As an extension of De Wit's competition theory a theoretical description has been developed of competition between plant populations with different rooting depths. This model shows that in mixtures of plants with different rooting depths the value of the Relative Yield Total can be expected to exceed one. Moreover, it predicts the frequency-dependence of the relative crowding coefficient of the deep rooting population with respect to the shallow rooting population. The relationship between properties of plant species and the environment required to establish a stable equilibrium turns out to be surprisingly simple. The shallow rooting species has to have a larger competitive ability sufficient to compensate for the extra nutrients that are exploited by the deeper rooting plants. The dependence of equilibrium plant frequencies on the properties of plant species and the characteristics of the environment is discussed.