Soil moisture meter on an Arduino Uno

// Soil moisture meter: an FC-28 resistive soil sensor, powered from a pin
// only while it is read, and the moisture on a 16x2 I2C LCD.
//
//   pin 7   the sensor board's VCC (powered from the pin, see below)
//   GND     the sensor board's GND
//   A0      the board's AO          pin 2   the board's DO
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The fork is two bare prongs, and the soil between them is a resistor: dry
// soil barely conducts, wet soil conducts well. The board puts the fork under
// a 10 k pull-up, so the reading is 1023 in the air and falls as the soil gets
// wetter. Its LM393 compares AO with the blue trimmer and pulls DO low, with
// its second lamp lit, once the soil is wet enough.
//
// # Why VCC is on a pin
//
// A current through wet soil all day is electrolysis: the prongs plate away
// in weeks. So the sensor is powered for a few milliseconds a reading, and
// off the rest of the time. Pin 7 can supply the few milliamps the board takes.
//
// # The numbers
//
// What a fork reads depends on the soil and the salts in it as much as on the
// water, so these bands are a starting point, from the tutorials: over 750 is
// dry, 500 to 750 is about right, under 500 is wet. Calibrate yours in dry and
// in just-watered soil.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int POWER = 7;
const int AO = A0;
const int DO = 2;
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: soil moisture meter");
  pinMode(POWER, OUTPUT);
  digitalWrite(POWER, LOW);
  pinMode(DO, INPUT);
  lcd.init();
  lcd.backlight();
}

void loop() {
  // Power the sensor, let it settle, read it, and switch it off again.
  digitalWrite(POWER, HIGH);
  delay(10);
  int moisture = analogRead(AO);
  bool wet = digitalRead(DO) == LOW;
  digitalWrite(POWER, LOW);

  const char *verdict;
  if (moisture > 750) verdict = "Dry, water me";
  else if (moisture >= 500) verdict = "Just right";
  else verdict = "Wet";

  lcd.setCursor(0, 0);
  lcd.print("Soil: ");
  lcd.print(moisture);
  lcd.print(wet ? "  DO wet " : "  DO dry ");
  lcd.setCursor(0, 1);
  lcd.print(verdict);
  lcd.print("        ");

  Serial.print("Moisture: ");
  Serial.print(moisture);
  Serial.print("  DO: ");
  Serial.print(wet ? "LOW" : "HIGH");
  Serial.print("  ");
  Serial.println(verdict);
  delay(1000);
}