Despite its interest for researchers and various practitioners in the last thirty years or so, a complete theoretical description of a drying bare soil has not been presented. When a bare soil is wet enough, evaporation occurs at its surface. But as drying proceeds, the soil hydraulic conductivity for liquid water declines steeply and evaporation occurs in a zone just below the surface. Proper treatment of the diurnal dynamics of this zone requires a theory of coupled heat and mass transfer (of water in both liquid and vapour phases). The energy balance at the soil surface varies dramatically when evaporation shifts from the surface to below it, although fluxes of heat and water vapour from the soil vary less. In this talk I will describe the relevant physics, mathematics, and solution using a commercially available finite-element program (FEMLAB).