This study developed and validated a simple method to study allelopathic effects of belowground root interactions between two plant species. The objective was to separate resource competition for water, nutrients and light from phytotoxic root interactions between an allelopathic donor plant (cover crop) and a sensitive receiver plant (annual weed). Experiments were done in non-sterile soil in pots under controlled conditions in a phytotron. Water and nutrient supply were kept constant in sufficient amounts throughout the experiment. The effect of shading was evaluated by the presence or absence of vertical nets between the competing plants. Growth repressing allelopathic root interactions were studied by separating the rhizospheres of the weed and the cover crop species with impenetrable plastic barriers in one condition and by allowing interplant root contacts in the other condition. The growth repressing effect of Fagopyrum esculentum Moench (common buckwheat) and Sorghum bicolor (L.) Moench x Sorghum sudanense (Piper) Stapf (sorghum sudangrass hybrid) on Amaranthus retroflexus (L.) (redroot pigweed) was very strong (91% and 68% respectively). It was due to both competitive shading effects (64% and 21% respectively) and root interactions (75% and 60% respectively) of potentially allelopathic nature.
