A mechanistic model is introduced for resource competition between plant species on the basis of the equations of De Wit (1960). The zero-isocline diagrams that can be drawn using this model produce qualitative predictions about the plant features that contribute to the success of plants in nutrient-poor and nutrient-rich environments, respectively. The relationship between the model presented and the Lotka-Volterra and the Tilman model is discussed. An example is presented where the presented theory applies to observations in the field and to the results of competition experiments.