Soil amendment with biochar is currently proposed as a management strategy to improve soil quality and enhance plant productivity. Relatively little is known about how biochar affects plant competition, although it has been suggested that it can increase the competitive ability of legumes. This study tested the impact of a biochar on the competitive ability of legumes through alterations to soil pH and/or nutrient availability. Biochar was produced from aboveground plant biomass from a species-rich semi-natural grassland pyrolysed at 400 °C. In a greenhouse experiment, a legume (red clover, Trifolium pratense L.); a grass (red fescue, Festuca rubra L.); and a forb (plantain, Plantago lanceolata L.) were grown in (1) monocultures, (2) in a mixed culture of red fescue and red clover, and (3) in a mixture of all three species. Soil treatments included fertilisation with nitrogen (N), potassium (K), phosphorus (P), or micronutrient fertiliser in the presence or absence of biochar; a pH-adjusted control soil; and a control (i.e. with no amendment). The competitive ability of red clover was quantified as the proportion of aboveground biomass of this species within the mixtures. Both biochar amendment and K fertilisation significantly (P <0.001) increased red clover biomass, and increased the competitive ability of red clover when grown with red fescue and plantain. Application of N fertiliser, irrespective of biochar amendment, resulted in significantly (P <0.001) greater red fescue and plantain biomass and eliminated the competitive advantage of red clover. The biochar-mediated pH increase did not affect red clover biomass or its competitive ability. We conclude that biochar has a beneficial effect on red clover under N limiting conditions due to an increase in K availability. Our results suggest a potential role for biochar to maintain the proportion of forage legumes in agricultural pastures or semi-natural grasslands.