We studied the symmetry of belowground competition at the level of species using a model of plant competition at the scale of individual roots. This model predicts the nutrient uptake of plants with varying root densities and uptake kinetics, competing for the same resource, at equilibrium between resource supply and plant uptake. A symmetry parameter is derived from this model. The analysis of the model behaviour suggests that the symmetry of belowground competition depends on the diffusive supply of the nutrient (buffer capacity × effective diffusion coefficient) for which competition occurs. As the diffusive supply is determined to a greater extent by soil water content and soil type rather than by nutrient type, this means that the symmetry of belowground competition should vary with soil water content and soil type. In particular, our model suggests that competition for nutrients that are primarily supplied by diffusion through the soil is (1) symmetrical at high diffusive supply when root uptake kinetics are identical or at low diffusive supply when roots are randomly distributed in the soil whatever their uptake kinetics are, (2) size-asymmetrical at high diffusive supply when root uptake kinetics are different, (3) partially symmetric or size-asymmetric at low diffusive supply when root are not randomly distributed in the soil.