6.4.6 Bioturbation
Organisms such as plants and burrowing animals can homogenize soil and dislodge soil probes.
A tree root can grow between the tines affecting the measurements and in some cases, tree roots
can bring a buried soil probe to the soil surface. Burrowing mammals and invertebrates may
decide that the Hydra Probes’ tine assembly makes an excellent home. If the Hydra Probe’s tine
assembly becomes home to some organism, the soil moisture measurements will be affected.
After the animal vacates, the soil will equilibrate and the soil measurements will return to
representative values.
The cable leading to the probe may also become a tasty treat for some animals. If communication
between the logger and the probe fails, check the cable for damage. A metal conduit like the one
shown in figure 3.7 is recommended.
6.4.7 Salt Affected Soil and the Loss Tangent
The Hydra Probe is less affected by salts and temperature than TDR or other FDR soil sensors
because of the delineation of the dielectric permittivity and operational frequency at 50Mhz.
While the Hydra Probe performs relatively well in salt affected soils, salts that are dissolved in
the soil water will influence both dielectric permittivities constants and thus the measurements.
The salt content will increase the imaginary dielectric constant and thus the soil electrical
conductivity. See Chapter 5. The Hydra Probe will not measure electrical conductivity beyond
1.5 S/m
In general, if the electrical conductivity reaches 1 S/m, the soil moisture measurements will be
significantly affected. The imaginary dielectric constant will have an influence on the real
dielectric constant because dissolve cations will inhibit the orientation polarization of water.
When addressing the Hydra Probes’ performance in salt affected soil, it is useful to use the loss
tangent equation [6.2].
Tan
δ
=
E
i
/E
r
[6.2]
The loss tangent (Tan
δ
) is simply the imaginary dielectric constant divided by the real dielectric
constant. If Tan
δ
become greater than 1.5 than the Hydra Probes calibration becomes unreliable.
It is interesting to note that the Hydra Probe will still provide accurate dielectric constant
measurements up to 1.5 S/m. If the salt content reaches a point where it is affecting the
calibrations, the user can use a custom calibration that will still provide realistic soil moisture
measurements in the most salt affected soils. See section 2.3 for custom calibrations.
6.4.8 Ped Wetting
A soil ped is single unit of soil structure. Ped shapes include granular, platy, blocky and
prismatic and ped sizes can range from 1mm granules to 10 cm prisms. The preferential pathway
water travels trough soil is between the peds. This is evident by clay film coatings that develops
around a ped. The clay film precursors become dissolved in the pore water, as the pore water
subsides, the clay film precursors fall out of solution and adhere to and surface of the peds
creating the clay film. The clay film will often times delay the infiltration of water into the ped
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