Detection and degradation of linuron [3-(3,4-dichlorophenyl) -1 - methoxy -1 - methylurea] in organic soils were studied using biological assays and chromatographic techniques. The level of linuron that caused 50% growth reduction of onion (Allium cepa L.) varied among three soils of greatly differing organic matter contents. Linuron and its metabolites were successfully separated with florisil column chromatography using a five-fraction solvent system; however, only linuron and 3 - (3,4 - dichlorophenyl) -1 -methylurea could be extracted from organic soil samples at satisfactory recovery rates. Soil samples from eight grower fields and from field plots were analyzed to determine the residue level of these two compounds. Quantitative assessment of the compounds was made with high pressure liquid chromatography. Results of the biological assay and chromatographic analysis showed that linuron and its phytotoxic metabolite, 3 - (3,4 - dichlorophenyl) -1 - methylurea, were not accumulating in the organic soils of Ontario and that the levels detected were not phytotoxic to onions when grown in pot bioassays.

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Field and laboratory experiments were conducted to investigate the behaviour of two preemergence herbicides, S-propyl butylethylthiocarbamate (pebulate) and N,N-dimethyl-2,2-diphenylacetamide (diphenamid) in field seeded tomatoes (Lycopersicon esculentum Mill.). In the growth chamber, tomatoes were seeded into three soil types and treated with three rates of pebulate and diphenamid and then grown under three temperature regimes for 25 days. The fresh weights of tops and roots were reduced as the concentration of herbicide was increased, and these weights were further reduced as the temperature was increased and as the clay content of the soil decreased. Under field conditions, neither pebulate nor diphenamid treatments caused these deleterious effects to tomato plants as measured by the number of emerging seedlings and the yield of fruit. Pebulate produced inconsistent and generally poor weed control. Diphenamid gave promising preemergence control of grasses but inconsistent control of broad-leaf weeds. Pebulate left no toxic residues in the soil, however, diphenamid residues remained in the soil to be phytotoxic to wheat planted in the fall of the same year. The penetration of pebulate and diphenamid down a soil column appeared to depend partly on the flow of water through the profile and partly on the adsorption by soil particles. Diphenamid moved down the profile more readily than pebulate.