Soil moisture - impedance method (Theta Probe)

Parameter to be measured:

Soil moisture, permitivity

Method:

Frequency Domain Sensor working with the so called impedance-method Usable in saline soils - up to 2000 mS.m^-1

Equipment:

• Delta-T Device: Theta-Probe (ML2x) with 4 rods of 600 mm length, HH2 display for manual reading or a data logger

Advantages:

• easy and fast measurement of punctual soil moisture
• manual as well as continuously logged measurements possible

Disadvantages:

• Each sensor has to be calibrated
• careful application of probe in stony and dry soils, because rods break easy
• during field application cable breakes often occures at the crossing between probe head and cable
• Underestimating of permitivity in media with low permitivity (ε > 40, clay and organic soils)
• unaccuarate values in saturated media

What to watch out for:

• Good contact between probe and medium has to be proved

Price:

• 1270 € for probe and reading device (2008)

Links

Projects that used the above equipment:

• The ECHO project
• COMTESS

Other related web sites:

• John Miller Theta Probe: Theta Probe ML2X Principles of operation and application
• Delta-T Devices: User Manual
• Test of 5 different soil moisture sensors
• sowacs.com - the central resource for soil moisture measurement

References

Blonquist, JM; Jones, SB; Robinson, DA. 2005. Standardizing characterization of electromagnetic water content sensors: Part 2. Evaluation of seven sensing systems. VADOSE ZONE JOURNAL 4(4), 1059-1069.

Gaskin, GJ; Miller, JD. 1996. Measurement of soil water content using a simplified impedance measuring technique. JOURNAL OF AGRICULTURAL ENGINEERING RESEARCH 63(2), 153-159.

Miller, JD; Gaskin, GJ; Anderson, HA. 1997. From drought to flood: Catchment responses revealed using novel soil water probes. HYDROLOGICAL PROCESSES 11(5), 533-541.

Seyfried, MS; Murdock, MD. 2004. Measurement of soil water content with a 50-MHz soil dielectric sensor. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL 68(2), 394-403.