A simple model is developed to investigate the role of spatial dynamics in the soil-atmosphere system. The model is constructed by considering the mass and energy balance equations for soil and atmosphere, closed with a two-dimensional, corrected quasigeostrophic approximation for large-scale atmospheric motions, and a suitable model for rainfall occurrence. The main result presented concerns the linear stability analyses of the homogenous equilibrium solutions for dry and wet climates. In the first case (desert) the system is stable and no spatial perturbation internal to the system can produce spatial heterogeneities. In the second case (wet climate) the dynamics can produce spatial instabilities of several scales, resulting in likely patterns of wet and dry zones. A key role in triggering this instability is played by the sensible heat flux to the atmosphere, which in turn is linked to soil moisture.