In this study, we present a radiative-transfer model to simulate reflectance and transmittance of light by soil slabs. The radiative-transfer model implemented, referred to as the beam-tracing model, uses cross-sectional images of scattering media to describe the geometry of the soil, i.e. soil texture and structure. This model traces the optical path of incident beams through such cross-sectional images. Tracing many light beams simulates the reflected and transmitted light intensities for the media. The accuracy of the beam-tracing model was tested by comparing its predictions with those of analytical solutions and with experimentally measured reflectance and transmittance data of soil samples. We implemented this model to explain the systematic deviation between measured data and those calculated with the four-flux model applied in Banninger (2004). With the beam-tracing model, we show that the small systematic inaccuracies of the four-flux model are caused by the simplified description of the geometry of the scattering medium. The beam-tracing model can be used to describe various optical properties of particulate media. As an application, we calculated the decrease of light intensity with depth.