EFFECTS OF PREEMERGENCE AND POSTEMERGENCE HERBICIDES ON UREA HYDROLYSIS AND NITRIFICATION OF UREA NITROGEN IN SOIL

The influence of 5 and 50 mg active ingredient kg-1 soil of nine preem ergence and nine postemergence herbicides on transformations of urea N in soil was studied in samples of two coarse-textured and two fine-te xtured soils incubated aerobically at 20-degrees-C. The effects of eac h herbicide on soil urea transformations was measured by determining t he amounts of urea hydrolyzed and the amounts of NO3- and NO2- produce d at various times after treatment with urea. Applied at the rate of 5 mg active ingredient kg-1 soil, none of the herbicides retarded urea hydrolysis in the four soils used, but four of the postemergence herbi cides (acifluorfen, diclofop methyl, fenoxaprop ethyl, and tridiphane) retarded nitrification of urea N in the two coarse-textured soils. At 50 mg active ingredient kg-1 soil, four of the postemergence herbicid es (acifluorfen, diclofop methyl, DPX-6202, and fenoxaprop ethyl) reta rded urea hydrolysis in the two coarse-textured soils. All the herbici des tested except siduron retarded nitrification in the two coarse-tex tured soils when applied at 50 mg of urea N active ingredient kg-1 soi l, and fenoxaprop ethyl and tridiphane markedly retarded nitrification of urea N in all four of the soils when applied at this rate. One-way analysis of variance and correlation analyses indicated that the inhi bitory effects of the 18 herbicides tested on nitrification of urea N in soil increased with a decrease in the organic-matter content and an increase in the sand content of the soil.