The contrast in the dielectric constant between a landmine and the surrounding soil is one of the most important parameters to be considered when using ground penetrating radar (GPR) for landmine detection. For most geologic materials the dielectric constant lies within a range of 3 to 30, with dry sand at the lower end of this range at about 3 to 5. Nonmetallic antitank landmines have dielectric constants within a range of about 3 to 10 depending on their composition. A model was developed to predict whether or not field conditions are appropriate for use of GPR instruments. The predictions of this model were validated using GPR profiles in field soils with different soil textures at various soil water contents. Model predictions and field measurements provide convincing evidence that increasing the soil water content around a nonmetallic landmine can improve detection in sand and silt soils. However, data for the clay soils suggest that under elevated soil water conditions detection of nonmetallic landmines are not improved; instead radar images in these soils become worse with increasing soil water content. Data suggest that detection of metallic landmines also degrades with increasing soil water content in sandy soils. The field data are in agreement with the model predictions. Our experimental and model results demonstrate the great potential and the pitfalls of landmine sensors based on GPR. Knowledge of soil texture, dry bulk density, and water content are necessary to determine or predict whether soil conditions are suitable or not for GPR mine detection. The model presented here can be useful for making this determination.