6.4.3 Soil Bulk Density
In general, the greater the soil density, the less water it will hold and the slower water will move
through it. There will often times be soil horizons that will be more dense than others giving the
soil different hydrological properties with depth. Occasionally, water will stop or slow down and
rest on a dense, less permeable layer of soil. This phenomenon is called perched water. If two
Hydra Probes 20 cm apart have very different soil moisture readings, chances are that one of the
probes is residing in perched water.
There is also a relationship between soil bulk density and the complex dielectric permittivity.
The soil dry bulk density (
ρ
b
) can be described by equation [6.1]
ρ
b
= m/V
[6.1]
where m is the mass of the dry soil in grams and V is the volume in cubic centimeters.
The bulk density is associated with the density of a soil ped or a soil core sample. The particle
density (
ρ
p
) is the density of a individual soil particle such as a grain of sand. The two densities
should not be confused with one an other. Because
E
r
and
Ei
of dry soil is a function of both the
bulk and particle densities (
ρ
b
,
ρ
p
), the soil density often creates the need for soil specific
calibrations.
6.4.4 Shrink/Swell Clays
Shrink/swell clays belong to the soil taxonomic order vertisol and are composed of smectite
clays. These clays have a large ion exchange capacity and will shrink and swell seasonally with
water content. The seasonal expansion and contraction homogenizes the top soil and the subsoil.
As the clay shrinks during a drying period, the soil will crack open and form large crevasses or
fissures. If a fissure forms in the measurement volume of the Hydra Probe, the probe will signal
average the fissure and potentially generate biased results. If the fissure fills with water, the soil
moisture measurement will be high, if the fissure is dry, the soil moisture measurement will be
lower than expected. If the Hydra Probe measurements are being affected by shrink/swell clays,
It recommend to relocate the probe to an adjacent location.
6.4.5 Rock and Pebbles
Often times, it will be obvious if a rock is encountered during an installation. Never use
excessive force to insert the probe into the soil. Some soils will have a distribution of pebbles. If
a pebble finds its way between the probe’s tines, it will create an area in the measurement
volume that will not contain water. The probe will signal average the pebble and thus lower the
soil moisture measurement. If the pebble is an anomaly, relocating of the probe would provide
more representative soil measurements. However, if it is revealed from the soil survey that there
exists a random distribution of pebbles, a pebble between the tines may provide realistic
measurements because of the way pebbles influence soil hydrology.
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