THE FATE OF N‐(4‐CHLOROPHENOXY)‐PHENYL‐NN‐DIMETHYLUREA (C‐1983) IN SOILS AND PLANTS*

Abstract

Summary. The adsorption of N'‐(4‐chlorophenoxy)‐phcnyl‐NN‐dimethylurea (C‐1983) and N‐butyl‐N‐(3,4‐dichlorophenyl)‐ N‐methylurea (ref. comp.) by different soils was investigated with a slurry‐type procedure. Freundlich isothermal equilibrium plots demonstrate that both herbicides, but especially C‐1983, were quite strongly adsorbed, on soil particles. The following concentrations of C‐1983 adsorbed were found to be in equilibrium with 1 ppm soil solutions: 14 γ/g (sandy loam), 40 γ/g (clay loam) and 110 γ/g (humus soil) The corresponding values for the ref. comp. were 6.3, 21 and 60 γ/g. Adsorption of C‐1983 was strong on bentonite (400 γ/g) and less pronounced on cellulose (48 γ/g). Soil pH had no significant effect on adsorption. Desorption equilibrium plots indicate that about 50% of C‐1983 adsorbed was released into solution according to adsorption equilibrium conditions. The remaining 50% came off at a much lower rate.Leaching of C‐1983 and the ref. comp. was studied with soil columns to which 200 mm of simulated rain was applied during a period of 45 hours. At a low soil moisture level (wilting capacity), the bulk of C‐1983 was not removed from the top 1 cm section of all three soils. Leaching was somewhat more pronounced in sandy loam (80% recovery in top 1 cm layer) than in clay loam (90%) or in humus soil (95%). Only traces of C‐1983 were recovered from depths below 10 cm and in water percolating from the columns. The ref. comp. leached more easily than C‐1983, especially in sandy loam (bulk of herbicide below 5 cm level). Leaching of both herbicides, but especially of the ref. comp., was less pronounced at the higher soil moisture level (field capacity wet) than at the lower one (wilting capacity).It is suggested that leaching is not a major factor in removing C‐1983 from the surface soil layers. On the other hand, its very strong adsorption seems to be a decisive factor in controlling its availability insoil solution and thus determining both the initial toxicity and and the residual activity in some types of soils. The practical implications for field use are discussed. Le comportement de la N'‐(4‐chllorophénoxy)‐phényl‐NN‐diméthylurée (C‐1983) dans les terres et dans les plantes 1. Adsorption et lessivage dans divers types de terres