How Does A Soil Moisture Sensor Work?

Soil Moisture sensors are a practical, quick and effective tool for monitoring soil moisture content out in the field. This article investigates four types of soil moisture sensors and how they work.

Soil Moisture Sensors

Our customers often ask us, “How does a soil moisture sensor work?”.

Our answer: It depends on the type of sensor!

Our scientists have prepared four diverse examples of soil moisture sensors, detailed how they work, set out the advantages and disadvantages of each sensor type, and provided a real-world product example.

Soil Moisture Sensors at Instrument Choice

Capacitance

How it works:

Capacitance is the ability of a component or circuit to collect and store energy in the form of an electrical charge. Capacitance soil sensors exploit this technique to measure the dielectric permittivity of a surrounding medium.

A capacitor consists of three pieces; a positive plate, a negative plate, and the dielectric (the space between the plates). When connecting this capacitor (made of the metal plates or rods to be inserted in the soil) with an oscillator to form an electrical circuit, changes in the circuit operating frequency are used to calculate soil moisture levels.

Example of a Capacitance Soil Moisture Sensor:

Aquaterr Soil Moisture Meter
Product code: IC-T-350

Brief: 

The IC-T-350 is a multifaceted soil moisture meter built to withstand years of industry use and abuse.

Key features:

• 2cm aircraft-grade stainless steel probe
• Weatherproof aluminium housing that protects the easy-to-read display
• Scales for measuring clay loam and sand.

Data Captured:

• Percent saturation: 0 to 100% saturation; and
• Temperature: 5°C to 65°C.

Great for:

• Determining moisture in compost and in soil during early planting to maintain ideal conditions
• Golf courses
• Parks and gardens
• Landscape managers
• Irrigation consultants; and
• Farmers.

Time Domain Reflector (TDR)

How it works:

Like capacitance sensors, Time Domain Reflector (TDR) sensors employ dielectric measurement techniques to obtain a soil moisture reading. What’s the difference? TDR sensors have two or three metal rods in parallel that, when inserted in the soil, pass an electromagnetic wave between them. The time taken for the wave to travel from one rod to the other indicates the charge storing capacity of the soil and the volumetric water content.

Example of a TDR Digital Soil Moisture Sensor:

TDR Digital Soil Moisture Sensor Reader
Product code: IC-RD1200

Brief:

The IC-RD1200:

• Captures a wide range of moisture and temperature information
• Highly functional - with an easy to use, handheld interface
• Stores and expors data; and
• Provides irrigation recommendations based on soil type.

Data Captured:

• Volumetric Water Content: 0 to 100% with 0.1% resolution
• Permittivity: 1 to 80 with 0.1 unit resolution
• Bulk EC: 0 to 5000 uS/cm - resolution depends on reading range.
• Pore Water EC: 0 to 55000 uS/cm; and
• Temperature: - 40 to +60°C with 0.1°C degree resolution.

Great for:

• Farmers
• Horticulturalists
• Parks and golf course management
• Landscapers and gardeners; and
• Irrigation planning and management.

TDT (Time Domain Transmissometry)

How it works:

TDT sensors operate in a similar fashion to TDR sensors. The defining difference of TDT sensors is that the sensor rod is connected to the electrical source at both the beginning and end (i.e. a closed circuit). TDT sensors measure the travel time of the wave propagation between the rods.

Example of a TDT Digital Soil Moisture Sensor:

Digital TDT SDI-12 Soil moisture and conductivity sensor
Product code: ACC-SEN-SDI

Brief:

The ACC-SEN-SDI:

• Does not require soil-specific calibration
• Works in all soils
• Is very stable; and
• Is made from durable inert materials - which makes it a robust soil moisture research tool.

Data Captured:

• Absolute volumetric water content: 0 to 100%
• Temperature: 1˚ C to 50˚ C for VWC (no ice), -20˚ C to 50˚ C for the other data

Great for:

• Research; and
• Irrigation planning and management.

Standing Wave Sensors

How it works:

A standing wave soil moisture sensor (also known as an amplitude domain reflectometer) uses the standing wave principle to indicate the ratio of two or more substances that form a body of material, with each substance having a different dielectric constant (Ka).

Standing Wave sensors consist of metal rods arranged in a circle around a central rod that acts as the transmission line. The sensor measures the impedance of a signal from the transmission probe to the receiving probes. Impedance contains two parts: the dielectric constant and the soil’s electrical conductivity. Electrical conductivity is minimised by the signal selection, leaving the dielectric constant, which is proportional to soil moisture.

Example of a Standing Wave Soil Moisture Sensor:

Instant Soil Moisture Reading Kit
Product code: IC-MPKit-406B

Brief:

Portable and convenient, the IC-MPkit-406B is exceptionally straightforward to use. Insert the moisture sensor to the required depth of measurement, and the moisture content displays on the meter. Moisture readings are also saved to the device for later recall or download to a computer.

Data Captured:

• Volumetric Soil Water: 0 to 100 VSW% 

Great for:

• Scientific research
• Irrigation monitoring and management
• Sports fields
• Mine sites
• Landfills 

Conclusion

Selecting the best soil moisture meter for your application will depend on your individual needs, soil composition, and your required outcome.

If you need more information about how soil moisture sensors work or need assistance finding the best for your application, speak with an Instrument Choice Scientist! Call 1300 737 871 or email [email protected].