Noncompetitive Effects of Johnsongrass (Sorghum halepense) on Soybeans (Glycine max)

Abstract

Studies were conducted in North Carolina to determine if johnsongrass [Sorghum halepense(L.) Pers.] rhizomes in the soil exude or produce substances that can affect soybean [Glycine max(L.) Merr.] seedling growth. Fresh and dry weights of soybean seedlings were dependent on the concentration of rhizomes present in the soil before soil samples were used for soybean growth, and on the time of year when soil samples were collected. In studies on rhizome residues, soybean seedling dry weight decreased as percent of dry, decayed rhizomes in the soil increased. For each rhizome concentration, soybean growth inhibition decreased as decay time increased. Fresh and dry weights of soybean plants were reduced by diluted fresh rhizome extract used for irrigation once a week for 3 weeks. Since no rhizomes were present in most of the soils during soybean growth and since nutrient concentrations, organic matter, and pH were not different among the soils used, it is suggested that johnsongrass rhizomes living or decaying in the soil exude, contain, or produce substances that exhibit allelopathic characteristics to soybean growth.