Which soil moisture sensor should I choose?

Standing wave, capacitance and Time Domain Reflectometry (TDR) measure the dielectric constant of the soil whereas the neutron probe measures the hydrogen content of soils.

The dielectric constant is the ratio of the permittivity of a substance to free space. Dielectric constant (Ka) in air is 1, water is 80 and soil is 3 to 4. Therefore any relative change between air, water and soil will change Ka.

Standing wave uses an oscillator to generate an electrical field in order to detect Ka. The parallel needles of the MP406 and MP306 act as a coaxial transmission line which is used to generate a signal. The amplitude of the signal which is generated by a reflected electrical signal is measured and converted to moisture content.

Capacitance determines Ka by measuring the charge time of a capacitor, which uses soil as a dielectric medium. The capacitance sensor forms a pair of electrodes and the soil acts as a dielectric. The capacitor charge time is a linear function of the dielectric permittivity of the soil.

TDR determines Ka by sending a high speed electromagnetic pulse down a line of known length and measure the reflectance of the pulse. The travel time of the pulse is used to directly calculate Ka.

Neutron probes do not measure Ka but measure hydrogen atoms in the soil. In most cases water is the only source of hydrogen in soils. Neutrons are emitted from a decaying source into the soil profile. The neutrons collide with hydrogen atoms in the soil (i.e. water) therefore the more collisions the more water in the soil. A measurement is made of the number of neutrons that have collided and this is related to soil moisture content.

MP306 Standing Wave sensors connected to a Soil Moisture Meter measuring volumetric moisture content down a soil profile.

SOIL MOISTURE SENSORS

MP306 and MP406- Standing Wave Sensors

The MP306 and MP406 can be used to take continuous measurements over time through permanent or temporary burial and connection to a soil moisture meter (SMM).

10HS – Capacitance Sensor

The sensor is 10cm in length, ideal in field applications for irrigation scheduling or catchment hydrology and ecology studies using wireless networks.

TDR Sensor

TDR sensors are some of the most accurate sensors. Soilmoisture Equipment Corp. (SEC) are manufacturers of TDR sensors.

Neutron Probe

ICT International provides scientists and irrigation farmers with a full and comprehensive neutron probe service. ICT has been actively supporting the many hundreds of neutron probe users worldwide since 1980.