Validity of Simple Form of Fick's First Law in Self and Tracer Diffusion

General thermodynamic equations describing diffusion flux have been developed, and the special cases of self-and tracer-diffusion are considered; on this basis, the validity of the simple form of Fick's first law in these latter cases is examined. Relations based on irreversible thermodynamics for transport processes are also considered for the purpose and also for discussing the behaviour of the background electrolyte in self-and tracer-diffusion. Implications of these various deductions in soil systems have been discussed.