Capacitive soil moisture sensor with Arduino Nano — calibrated % on A6

The capacitive soil sensor is the one to buy for pots and garden beds: unlike the old two-prong resistive probe, nothing on it corrodes. It outputs a voltage that drops as the soil gets wetter.

The Nano reads it on A6 — one of its two analog-only pins, which is exactly what this sensor needs. The sketch averages 16 readings, converts them to a 0–100 % figure using your own dry and wet calibration, and suggests watering below 30 %.

Wiring

Every connection
Arduino Nano	Capacitive soil moisture sensor v1.2	Wire	Notes
GND	GND	black
5V	VCC	red
A6	AOUT	yellow	A6 is analog-only — a good home for a sensor that only gives a voltage.

3.3–5.5 V part, 5 V board. The output is an analog voltage that stays below the supply. Power it from 3.3 V on ESP32 and Pico so AOUT can never exceed the ADC input range; on an Uno 5 V is fine.

Board quirks

The output falls as the soil gets wetter. Every sensor is different, so calibrate: send d with the probe in air and w with it in a glass of water.
On a 5 V Nano the output (about 1.2–3 V) goes straight to A6 — no divider.
Some cheap v1.2 boards are mis-built (wrong regulator or a missing resistor), so the reading barely changes or is very slow. If dry and wet differ by less than ~150 counts, try another sensor.
Push the probe in only up to the line printed on it, and seal the top edge and components with nail polish or hot glue — monsoon rain kills the exposed circuit, not the probe.

Code

soil-nano.inoArduino C++Open in Playground Flash

// Capacitive soil moisture sensor v1.2 on an Arduino Nano, read on the analog-only pin A6.
// VCC → 5V, GND → GND, AOUT → A6. Calibrate DRY and WET with your own sensor, then it prints %.
const uint8_t SOIL_PIN = A6;
int DRY = 600;  // reading in dry air       (send 'd' with the probe in air to store)
int WET = 280;  // reading in a glass of water (send 'w' with the probe in water)

int readAverage() {
  long sum = 0;
  for (uint8_t i = 0; i < 16; i++) {
    sum += analogRead(SOIL_PIN);
    delay(5);
  }
  return sum / 16;
}

void setup() {
  Serial.begin(115200);
  Serial.println(F("Soil sensor on A6 - send d (dry) or w (wet) to calibrate"));
}

void loop() {
  int raw = readAverage();
  if (Serial.available()) {
    char c = Serial.read();
    if (c == 'd') DRY = raw;
    if (c == 'w') WET = raw;
    if (c == 'd' || c == 'w') {
      Serial.print(F("Calibrated: dry="));
      Serial.print(DRY);
      Serial.print(F(" wet="));
      Serial.println(WET);
    }
  }
  int pct = constrain(map(raw, DRY, WET, 0, 100), 0, 100);
  Serial.print(F("Raw: "));
  Serial.print(raw);
  Serial.print(F("  Moisture: "));
  Serial.print(pct);
  Serial.println(pct < 30 ? F(" %  -> water the plant") : F(" %"));
  delay(1000);
}

// Capacitive soil moisture sensor v1.2 on an Arduino Nano, read on the analog-only pin A6.
// VCC → 5V, GND → GND, AOUT → A6. Calibrate DRY and WET with your own sensor, then it prints %.
const uint8_t SOIL_PIN = A6;
int DRY = 600;  // reading in dry air       (send 'd' with the probe in air to store)
int WET = 280;  // reading in a glass of water (send 'w' with the probe in water)

int readAverage() {
  long sum = 0;
  for (uint8_t i = 0; i < 16; i++) {
    sum += analogRead(SOIL_PIN);
    delay(5);
  }
  return sum / 16;
}

void setup() {
  Serial.begin(115200);
  Serial.println(F("Soil sensor on A6 - send d (dry) or w (wet) to calibrate"));
}

void loop() {
  int raw = readAverage();
  if (Serial.available()) {
    char c = Serial.read();
    if (c == 'd') DRY = raw;
    if (c == 'w') WET = raw;
    if (c == 'd' || c == 'w') {
      Serial.print(F("Calibrated: dry="));
      Serial.print(DRY);
      Serial.print(F(" wet="));
      Serial.println(WET);
    }
  }
  int pct = constrain(map(raw, DRY, WET, 0, 100), 0, 100);
  Serial.print(F("Raw: "));
  Serial.print(raw);
  Serial.print(F("  Moisture: "));
  Serial.print(pct);
  Serial.println(pct < 30 ? F(" %  -> water the plant") : F(" %"));
  delay(1000);
}

Expected serial output

Serial Monitor · 115200Output

Soil sensor on A6 - send d (dry) or w (wet) to calibrate
Raw: 589  Moisture: 3 %  -> water the plant
Raw: 402  Moisture: 61 %

Soil sensor on A6 - send d (dry) or w (wet) to calibrate
Raw: 589  Moisture: 3 %  -> water the plant
Raw: 402  Moisture: 61 %

Common errors

Moisture always 0 % or 100 %

DRY/WET not calibrated for your sensor — the defaults are only a guess. Use the d and w commands.

digitalRead on A6 always 0

Correct — A6/A7 have no digital input on the Nano. Use analogRead(), as the sketch does.