Soil Moisture Sensor is a popular device in Smart Farm projects or automatic watering systems. It works with Arduino easily, is cheap, and is suitable for learning and experimenting.

Soil Moisture Sensor

Soil moisture sensor is a sensor that detects the moisture level in the soil by measuring the conductivity between the probe legs (similar to measuring the resistance of soil).

Dry soil → low conductivity → low value obtained
Moist soil → High conductivity → High obtained value

Sensors often come with a conversion module that provides both analog and digital pins.

Soil Moisture Sensor Specifications

list	details
Supply voltage	3.3V – 5V
Current used	About 10mA
Output signal	Analog (A0) and Digital (D0)
Detection range	General soil moisture levels
structure	2 probe legs (ground plug), signal converter board has a threshold adjustment potentiometer

The Digital Output will give a HIGH or LOW value according to the threshold we set.

Using with Arduino

Wiring:

Soil Sensor Pin	Arduino Pin
VCC	5V
GND	GND
A0 (Analog)	A0
D0 (Digital)	D2 (if used)

Analog code example:

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int sensorPin = A0;

void setup() { Serial.begin(9600); }

void loop() { int value = analogRead(sensorPin); Serial.print("Soil moisture : "); Serial.println(value);

delay(1000); }

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The value returned by analogRead() will be between 0–1023.

High value = dry soil

Low value = moist soil

Digital code example:

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int sensorDigital = 2;

void setup() { pinMode(sensorDigital, INPUT); Serial.begin(9600); }

void loop() { int value = digitalRead(sensorDigital);

if (value == LOW) { Serial.println("Moist soil"); } else { Serial.println("Dry soil"); }

delay(1000); }

Additional tips:

Do not leave the sensor plugged into the ground for a long time (rust may occur).
Use the relay together with a water pump or electric valve to automatically water plants.
If you want more accuracy, try using a capacitive soil moisture sensor instead of the regular one (it is more durable and does not rust).