The motive forces of ion diffusion in soils as a non-uniform electric-field system come from two aspects: concentration gradient and electric potential gradient So the Fick's diffusion laws is unsuitable for describing ion diffusion in soils directly. It is necessary to advance new fundamental equations for describing ion diffuiom in soils. Based on the fundamental theory of thermodynamics and dynamics of material diffusion, a set of fundamental equations descriding ion diffusion in soils has been obtained under the given conditions of experiment and through solving the fundamental equations, suecessful predication of ion diffusion in soils has been made.