PERSISTENCE OF THE INHIBITORY EFFECTS OF PHOSPHOROAMIDES ON UREA HYDROLYSIS IN SOILS

The persistence of the inhibitory effects of three phosphoroamides [N-(n-butyl) thiophosphoric triamide (NBPT), phenylphosphorodiamidate (PPD), and thiophosphoryl triamide (TPT)] on urea hydrolysis in soils was assessed by measuring the ability of four soils to hydrolyze urea after they had been treated with 5-mu-g phosphoroamide/g soil and incubated at 15-degrees-C or 30-degrees-C for 0, 3, 7, 14, or 28 days. The soils used differed markedly in pH, texture, and organic-matter content. The data obtained showed that the persistence of the effects of the phosphoroamides studied decreased with increase in soil temperature from 15-degrees-C to 30-degrees-C and that whereas the effect of PPD decreased with increase in the time of incubation, the effects of NBPT and TPT sometimes increased before decreasing with increased time of incubation. These observations are in harmony with the recent findings that PPD is a potent inhibitor of urease activity, but decomposes in soils with formation of phenol, which is a relatively weak inhibitor of urease activity, whereas NBPT and TPT do not inhibit urease activity but decompose in soil with formation of their oxon analogs, which are potent inhibitors of urease activity. The inhibitory effect of NBPT on urea hydrolysis was considerably more persistent than that of PPD or TPT and was significant even after incubation of NBPT-treated soil at 15-degrees-C or 30-degrees-C for 28 days.