South African lovegrass (Eragrostis plana Nees) is an aggressive and difficult-to-control species in grazing areas of the south of South America, whose invasion capacity is increased by its phytotoxic capability. The objectives of this work were to identify and quantify chemicals produced by the plant shoots, to evaluate the inhibitory capability of plant extracts on development of lucerne (Medicago sativa L.) and wheat (Triticum aestivum L.), and to determine the spatial distribution of phenolics in rhizosphere soil in an infested field. Extracts were obtained by exhaustive extraction of dry shoot biomass, using solvents in increasing order of polarity. Soil samples contained in the rhizosphere were collected from an infested field. Bioassays for determination of phytotoxicity of extracts on lucerne and wheat seedlings were conducted in a completely randomised design with four replicates. The extracts showed suppressive, stimulating or neutral effects on the development of the radicle, shoots, and total lucerne and wheat seedlings, with greater phytotoxicity, generally, of ethyl acetate and methyl alcohol extracts. In the shoot extracts, nine phenolic compounds were identified and quantified: caffeic acid, coumaric acid, ferulic acid, gallic acid, vanillic acid, catechin, epicatechin, resveratrol, rutin. Of these, four were identified in the soil: catechin, epicatechin, coumaric acid, ferulic acid. Catechin, epicatechin and ferulic acid showed similar distribution patterns in the soil profile, with higher concentrations detected on the surface and on the side opposite the plant crown, which indicates release by the decomposition of tissues deposited on the soil surface. Coumaric acid showed higher concentrations on the surface and in the deeper layer of the soil next to the plant, indicating release both from decomposition of the shoots and from the root system. Future work may explore the phytotoxicity of the analytes identified, either isolated or in admixture, using dose curves and an inhibitory response to target plant species.