Soil amended with plant debris and (or) residues has often been used in bioassays for allelopathy. The objective of the present study was to determine if washed test material could be used as a control for keeping organic matter constant when testing for the presence of allelochemicals in suspected allelopathic plants. Leaves of an allelopathic perennial weed Pluchea lanceolata, were soaked in distilled water for 72 h. The filtrate was identified as leaf leachate of unwashed leaves, and resultant leaves were identified as leaves with one washing. Leachates from sequential washings of P. lanceolata leaves with water, and soil amended with leaves of these washings showed variations in total phenolic content. Results demonstrated that phenolic concentrations of aqueous leaf leachate decreased with each washing. However, phenolic concentration was not different among soils amended with leaves with two, three, four, five, and six washings. In soils amended with leaves of different washings, the growth responses of mustard and wheat showed similar trends to the phenolic concentration in the amended soils.