Existing continuum models of multiphase flow in porous media are unable to explain why preferential flow (fingering) occurs during infiltration into homogeneous, dry soil. Following a phase-field methodology, we propose a continuum model that accounts for an apparent surface tension at the wetting front and does not introduce new independent parameters. The model reproduces the observed features of fingered flows, in particular, the higher saturation of water at the tip of the fingers, which is believed to be essential for the formation of fingers. From a linear stability analysis, we predict that finger velocity and finger width both increase with infiltration rate, and the predictions are in quantitative agreement with experiments.