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How Do Soil Moisture Sensors Work – A Basic Guide to Measuring Soil Moisture Content

How Do Soil Moisture Sensors Work? Inarguably, “smart gardens” are taking over farming. They are autonomous irrigation systems that monitor soil moisture levels. As such, having a soil moisture sensor is crucial for constructing such a system.

Soil moisture sensors are convenient, help save water, and can benefit new and inexperienced gardeners. The question is, how do soil moisture sensors work?

Stick around as we introduce several types and applications of the sensor and its pin configuration. 

What is a Soil Moisture Sensor?

A soil moisture sensor is a device that monitors the amount of volumetric water present in the soil.

measuring soil humidity

(measuring soil humidity)

Additionally, they help us design improved agricultural systems that automatically adjust to your environment. Thus, an Arduino MCU sensor may automatically switch on sprinklers when the soil moisture is insufficient.

Pin Configuration 

The soil moisture sensor requires the connection of just four pins below;

(soil moisture sensor pinout)

GND/Ground pin; It functions as the ground connection.

VCC pin; It acts as the sensor’s power supply pin, providing a voltage between 3.3V to 5V to the sensor. Often, the analog output depends on the sensor voltage provided. 

Digital Output/DO pin; It produces the internal comparator circuit’s digital output. Either link it directly to a 5V relay or any Arduino digital pin. 

Analog output/AO pin; It offers an analog signal spanning from 0V and supply value and connects to an Arduino analog input.

Soil Moisture Sensor Type

Generally, different types of soil moisture sensors have distinct working principles. 

Resistive Soil Moisture Sensor

A resistive soil moisture sensor measures the connection between the soil’s total water content and its electrical resistance to determine its moisture quantity.  

You can distinguish the sensors by two protruding probes, each of which you put into a soil sample directly.

A resistive soil moisture sensor with dual probes

(A resistive soil moisture sensor with dual probes)

Working principle

  • First, an electrical current passes one probe to the next, allowing the resistive sensor to quantify the soil’s resistance between the probes.
  • When the soil has low water content, the electrical conduction is poor. As a result, you’ll obtain a high reading in resistance which means your soil moisture is low. 
  • Contrarily, a high electrical conductivity indicates high soil moisture content. Thus, you’ll get a low resistance reading. 

Capacitance Soil Moisture Sensors

The second soil water content sensor measures the variation in capacitance to determine soil moisture. In other words, the capacitance quantifies the electrical charge amount held crossway an electrical potential. 

capacitive soil moisture meter

(capacitive soil moisture meter)

Typically, the capacitive soil humidity sensor consists of a negative and positive plate with a dielectric medium at the center.

Working principle

  • For the capacitive sensor, soil moisture content affects the capacitance of the soil, which acts as the dielectric medium
  • If you pair a timer circuit with the sensor, you’ll get an analog voltage that an Arduino board can read.
  • Consequently, the analog voltage correlates well with soil moisture levels by indicating the amount. 

Time Domain Reflector (TDR) Soil Moisture Sensors

TDRs measure erodible media and soil moisture. TDR can identify a material’s permittivity from any wave propagation due to the tight link between its water content and the material’s dielectric constant (permittivity).

How Do Soil Moisture Sensors Work: TDT (Time Domain Transmissometry) Soil Moisture Sensors

TDT sensor (Time Domain Transmissometry) is in the third category. It estimates the duration an electromagnetic wave takes to travel down the soil’s transmission line. Usually, increased moisture content slows down a probe’s signal. 

Furthermore, TDT sensors are more accurate and use less power than TDR sensors. Additionally, its measurement bandwidth benefits users due to its reduced sensitivity to capacitance-affecting interference.

Standing Wave Sensors (SWS)

Standing wave sensors use four probes, embedded-coplanar-strip, cylindrical, 3-rod, and 4-rod, to measure the standing wave ratio (SWR) in the transmission line.

Normally, a high-frequency SWR can evaluate moisture in agricultural goods with unsteady conductance. SWS also operates at 85°C, compared to roughly 65-75°C for other market sensors. Then, they are applicable in measuring organic fertilizer pellets in silos and wet plantation soil.

Soil Moisture Sensor with Arduino

Now, let’s learn how we can integrate our sensor with an Arduino board.

How Do Soil Moisture Sensors Work: Components needed 

How Do Soil Moisture Sensors Work: Setting up the hardware

First and foremost, let us consider the following pins;

Pin7: It is the highest position line

Pin6: LM358ID IC Op-Amp

Pin5: NE555DR IC

Pin4/GND: Connects the Arduino system’s GND pin

Pin3/VCC: 5V or 3.3V applicable. When using Arduino UNO, ensure your mains is a DC power supply to avoid the maximum ripple exceeding 100mV. 

Pin2/NC: Not connected 

Pin1/SIG: Analog output pin


  • Start by connecting the moisture sensor to the A0 port of the Arduino or base shield.
  • Next, plug your base shield into the grove port of the Arduino.
  • Then, place your sensor, a few inches deep, into a soil sample. Do not pass the white line in the sensor. 
  • Finally, use a USB cable to connect the Arduino device to your computer.
Interfacing soil moisture sensor with Arduino board

(Interfacing soil moisture sensor with Arduino board) 


A few of the soil sensor applications include;

watering system using a soil moisture sensor

(watering system using a soil moisture sensor)


All in all, soil moisture sensors help several farming enthusiasts to achieve their goals with minimum effort. 

If you still have any technical questions on soil moisture sensors, kindly contact us for assistance. 

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Emma Lu
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