Path tracking controllers for an autonomous vehicle are often designed by using either a dynamic model or a kinematic one and some models are related to wheel-ground contact, that makes the efficiency of the controller highly dependent on the ground parameters estimation, especially for off-road mobile robots intended to navigate in open environments. This paper proposes a new nonlinear observer designed to estimate the front and rear contact cornering stiffnesses in real time, that are related both on tire and soil proprieties. The latter is estimated using steering angles as well as yaw rate and lateral velocity, which are provided by a preliminary Kalman-Bucy observer. The performance of the proposed nonlinear observer combined with the LQR controller is evaluated by both advanced simulations and experiments in real conditions at different speeds.