I picked up some inexpensive soil moisture monitors from Banggood. I found a really nice youtube tutorial where the guy hooks up an Arduino and an oled to sense moisture. The readings are scalled to 4 text values: dry, damp, moist and wet. This text is then displayed on an OLED.

The sketch and schematic is in the Arduino Projects directory at:

_tgDocuments/Arduino-projects/Capacitive_Soil_moisture

The sample code is also here:

/*********************************************************************
  This is an example for reading the analog sensor output of a
  capacitive soil moisture probe and displaying the result on a
  128x64 OLED display over the I2C bus.

  This example requires the SSD1306 display driver by Adafruit:
  https://github.com/adafruit/Adafruit_SSD1306

  Video detailing the capacitive soil moisture measurement probe:
  https://youtu.be/pdGRs7GXBeE

*********************************************************************/

#include <Wire.h>
#include <Adafruit_SSD1306.h>

#define OLED_RESET 4
Adafruit_SSD1306 display(OLED_RESET);

// make sure the SSD1306 driver is configured for 64 line height
#if (SSD1306_LCDHEIGHT != 64)
#error("Height incorrect, please fix Adafruit_SSD1306.h!");
#endif

const int wetProbe      = 300;   // wet readings are around 1.5v or analog input value ~300
const int dryProbe      = 620;   // dry readings are around   3v or analog input value ~620


// Analog Sensor Input Pin
int sensorInput         = 0;    // soil probe is on analog input 0

// Variables
int validSensorReading  = 0;    // valid sensor analog reading to record
int sensorReading;              // new sensor reading to evaluate and record or discard
int sensorResult;               // scaled sensor data [0..3] = [wet, damp, moist, dry]


void setup()   {

  Serial.begin(9600);           // some debug messages will display sensor raw data

  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);  // initialize with the I2C address of the display
  display.clearDisplay();                     // clear the display buffer
}


void loop() {

  sensorReading = analogRead(sensorInput);
  if (abs(validSensorReading - sensorReading) > 10) {
    validSensorReading = sensorReading;
  }

  Serial.print ("Old Sensor Reading: ");
  Serial.println (validSensorReading);
  Serial.print ("New Sensor Reading: ");
  Serial.println (sensorReading);

  sensorResult = map(validSensorReading, wetProbe, dryProbe, 0, 4);  // scale analog input to a smaller range for wet to dry

  Serial.print ("Scaled Sensor Reading 0-4: ");
  Serial.println (sensorResult);
  Serial.println ();


  // display the correct soil moisture level on the display
  // lower voltages represent more wet levels
  switch (sensorResult) {
    case 0:
      displayTextProbe("Wet");
      break;
    case 1:
      displayTextProbe("Damp");
      break;
    case 2:
      displayTextProbe("Moist");
      break;
    case 3:
      displayTextProbe("Dry");
      break;
    case 4:    // same as case 3, due to how map works.
      displayTextProbe("Dry");
      break;
  }
  delay(500);
}

void displayTextProbe(const char * sensorDisplay) {
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(0, 0);
  display.clearDisplay();
  display.println("Soil: ");
  display.println(sensorDisplay);
  display.display();
  delay(1);
}