三、 Surface charge and potential of soil (-) Cause and categories of the soil charge 1. Permanent charge-The net negative(or positive charge of clay particles inherent in the crystal structure of the particle; not affected by changes in ph or by ion exchange reactions 2, Variable charge-pH dependent charge Zero point of charge(ZPC)-The pH value of a solution in equilibrium with a variable charge material or mixture of materials whose net charge from all sources is zero三、Surface charge and potential of soil （一）Cause and categories of the soil charge 1、Permanent charge-The net negative (or positive) charge of clay particles inherent in the crystal structure of the particle; not affected by changes in pH or by ion￾exchange reactions. 2、Variable charge- pH dependent charge Zero point of charge (ZPC) –The pH value of a solution in equilibrium with a variable charge material or mixture of materials whose net charge from all sources is zero