Soil Moisture Sensor: Automatic Plant Watering with Arduino

A soil moisture sensor is a simple sensor that measures how wet the soil is in a pot or garden. It's the basis of many projects, from a warning light that tells you when a plant is thirsty to a system that waters your flowers while you're on holiday. It's cheap, connects to Arduino with three wires, and once you add a relay and a small pump you have an automation that's genuinely useful.

How does a soil moisture sensor work?

Water in the soil changes how well the soil conducts electricity and stores charge. Resistive sensors measure the resistance between two metal prongs pushed into the soil: as the soil gets wetter, the resistance drops. Capacitive sensors measure the capacitance around a coated electrode; the metal never touches the soil. On most modules the reading is high when the soil is dry and low when it's wet.

Wiring

Reading moisture with Arduino and calibrating

Every sensor and every soil gives different values, so calibrate your own sensor first: note the reading with the sensor in the air (completely dry) and dipped in a glass of water (completely wet). Then use map() to turn that range into a percentage from 0 to 100. Replace DRY and WET in the code with your own readings:

1const int SENSOR_PIN = A0;
2const int POWER_PIN = 7;       // sensörü yalnızca ölçerken besle / power only while measuring
3const int DRY = 850;           // havadayken okunan / reading in air (kendin ölç / measure yours)
4const int WET = 380;           // sudayken okunan / reading in water (kendin ölç / measure yours)
5
6void setup() {
7  pinMode(POWER_PIN, OUTPUT);
8  Serial.begin(9600);
9}
10
11int readMoisture() {
12  digitalWrite(POWER_PIN, HIGH);
13  delay(10);                                              // değerin oturmasını bekle / let it settle
14  int raw = analogRead(SENSOR_PIN);
15  digitalWrite(POWER_PIN, LOW);
16  return constrain(map(raw, DRY, WET, 0, 100), 0, 100);   // % nem / % moisture
17}
18
19void loop() {
20  int moisture = readMoisture();
21  Serial.print("Toprak nemi / Soil moisture: %");
22  Serial.println(moisture);
23  delay(2000);
24}

Automatic watering: relay and pump

Now let's turn the reading into real action: when soil moisture drops below 30%, run the pump for 4 seconds, then wait 1 minute for the water to spread through the soil before measuring again. This wait stops the pump from running non-stop and flooding the pot because the sensor hasn't got wet yet. The pump can't be powered straight from an Arduino pin; drive it through a relay or a MOSFET. For relay wiring, see What Is a Relay Module?

1const int SENSOR_PIN = A0, POWER_PIN = 7, PUMP_PIN = 8;   // PUMP_PIN → röle IN / relay IN
2const int DRY = 850, WET = 380;
3const int START_BELOW = 30;          // %30'un altında sula / water below 30%
4const unsigned long PUMP_MS = 4000;  // her seferde 4 sn / 4 s per run
5const unsigned long SOAK_MS = 60000; // suyun yayılması için bekle / wait for the water to soak in
6
7int readMoisture() {
8  digitalWrite(POWER_PIN, HIGH);
9  delay(10);
10  int raw = analogRead(SENSOR_PIN);
11  digitalWrite(POWER_PIN, LOW);
12  return constrain(map(raw, DRY, WET, 0, 100), 0, 100);
13}
14
15void setup() {
16  pinMode(POWER_PIN, OUTPUT);
17  pinMode(PUMP_PIN, OUTPUT);
18  digitalWrite(PUMP_PIN, HIGH);      // aktif LOW röle: kapalı başla / active-LOW relay: start off
19}
20
21void loop() {
22  if (readMoisture() < START_BELOW) {
23    digitalWrite(PUMP_PIN, LOW);     // pompayı çalıştır / pump on
24    delay(PUMP_MS);
25    digitalWrite(PUMP_PIN, HIGH);    // pompayı durdur / pump off
26    delay(SOAK_MS);                  // tekrar ölçmeden önce bekle / wait before measuring again
27  }
28  delay(5000);
29}

Soil moisture with the ESP32 and Raspberry Pi Pico

On the ESP32, connect the sensor to one of the ADC1 pins (GPIO 32–39), which still work while Wi-Fi is on; values range from 0 to 4095, so calibrate again. With an ESP32 you can send the moisture level to your phone as a notification, or build a pot sensor that measures once an hour in deep sleep and runs on batteries for months. Reading it on the Pico with MicroPython looks like this:

1from machine import ADC, Pin
2import time
3
4sensor = ADC(26)               # GP26 → AOUT
5power = Pin(15, Pin.OUT)       # sensörü yalnızca ölçerken besle / power only while measuring
6DRY, WET = 52000, 24000        # kendi değerlerinle değiştir / replace with your readings
7
8def moisture():
9    power.value(1)
10    time.sleep_ms(10)
11    raw = sensor.read_u16()
12    power.value(0)
13    pct = (DRY - raw) * 100 // (DRY - WET)
14    return max(0, min(100, pct))
15
16while True:
17    print("Toprak nemi / Soil moisture: %", moisture())
18    time.sleep(2)

What should soil moisture be?

These percentages are relative to your sensor's own dry-to-wet range; they're not the scientific water content of the soil. The best threshold is found by watching for a few days what the sensor reads when your plant actually needs watering.

Soil moisture sensor project ideas

• Automatic pot watering: a system that waters your flowers by itself, even when you're on holiday.
• "I'm thirsty" alert: an LED that lights or a buzzer that beeps when the soil dries out.
• Phone notification: send a Telegram message from an ESP32 when moisture drops.
• Multi-pot watering: water plants individually by giving each pot its own sensor and a small valve.
• Mini greenhouse station: also measure air humidity with a DHT11 and light with an LDR, and show everything on one display.
• Smart garden: automatically skip watering when a rain sensor detects rainfall.

Common mistakes and how to fix them

• The value is always 1023 or always 0: the DO pin is being read instead of AO, there's no power, or the wire to the sensor board is broken.
• The prongs have corroded and the readings make no sense: the resistive sensor was powered continuously; power it from a pin or switch to a capacitive model.
• Readings drift over time: conductivity changes as the soil compacts or is fertilised; recalibrate now and then.
• The pump won't stop: the relay is active LOW and HIGH/LOW are swapped in the code, or it measures again before the water has soaked in; add a wait between readings.
• The capacitive sensor gives strange values: the electronics at the top have got wet; make the board waterproof.
• Readings go wrong when Wi-Fi starts on the ESP32: the sensor is on an ADC2 pin; move it to a pin between GPIO 32 and 39.

A soil moisture sensor can be your first project that goes beyond measuring and actually changes something in the real world. In the BlueGrays app you'll find a smart watering station and an ESP32 smart plant watering system, step by step with circuit diagrams and code.