Previous laboratory studies have shown that nitrapyrin (2‐chloro‐6‐(trichloromethyl) pyridine) can produce phytotoxicity in certain plants. Nitrapyrin hydrolyzes in soil to 6‐chloropicolinic acid; however, the phytotoxic properties of the latter compound have not been compared with those of the former. Therefore, studies were conducted to compare the relative toxicity of nitrapyrin and 6‐chloropicolinic acid to the seedlings of various crop plants, to compare their phytotoxicity in a variety of soils, and to estimate if the soil concentration, of either chemical, required to produce significant growth reduction is less than recommended field application rates for nitrapyrin. Nitrapyrin was more toxic than 6‐chloropicolinic acid to the graminaceous species tested which included corn (Zea mays L.), sorghum (Sorghum vulgate L.), wheat (Triticum aestivum L.), and rice (Oryza sativa L.). 6‐Chloropicolinic acid was more toxic than nitrapyrin to the dicotyledenous species tested which included cotton (Gossypium hirsutum L.), sugar beet (Beta vulgaris L.), tomato (Lycopersicon esculentum L.), alfalfa (Medicago sativa L.), and soybean (Glycine max L.). The dicotyledenous species were more sensitive to both compounds than were the graminaceous species. All species tested except alfalfa and tomato tolerated soil concentrations of nitrapyrin estimated to be greater than the highest recommended field application rates. Nitrapyrin was more highly adsorbed and therefore detoxified to a greater extent than 6‐chloropicolinic acid when soil organic matter increased.

Nitrapyrin (2‐Chloro‐6‐(trichloromethyl) pyridine) is a selective nitrification inhibitor and 6‐chloropicolinic acid is its principal metabolite in plants and soil. Nitrapyrin has been found to produce more phytotoxicity on some graminaceous species than 6‐chloropicolinic acid, and the opposite has been found for some dicotyledenous species. This study was conducted to determine if the differences in activity observed with nitrapyrin and 6‐chloropicolinic acid on some monocotyledenous and dicotyledenous species could be due, at least in part, to differences in the site of exposure of the seedlings to the two chemicals. Chemically treated bands of soil were placed either above or below the seeds of cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.), and the effect of the chemicals was assessed by weighing top growth of the plants. Nitrapyrin reduced the top growth of both cotton and wheat when exposure occurred through both the roots and shoots of the germinating seedlings. 6‐Chloropicolinic acid reduced the top growth of both species when exposure occurred through the roots only. The site of exposure of the germinating seedlings to the chemicals influences the degree of phytotoxicity obtained. The inherent activity of each compound against each species, however, appears to be a more important factor with regard to phytotoxicity.