Soil-Moisture Calibration Study

Discover why a soil sensor never gives a 'real' percentage. Record the raw reading in air and in water, then test dry, damp and wet soil, probe depth and packing — and graph how much those choices change the number.

One of 7 projects in the ESP32 Plant Monitoring Learning Kit — the parts below are from that kit.

What you build and learn

• How a capacitive soil sensor works
• Raw ADC values versus calibrated percentages
• Repeating a measurement to check it is reliable
• Turning observations into a simple graph

What this project uses

Every item is in the kit — nothing extra to buy.

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ESP32U

In the kit

DIY-002 capacitive soil sensor Capacitive Soil Moisture Sensor Module v2.0, 3.3-5.5V, for Arduino - Moisture Detection Garden Watering DIY DIY-002

In the kit

breadboard Transparent 830 Points Solderless Breadboard Clear Prototype Circuit Board MB-102 High Quality DIY Electronic… DIY-295

In the kit

jumper wires Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212

In the kit

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user-supplied dry and wet soil references

In the kit

Build this with

ESP32 Plant Monitoring Learning Kit

All 6 parts in one box, with this project and 6 more to build from them.

AED 65.00 See the kit Add kit to cart

Need only these parts? Order them loose · from AED 52.76 ▾

Used in this project tick what you still need

DIY-002 capacitive soil sensor Capacitive Soil Moisture Sensor Module v2.0, 3.3-5.5V, for Arduino - Moisture Detection Garden Watering DIY DIY-002 · ×1

AED 11.99

In stock

breadboard Transparent 830 Points Solderless Breadboard Clear Prototype Circuit Board MB-102 High Quality DIY Electronic… DIY-295 · ×1

AED 15.75

In stock

jumper wires Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212 · ×1

AED 25.02

In stock

Selected parts AED 52.76

Add 3 parts to cart

One of each part. Packs (resistor assortment, jumper wires) already hold more than a single build needs. ESP32U, user-supplied dry and wet soil references: included in the kit, ask us for it loose. Delivery is AED 20, free on orders AED 100+.

Connections

Unplug USB before you change any wire. Pin names are the labels printed on the board.

Step by step

1. In Arduino IDE install the ESP32 boards package (Boards Manager → esp32 by Espressif) and, from Library Manager, Adafruit BME280 and Adafruit Unified Sensor. Select the ESP32 Dev Module board and your USB port.
2. With USB unplugged, wire the soil sensor: VCC to 3V3, GND to GND, AOUT to GPIO34.
3. Upload the Soil-moisture calibration study sketch. Open Tools → Serial Monitor at 115200 baud, with the line ending set to Newline.
4. Hold the probe in the air, wait for the raw value to settle, and type d. Then dip the blade in a glass of water up to the line (never over the electronics) and type w.
5. Push the probe into the dry, damp and wet soil cups in turn, to the same depth. Wait 10 seconds in each and write down raw and percent.
6. Repeat in the damp cup at half depth, and again with the soil pressed down firmly. Note how much the reading moves.
7. Plot raw value against cup in a spreadsheet. Type r to reset the endpoints and try calibrating with dry and wet soil instead of air and water.

Sketch for this project

Written for the board in the kit. Copy it into the Arduino IDE; change the pin numbers if you wire it differently.

/*
  ESP32 Plant Monitoring Learning Kit
  Project 2: Soil-moisture calibration study

  Wiring: capacitive soil sensor VCC 3V3, GND, AOUT GPIO34.

  What you learn: the sensor does not give a percentage - it gives a raw
  number that depends on the soil, the depth and the sensor coating. Hold
  it in the air (dry), then in a glass of water (wet), and the sketch
  records both ends. Everything after that is a percentage BETWEEN those
  two readings, which is why two kits never agree on "40 %".
*/

const int SOIL_PIN = 34;
const int ADC_SAMPLES = 24;
const int MIN_SPAN = 150;
const unsigned long REPORT_MS = 1000;

int dryRaw = -1, wetRaw = -1;
int lowest = 4095, highest = 0;
unsigned long lastReport = 0;

int readAverage() {
  long sum = 0;
  for (int i = 0; i < ADC_SAMPLES; i++) { sum += analogRead(SOIL_PIN); delay(2); }
  return sum / ADC_SAMPLES;
}

void setup() {
  Serial.begin(115200);
  pinMode(SOIL_PIN, INPUT);
  analogReadResolution(12);
  Serial.println("Type d (sensor in air) or w (sensor in water) in the Serial input, r to reset.");
  Serial.println("t_s,raw,lowest,highest,percent");
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == 'd') { dryRaw = readAverage(); Serial.print("dry endpoint = "); Serial.println(dryRaw); }
    if (c == 'w') { wetRaw = readAverage(); Serial.print("wet endpoint = "); Serial.println(wetRaw); }
    if (c == 'r') { dryRaw = wetRaw = -1; lowest = 4095; highest = 0; Serial.println("reset"); }
  }
  if (millis() - lastReport < REPORT_MS) return;
  lastReport = millis();
  int raw = readAverage();
  if (raw < lowest) lowest = raw;
  if (raw > highest) highest = raw;
  Serial.print(millis() / 1000); Serial.print(",");
  Serial.print(raw); Serial.print(",");
  Serial.print(lowest); Serial.print(",");
  Serial.print(highest); Serial.print(",");
  if (dryRaw >= 0 && wetRaw >= 0 && abs(dryRaw - wetRaw) >= MIN_SPAN) {
    int percent = map(raw, dryRaw, wetRaw, 0, 100);
    Serial.println(constrain(percent, 0, 100));
  } else {
    Serial.println("uncalibrated");
  }
}

• Commands: d = dry endpoint (in air), w = wet endpoint (in water), r = reset.
• MIN_SPAN (150) — if the two endpoints are closer than this the sketch keeps saying uncalibrated.
• ADC_SAMPLES (24) readings are averaged for each value. REPORT_MS (1000) prints once a second.

What you should see

• A header t_s,raw,lowest,highest,percent, then one row per second.
• 'uncalibrated' in the percent column until both endpoints are set.
• Dry soil reading near 0 %, wet soil near the top end — and different numbers when you change depth or packing.

If it doesn't work

Try next

• Repeat with a sandy soil and a potting mix and compare.
• Use your best dry/wet soil endpoints in the main build's soil-dry and soil-wet captures.

More from this kit

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