The growing importance of urea fertilizer in world agriculture has stimulated research to find compounds that will retard hydrolysis of urea by soil urease and thereby reduce the problems encountered in use of this fertilizer. Numerous compounds have been patented or proposed as inhibitors of urea hydrolysis in soil, but only phenylphosphorodiamidate (PPD) has attracted significant attention. Recent work showed that PPD was the most effective of 12 phosphoroamides evaluated as soil urease inhibitors and was considerably more effective than other compounds proposed for inhibition of urease activity in soil;The potential value of six new phosphoroamides for reduction of the problems encountered in use of urea fertilizer was studied by determining the effects of different amounts of these compounds on urea hydrolysis, nitrification, denitrification, nitrite accumulation, mineralization of organic N, and gaseous loss of urea N as NH(,3) in soils treated with urea. The phosphoroamides used were N-(diaminophosphinyl)-cyclohexylamine (DPCA), N-benzyl-N-methyl phosphoric triamide, diethyl phosphoric triamide, trichloroethyl phosphorodiamidate (TEPD), dimethyl phosphoric triamide, and N-butyl phosphorothioic triamide (NBPT). The soils used were selected to obtain a range in properties, and the effects of the six phosphoroamides studied were compared with those of two compounds known to be among the most effective compounds thus far proposed for retarding urea hydrolysis in soils (PPD and hydroquinone);All six of the phosphoroamides evaluated compared favorably with hydroquinone as soil urease inhibitors and two of them, NBPT and DPCA, were superior to PPD for retarding urea hydrolysis, nitrite accumulation, and ammonia volatilization in soils treated with urea. None of the phosphoroamides studied significantly affected denitrification of nitrate or mineralization of organic N (as alanine) when applied at the rate of 10 or 50 (mu)g g('-1) soil, and only TEPD had a significant inhibitory effect on nitrification when applied at the rate of 5 or 10 (mu)g g('-1) soil;The work reported indicates that NBPT is the most effective compound thus far proposed for inhibition of soil urease activity and deserves consideration as a fertilizer amendment for retarding hydrolysis of urea fertilizer in soil.