Soil influence on the performance of ground-penetrating radar (GPR) for landmine detection is investigated. Electric resistivity tomography (ERT), time-domain reflectometry (TDR) and GPR are used to measure the electric and dielectric properties of a variety of soils in the field. The results are used to generate soil models that are the basis for the simulations of realistic GPR data. Spatial soil variability shows to be high and causes a significant impact. Permittivity fluctuations show to have a larger influence than fluctuations of conductivity. The soils of a test facility are investigated with geophysical techniques and are classified according to the expected impact on landmine detection. The classification is congruent to the results of a dual-sensor test that was carried out on the facility. Geophysical measurements on soils and geostatistical data analysis can be used to predict the performance of GPR.