Soil Moisture sensor

Using Arduino Sensors
13

I'm confused, I thought if you were using hardware serial, it was not recommended to use digital pins 0 and 1 as GPIO.

#define sensorPin3 1
#define sensorPin4 0
...
pinMode(sensorPin3, INPUT);
pinMode(sensorPin4, INPUT);

What happens to sensorPin3 when the first Serial.print() is executed?