielectric affects of sand and some silts are predicable, the
temperature corrected real dielectric permittivity is used for the sand and silt calibrations.
The temperature/dielectric properties of clay are not well understood and are highly dependent
on the type of clay and the amount of cations present. The real dielectric permittivity of some
clays go up with temperature because molecular vibrations of cations bound to negative receptor
sites increases thus increasing the polarization of the clay with temperature. In other words, with
some clays, the dielectric permittivity will increase with temperature while the dielectric
permittivity of water will decrease with temperature. This is why dielectric temperature
correction works better for sand.
If you are calibrating the Hydra Probe in a clay rich soil with large diurnal temperature
fluctuations, we recommend the temperature uncorrected real dielectric permittivity.
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