Work reported showed that N‐butyl phosphorothioic triamide (NBPT) is considerably more effective than phenylphosphorodiamidate (PPD) as a soil urease inhibitor and merits consideration as a fertilizer amendment for retarding hydrolysis of urea fertilizer in soil. Studies to determine the factors influencing the effectiveness of NBPT for retardation of urea hydrolysis in soil showed that the inhibitory effect of NBPT on hydrolysis of urea by soil urease increased markedly with the amount of NBPT added and decreased markedly with time and with increase in temperature from 10 to 40°C. They also showed that the ability of NBPT to retard urea hydrolysis in 13 surface soils selected to obtain a wide range in properties was significantly correlated with organic C content (r = ‐0.70**), total N content (r = ‐0.76**), cation‐exchange capacity (r = ‐0.67* ), sand content (r = 0.61*), clay content (r = ‐0.63*),and surface area (r = ‐0.66*), but was not significantly correlated with pH, silt content, urease activity, or CaCO3equivalent. Multiple‐regression analyses indicated that the effectiveness of NBPT for retardation of urea hydrolysis in soil tends to increase with decrease in soil organic‐matter content.