Plant roots have considerable impact on the mechanical stability of soil, but to date the underlying mechanisms have been poorly quantified. In this study, controlled laboratory studies of soil reinforced with willow trees (Salix viminalis cv;Tora) found a strong correlation berween the cross-sectional area of soil covered by roots and shear reinforcement. We separated broken versus pulled-out roots and measured individual root diameters crossing the shear-plane. The shear strength of planted specimens compared with non-planted specimens increased eight-fold at 0.10-m shear depth, more than four-fold at 0.25-m depth, and more than doubled at 0.40-m depth. These data were used to evaluate several models of root-reinforcement. Models based on catastrophic and simultaneous failure of all roots overpredicted reinforcement by 33% on average. Better agreement berween experimental and model results was found for a stress-based fiber-bundle-model, in which roots break progressively from weakest to strongest, with the load shared on the remaining roots at each step. Roots have a great capacity to reinforce soils, with existing models providing reasonable predictions of increased shear strength. However, deterministic understanding and modeling of the processes involved needs to consider root failure mechanisms. In particular, the role of root stiffness and root-soil adhesion is not considered in existing models of soil reinforcement by plant roots.