The solution of the radiation transfer equation in a vegetation canopy requires the specification of appropriate boundary conditions, especially at the interface between the vegetation layer and the soil. This short paper documents the effect of soil brightness and anisotropy on the field of radiances emerging at the top of the vegetation canopy in the solar domain. The study is based on the analysis of the sensitivity of an advanced radiation transfer model. It is shown that the scattering properties of the soil must be carefully assigned to ensure a correct interpretation of data collected over optically thin vegetation canopies. ¿ 1998 American Geophysical Union