In multi-species grassland communities, plant growth and development are modified by both a number of habitat-related factors and physico-chemical processes resulting from the neighbourhood of other species. Plant interactions mediated through chemical substances are identified within the allelopathic processes. The allelopathic process involves excretion of bioactive compounds from plant or microorganisms that inhibit or stimulate physiological processes of the neighbour plants. The allelopathic compounds can exert a harmful impact on the emergence of seedlings, initial development and installation. Therefore, it is essential to reduce the allelopathic influence of the old sward on the plants sown as well to limit the interaction between these new plants. It is claimed that, among others, fertilizer components may partially reduce effects of allelopathic influences In this study we show the differences in growth inhibition of Ph. pratense seedlings caused by the water extracts of leaves of selected grass species and the amelioration of growth inhibition by addition of magnesium sulfate. The bioassays were performed on Petri dishes under the laboratory conditions. The activity of allelopathic substances in the leaf extracts was evaluated by the degree of inhibition of seed germination, seedling height and root length compared to the control objects (supplied with distilled water). The amelioration of the negative allelopathic effects by a complete nutrient component with or without the addition of magnesium sulfate was also evaluated against the appropriate controls and compared to the objects where blotting-paper was moistened only with leaf extracts.The present studies confirmed the defensive activity of magnesium sulfate against the allelopathic compounds of the tested grass species affecting the initial growth and development of Phleum pratense. The obtained results indicate potential elimination of the allelopathic negative influence of plants through suitable fertilization.