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Arduino - Soil Moisture Sensor

In this tutorial, we are going to learn how to use the moisture sensor with Arduino. In detail, we will learn:

Resistive Moisture Sensor vs Capacitive Moisture Sensor
How to read the value from a capacitive moisture sensor using Arduino
How to calibrate a capacitive moisture sensor
How to determine if the soil is wet or dry

Hardware Required

1	×	Official Arduino Uno
1	×	Alternatively, DIYables ATMEGA328P Development Board
1	×	USB 2.0 cable type A/B (for USB-A PC)
1	×	USB 2.0 cable type C/B (for USB-C PC)
1	×	Capacitive Soil Moisture Sensor
1	×	Jumper Wires
1	×	Recommended: Screw Terminal Block Shield for Arduino Uno
1	×	Recommended: Breadboard Shield for Arduino Uno
1	×	Recommended: Enclosure for Arduino Uno
1	×	Recommended: Prototyping Base Plate & Breadboard Kit for Arduino UNO

Or you can buy the following sensor kits:

1	×	DIYables Sensor Kit (30 sensors/displays)
1	×	DIYables Sensor Kit (18 sensors/displays)

Disclosure: Some links in this section are Amazon affiliate links. If you make a purchase through these links, we may earn a commission at no extra cost to you.
Additionally, some links direct to products from our own brand, DIYables .

Buy Note: Many capacitive soil moisture sensors on the market are low-quality, regardless of the version. We highly recommend buying the sensor with TLC555I Chip from the DIYables brand using the link above. We tested it, and it worked well.

About Soil Moisture Sensor Sensor

capacitive moisture sensor vs resistive moisture sensor

There are two kinds of moisture sensors:

Resistive moisture sensor
Capacitive moisture sensor.

Both sensors provide soil moisture information. However, their working principles are different. We highly recommend using the capacitive moisture sensor, because of the following reason:

The resistive soil moisture sensor corrodes over time. That is because the electrical current flows between its probes. This causes electrochemical corrosion.
The capacitive soil moisture sensor does NOT corrode over time. That is because its electrodes are not exposed electrodes and is comparatively corrosion-free.

The below image shows a resistive soil moisture sensor that corroded over time.

The rest of this tutorial will use the capacitive soil moisture sensor.

Capacitive Soil Moisture Sensor Pinout

A capacitive soil moisture sensor has three pins:

GND pin: needs to be connected to GND (0V)
VCC pin: needs to be connected to VCC (5V or 3.3v)
AOUT pin: Analog signal output pin outputs the voltage in inverse proportion to the soil moisture level. Connect this pin to an Arduino's analog input pin.

How It Works

The more the water in the soil, the lower the voltage in the AOUT pin is

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

Arduino Code

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-soil-moisture-sensor */ #define AOUT_PIN A0 // Arduino pin that connects to AOUT pin of moisture sensor void setup() { Serial.begin(9600); } void loop() { int value = analogRead(AOUT_PIN); // read the analog value from sensor Serial.print("Moisture: "); Serial.println(value); delay(500); }

Quick Steps

Copy the above code and open it with Arduino IDE
Click Upload button on Arduino IDE to upload code to Arduino
Bury the sensor in soil, then pour water into the soil. Or slowly submerge it into a cup of salt water.
Check out the result on Serial Monitor. It looks like the below:

COM6

Send

Moisture: 581 Moisture: 581 Moisture: 575 Moisture: 566 Moisture: 556 Moisture: 547 Moisture: 539 Moisture: 530 Moisture: 521 Moisture: 513 Moisture: 506 Moisture: 500 Moisture: 495 Moisture: 492 Moisture: 490 Moisture: 489 Moisture: 488

Autoscroll Show timestamp

Clear output

9600 baud

Newline

※ NOTE THAT:

Do NOT use pure water for testing since it doesn't conduct electricity, which means it won't impact the sensor readings.
The sensor readings typically don't drop to zero. It's normal for them to stay within the range of 500 to 600, but this might change based on factors such as how deep the sensor is placed, the type of soil or water, and the voltage of the power supply.
Never bury the circuit part (found on top of the sensor) in soil or water, as this could harm the sensor.

Calibration for Capacitive Soil Moisture Sensor

The measured value from the moisture sensor is relative. It depends on the soil's composition and water. In practice, we need to do calibration to determine a threshold that is a border between wet and dry.

How to do calibration:

Run the above code on Arduino
Embed the moisture sensor into the soil
Embed the moisture sensor into the soil
Pour water into the soil slowly
Watch Serial Monitor.
Write down a value at the time you feel that the soil changes its moisture from dry to wet. This value is called THRESHOLD.

Determine if the soil is wet or dry

After the calibration, update the THRESHOLD value you wrote down to the below code. The below code determines if the soil is wet or dry

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-soil-moisture-sensor */ #define AOUT_PIN A0 // Arduino pin that connects to AOUT pin of moisture sensor #define THRESHOLD 530 // CHANGE YOUR THRESHOLD HERE void setup() { Serial.begin(9600); } void loop() { int value = analogRead(AOUT_PIN); // read the analog value from sensor if (value > THRESHOLD) Serial.print("The soil is DRY ("); else Serial.print("The soil is WET ("); Serial.print(value); Serial.println(")"); delay(500); }

The result on Serial Monitor.