Purple nutsedge (Cyperus rotundus) foliage and tubers were tested for allelopathic potential against the following weeds; jute Chorchorus olitorius (broad leaved weed), and barnyard grass Echinochloa crus-galli (grassy weed) associated with soybean. In the greenhouse, during 2008 and 2009, foliage and tubers of C. rotundus were mixed with soil surface at 20, 40, 60 and 80 g/kg. C. rotundus negatively affected those of jute and barnyard grass particularly at 80 g. Jute was more susceptible to allelopathy by C. rotundus than barnyard grass. Inhibition in weed dry matter was higher with tuber than foliage residues. Tuber residues reduced the dry weight of jute and barnyard grass by 85.96% of the control and by 58.28% with 80 g, respectively. On the other hand, soybean growth and yield showed a high significant increase compared with unweeded pots. A high-performance liquid chromatography analysis showed that C. rotundus foliage contained the following phenolic acids: caffeic, ferulic, coumaric, benzoic, vanelic, chlorogenic and cinnamic.Tubers contained hydroxybenzoic, caffeic, ferulic, vanelic and chlorogenic.