The main goal of this work is to analyze the soil ionization effects in different types of grounding grids. The analysis was performed by implementing a software based on the FDTD method, for modeling the non-linear behavior of the ground due the application of high intensity lightning currents. Step voltages on the soil surface were calculated from the border of the grounding grid to a point 1 m away from it. The ionization influence is determined by comparing the results to data obtained by modeling linear ground. The obtained results are physically consistent.