This is because the negative charge on the Earth has two parts. Part one is the negative charge permanently (permanent charge) and the second part is a pH dependent charge.
This is because the negative charge on clay has two parts. Part One is a permanent negative charge (permanent charge) and the second part is pH dependent charge.
Because of the negative ions in the soil has two parts. The first part is a permanent negative charge (permanent charge) and the second part is a pH dependent charge.