PASCO PS-2163 Soil Moisture Sensor - Manual (Page 8)

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Soil Moisture Sensor PASCO Soil Water Potential Probe (PS-2513)
8
PASCO Soil Water Potential Probe
(PS-2513)
Introduction
There are two basic parameters that describe the state of water in soil: one is soil
water content, or the amount of water per unit of soil, and the other is soil water
potential, or the energy state of the water in the soil. Water potential is often preferred
over water content because it shows how water will move in a soil or from the soil to
the plant. In an object comes into hydraulic contact with soil, the water potential of
the object will come into equilibrium with the soil water potential.
The Soil Water Potential Probe uses a solid matric equilibration technique to measure
the water potential of the soil. The probe has a static matrix - a porous ceramic disk -
that is allowed to reach hydraulic equilibrium with the soil that is being measured.
The probe measures the dielectric permittivity of the ceramic disk to determine its
water potential and thereby the water potential of the soil.
Installing the Probe
Because it measures water potential, the probe needs good hydraulic contact with the
surrounding soil. The preferred method for installing the probe is to take some native
soil, wet it, and pack it in a ball around the entire probe, making sure that the moist
soil is in contact with all surfaces of the ceramic disk. The probe and moist soil are
then packed into the soil at the desired depth.
After the probe is installed, the hole that was excavated to bury the probe should be
back-filled with care taken to re-pack the soil back to its native bulk density. Leave at
least 15 cm of cable beneath the soil. Do not bend the cable in a tight radius as it
leaves the probe. At least 10 cm of cable nearest the probe should remain in a straight
line.
Removing the Probe
When removing the soil water potential probe from the soil, do not pull it out of the
soil by the cable! Doing so may break internal components and make the probe unus-
able.
Handling and Care
For the probe to accurately measure water potential, the ceramic disks must readily
take up water. If the ceramic disks are exposed to oils or other hydrophobic sub-
stances, then the ability of the disks to take up water can be compromised. As much as
possible, minimize the exposure of the ceramic disks to skin oils, synthetic oils, or
other hydrophopic compounds.
The probe consists of two engineered ceramic disks sandwiched between stainless
steel screens and the probe’s circuit board. The disks are brittle and can chip or crack
if mishandled. Sharp trauma such as dropping the probe onto a hard surface can cause
the ceramic to break. A cracked ceramic will cause a loss of accuracy.
In the engineering community,
“soil suction” is used instead of
soil water potential. Soil water
potential is simply the negative
of soil suction.
Frozen Soil
Under frozen soil conditions, the
probe cannot accurately mea-
sure the water potential of soil.
However, the probe is unaffected
by repeated freeze-thaw cycles,
so the probe can remain in the
soil year round.