Arduino capacitance measurement

I`m in the need to measure my plants soil moisture. I've read a lot about the possible options to measure soil moisture like sensing resistance or capacitance. Using the resistance measurement, the probes are sensitive to corrosion due electrolyse so I want to give it a go to measure the capacitance and see what happens.

Here is what I did for the hardware part on my Arduino nano:

This is the sketch I used

#include

// analog defines

// defines for setting and clearing register bits

#ifndef cbi

#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))

#endif

#ifndef sbi

#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

#endif

void setup() {

  Serial.begin(9600);

  Serial.println("StartUp\n");

  // analog init

    analogReference(INTERNAL);

  // set prescale to 16

  sbi(ADCSRA,ADPS2) ;

  cbi(ADCSRA,ADPS1) ;

  cbi(ADCSRA,ADPS0) ;

    

  Serial.println("Init OK \n");

}

void capMeasurement(void)

{

  uint16_t sensorValue = 0;  // variable to store the value coming from the sensor

  static uint16_t sensorHistory;

  sensorValue = 0;

  

  for(int count = 16; count != 0; count--)

  {

    // note that default Wire.h was to slow to use, so these are direct MCU commands.

    // should make a Arduino library from it.

    //pinMode(A0, OUTPUT);

    //pinMode(A1, OUTPUT);

    // the above commented text is done with direct MCU register commands (faster)

    DDRC = DDRC | ~B11111100; // make A0/A1 output

    

    //digitalWrite(A0, HIGH);

    //digitalWrite(A1, HIGH);

    // the above commented text is done with direct MCU register commands (faster)

    PORTC = PORTC | B00000011; //charge external Cx

    delayMicroseconds(10); // let the Cx charge 10uS

    

    //pinMode(A1, INPUT); 

    DDRC = DDRC & B11111101; // make A1 input

    

    //digitalWrite(A0, LOW);

    PORTC = PORTC & B11111100; // transfer charge to C1 (by grounding C1)

    

    // read the value from the sensor:

    sensorValue += analogRead(A1); 

  }

  PORTC = PORTC | ~B11111100; // done 

  Serial.print(" cap=");

  Serial.print(sensorValue);

}

void loop() {

  capMeasurement();

  Serial.println();

}