When crop plants were competing with well-established quackgrass (Agropyron repens) for the same nutrient supply, chlorosis and stunting of crop plants occurred. Apparently this was due to the greater ability of quackgrass to withdraw available nutrients, especially nitrogen, and to a lesser extent phosphorus and potassium, from the nutrient solution. No evidence was obtained for a phytotoxic substance from living quackgrass roots or rhizomes that could inhibit germination or growth of alfalfa and oats. Quackgrass residues in large amounts caused chlorosis and stunting in alfalfa and oats when added to soil. The evidence obtained linked this behavior to the high C:N ratio with consequent temporary deficiency of available nitrogen. Water extracts of quackgrass leaves or rhizomes were not phytotoxic unless subjected to high temperatures during or subsequent to extraction. If soils contained decomposing quackgrass or crop plant residues, extremely phytotoxic extracts of such soils were obtained, but only when soils were saturated and decomposition was anaerobic.