Plant symptoms indicating threshold soil herbicide levels on a Typic Quarzipsamment, Eufaula fine sand, are described and illustrated for five long‐residue phytotoxic compounds from three chemical families: the substituted ureas, the triazines, and the pyridines. Cucumis sativa developed marginal chlorosis with 1 ppmw fluometuron (3 (m‐trifluoromethylpheny1)‐1,l‐dimethylurea); palmate veinal chlorotic pattern with 1 ppmw prometryne (2‐methylmercapto‐4,6‐bis (isopropylamino)‐1,3,5‐triazine); auxin‐type manifestations of meristematic tissues with 0.01 ppmw picloram (4‐amino‐3,5,6‐uichloro‐picolinic acid); and an interveinal chlorotic pattern with 1 ppmw pyriclor (2,3,5‐trichloro‐4‐pyridinol). Robinia pseudoacacia developed severe leaf curling and stem twisting with tendril type appearance and restricted club‐like root system at 1 ppmw N‐serve (2‐chloro‐6‐(trichloromethyl) pyridine).

Bioassay procedures with leaf disks from Phaseolus vulgaris L., Indian bean, were used to determine factors influencing ( 1,1′‐dimethyl‐4,4'bipyridinium) paraquat activity. Relative phytotoxicity was determined by varying herbicide levels from 0 to 1.0 ppm ai with light intensity, temperature and reaction time. Reduction of extractable chlorophyll was closely related to increased paraquat concentrations. Increased time, temperature and light intensity accelerated rate and magnitude of leaf disk chlorosis induced with active paraquat levels.