Soil moisture capacitance meter sketch

/*
This ia a Modified sketch of RCTiming_capacitance_meter by Paul Badger 2008
http://www.arduino.cc/en/Tutorial/CapacitanceMeter

To measure the capacitance of a DIY soil moisture sensor
Alexshu.com
 */

#define analogPin      0          // analog pin for measuring capacitor voltage
#define chargePin      12         // pin to charge the capacitor - connected to one end of the charging resistor
#define dischargePin   11         // pin to discharge the capacitor

unsigned long startTime;
unsigned long elapsedTime;
long sum;

void setup(){
  pinMode(chargePin, OUTPUT);     // set chargePin to output
  digitalWrite(chargePin, LOW);

  Serial.begin(9600);             // initialize serial transmission for debugging
}

void loop(){
  for(int i=0; i<50; i++){
  digitalWrite(chargePin, HIGH);  // set chargePin HIGH and capacitor charging
  startTime = micros();

  while(analogRead(analogPin) < 648){       // 647 is 63.2% of 1023, which corresponds to full-scale voltage
  }

  elapsedTime= micros() - startTime;
  sum = sum +  elapsedTime;

  /* dicharge the capacitor  */
  digitalWrite(chargePin, LOW);             // set charge pin to  LOW
  pinMode(dischargePin, OUTPUT);            // set discharge pin to output
  digitalWrite(dischargePin, LOW);          // set discharge pin LOW
  while(analogRead(analogPin) > 0){         // wait until capacitor is completely discharged
  }

  pinMode(dischargePin, INPUT);            // set discharge pin back to input

  }
  Serial.print("Charge time: ");
  Serial.print(sum/50);
  Serial.println(" \t micro seconds");
  sum = 0;
}