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Enabling better global research outcomes in soil, plant & environmental monitoring.

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.

MP406 Sensors
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).

MP306 and MP406

 

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.

Decagon 10HS

 

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.

503elite-01

 

How do soil moisture sensors actually measure moisture?

Which soil moisture sensor should I use?

Advantages and disadvantages of four types of soil moisture measurement techniques:

Standing Wave

Capacitance

TDR

Neutron Probe

Accuracy

Good, excellent with

Satisfactory (good with

Excellent

Excellent

calibration

calibration)

Resolution

Excellent

Good

Excellent

Excellent (with

calibration)

Measurement Zone

2cm from central needle

<1cm from sensor

<1cm from sensor

30cm diameter sphere

of Influence

Cost

Moderate

Low

High

High

Does sensor need

YES for different soil

YES for different soil

No

YES for different soil

calibration?

types

types

types

Life Expectancy

20 years

2 to 5 years

20 years

20 years

Affected by:

Temperature?

No

YES

No

No

Salinity?

No (unless greater than

YES (careful calibration

No (unless greater than

No

9dS/m but can be

can improve Decagon

16dS/m)

corrected with

sensors but not other

calibration)

sensors)

Can be used in:

Agriculture?

YES

YES

YES

YES

Compost?

YES

No

YES

YES

Glasshouses?

YES

YES (only if temperature

YES

No

compensated)

Mine sites and

YES

No

YES

YES

landfills?

Soil columns?

YES

Not typically

YES

No