9. Soil Moisture Monitoring

Map a capacitive sensor reading between dry and wet reference values. Calibration is essential because sensor modules, soil, and placement vary.

Level: Beginner · Time: 30 minutes

Parts and wiring

• ESP32 DevKit V1 and a common 3.3 V-compatible capacitive soil-moisture sensor

Verify that the analog output cannot exceed 3.3 V. Modules vary slightly by manufacturer, so keep the calibration values for the exact probe you use.

Calibration

Record a reading with the probe in air as DRY_VALUE. Then insert only the probe section into fully wet soil or water and record WET_VALUE; never wet the electronics.

Complete sketch

#include <PxServ.h>
 
const char* WIFI_SSID = "your_wifi_ssid";
const char* WIFI_PASSWORD = "your_wifi_password";
const char* PXSERV_API_KEY = "your_pxserv_api_key";
const int SOIL_PIN = 34;
const int DRY_VALUE = 3000;  // Replace after calibration
const int WET_VALUE = 1400;  // Replace after calibration
 
PxServ client(PXSERV_API_KEY);
 
void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
  PxServ::connectWifi(WIFI_SSID, WIFI_PASSWORD);
}
 
void loop() {
  long total = 0;
  for (int i = 0; i < 10; i++) {
    total += analogRead(SOIL_PIN);
    delay(20);
  }
 
  int rawValue = total / 10;
  int moisture = map(rawValue, DRY_VALUE, WET_VALUE, 0, 100);
  moisture = constrain(moisture, 0, 100);
 
  client.setData("soil/raw", String(rawValue));
  client.setData("soil/moisture-percent", String(moisture));
  Serial.print("Soil moisture: ");
  Serial.println(moisture);
  delay(15000);
}

Test

Compare dry and moist soil while keeping insertion depth and compaction consistent. The value should approach 0% when dry and 100% when wet.

Use the exact calibration values from the probe you have, because the same circuit can behave differently between soil types and sensor batches.