A field experiment compared the ability of several phosphoroamide compounds to retard urea hydrolysis on a fallowed notill (NT) and conventionally tilled (CT) silt loam soil (pH 5.70). Treatments consisted of urea prills (200 kg N ha-1) with and without inhibitors (4.0 kg ha-1) and a no-fertilizer check. Fertilizers were surface-applied to 10-cm-diameter polyvinyl chloride (PVC) cylinder microplots partially embedded in the soil. Duplicate microplots were removed at intervals after fertilization and analyzed for the quantity of urea remaining. Results indicate that, of the compounds tested, phenyl phosphorodiamidate (PPD) most effectively inhibited urea hydrolysis. Addition of PPD reduced the initial (first 4 to 10 d after fertilization) rate of hydrolysis by 60 to 70% in three of four trials conducted. A laboratory study conducted on a soil at two contrasting initial pH values (5.6 and 7.4) showed that PPD was more effective in the acidic than in the alkaline soil. Two other inhibitors (UI4 and UI5) retarded hydrolysis to a greater degree than did PPD in the alkaline soil. In the same laboratory study phosphoroamide (UI6), not tested in the field, was found to be least effected by soil pH and showed promise as a urease inhibitor in both acidic and alkaline soils. Field and laboratory studies indicate that urea was hydrolyzed 2.3 to 3 times faster when added to corn-residue-covered surfaces than to bare soil. This finding suggests that the residue contained high levels of urease activity.
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