In this paper, the potential use of P-band radar signals for the estimation of soil roughness parameters is analyzed. The Integral Equation Model (IEM) is used to study the sensitivity to soil surface parameters of backscattered P-band signals. A new roughness parameter referred to as Zp, combining the root mean square surface height and the correlation length, is proposed to describe the behavior of P-band radar signals as a function of soil roughness. The IEM model is validated using real data covering a large range of roughness values, derived from experimental airborne P-band SAR campaigns made over agricultural fields. Discrepancies between the measurements and simulations led to the analysis of the influence of low frequency roughness structures on backscattering simulations. The analysis of the behavior of P-band radar signals as a function of multi-scale soil roughness (micro topography and large roughness structures) reveals the complexity of using P-band data for the analysis of bare surface soil parameters.