First, this paper discusses ion diffusion kinetics in soil systems theoritically. Ion diffusion kinetics in soils can be classified into three types: negative first-order, zero-order and first-order kinetics. Based on this, an experiment on Mg²⁺ diffusion kinetics was done in soil and quartz sand systems. The results showed that Mg²⁺ diffusion kinetics in the two systems were a first-order kinetic process. But at the temperature of 298K and in the initial stage of diffusion, first-order and negative first-order kinetic processes coexisted in Mg²⁺ diffusion in the soil system. Through the study of Mg²⁺ diffusion in quartz sand systems, we also found that an exchange reaction might take place between Mg²⁺ and hydrogen at the hydration SiO₂ surface, and the Mg²⁺ which took part in exchange reaction was at diffused in the quartz sand within the initial ten minutes.