Noninvasive and nondestructive root phenotyping techniques under field conditions are needed to advance plant root science. Soil electrical parameters including capacitance and resistance hold potential to meet this need, although their specific ability to detect roots remains uncertain. In this study, we developed a two-phase root and soil permittivity model at high frequency enabling accurate and noninvasive prediction of root volume. The validation calculation showed that the two-phase model successfully predicts root volume with a minimal error of less than 1 % under the control conditions with roots placed in wet soil and water. Furthermore, we demonstrated that the high-frequency bulk soil capacitance when integrated into an empirical model, can accurately estimate root volume for summer maize (Zea mays L.) in both the field (root mean square error (RMSE) = 1.52 cm3) and in root washing-free measurement applications in the absence of soil (RMSE = 0.36 cm3). When the empirical relationship between high-frequency bulk soil capacitance and root volume is not adequate, the two-phase model can be used to calibrate the empirical model. To this end, a series of soil pot experiments with winter wheat (Triticum aestivum L.) showed that the measurement accuracy can be fundamentally improved. These results highlighted the effectiveness of the two-phase permittivity model, making high-frequency bulk soil capacitance a reliable method for in situ measurement of root volume. It is anticipated that the availability of new equipment for measuring high-frequency soil electrical parameters will lead to widespread adoption of this root phenotyping technique in the future.