Landmine detection by ground-penetrating radar (GPR) becomes challenging when soil is inhomogeneous. Soil inhomogeneity causes unwanted reflections (clutter) which disturb reflections from landmines. Thorough investigations on the influence of soil inhomogeneity and clutter on GPR are important for the use in demining to assess the performance and ensure the safety of the operation. In order to observe the influence of soil inhomogeneity an irrigation test was carried out and GPR data were collected after the irrigation while soil water content distribution was changing. Correlation length and variability of soil electric properties are determined by geostatistical analysis from GPR data. The theoretical calculation of Mie scattering using the determined parameters is in good agreement with power of clutter extracted from GPR data. Therefore it is demonstrated that scattering by soil inhomogeneity is governed by Mie scattering.