Factors influencing transformation rates and formation of products of phenylurea herbicides in soil.

Transformation rates of phenylurea herbicides and their products were measured in native soil, sterile soil, soil suspensions, and soil inoculated with pure cultures of microorganisms. In native soil, transformation rates generally increased with decreasing adsorption of the herbicides, but correlations with adsorption coefficients were poor. In sterile soil, substitution patterns of the compounds influenced transformation rates. In soil suspensions, transformation rates increased with lipophilicity of the herbicides. In sterilized soil inoculated with specific microorganisms, transformation was mainly influenced by substrate specificities of the microorganisms to reactive sites of the phenylureas. In all cases, N-demethylation was an important, but not the only, transformation pathway. The data indicate that transformation rates of phenylureas in soils are affected by several parameters, related to the soil, the compounds, and the type of transformation. Although the results were gained in the laboratory under artificial conditions, they form a basis to establish quantitative structure-reactivity relationships and provide explanations for quality and quantity of the formed products.