In this paper the results of the research being carried out on the impulse characteristics of soils and more specifically the ionization voltage gradient are presented. The determination of the soil ionization voltage and the respective gradient using a universally accepted method is yet to be established. Aiming at meeting this need, the present paper critically presents these methods and records the ionization gradient values for soil types, which have derived from different experimental approaches. The comprehensive study of these methods is intended to serve as a guide for researchers studying the nonlinear phenomena developed in the soil.