EVALUATION OF UREA FERTILIZERS AND UREASE INHIBITORS FOR CORN AND WHEAT PRODUCTION (PHOSPHOROAMIDE, HYDROLYSIS)

Urea is the major solid N fertilizer in world agriculture, however, ammonia volatilization from surface-applied urea fertilizers can result in urea producing lower crop yields than other N fertilizers. Urease inhibitors retard urea hydrolysis and, in laboratory studies, reduce ammonia volatilization loss from urea fertilizers. Six phosphoroamide urease inhibitors trichloroethyl phosphorodiamidate, diethyl phosphoric triamide, dimethyl phosphoric triamide, N-(diaminophosphinyl)-cyclohexylamine, N-benzyl-N-methyl phosphoric triamide, and phenylphosphorodiamide were added to urea and urea-ammonium nitrate (UAN) fertilizers that were surface-applied in the fall and spring of two years to winter wheat (Triticum aestivum L.) grown on Ryker silt loam (Typic Paleudalf), Rensselaer loam (Typic Argiaquoll), and Avonburg silt loam (Aeric Fragiaqualf). Grain yield and N uptake was generally higher from application of urea or (NH(,4))(,2)SO(,4) than from UAN, presumably due to greater NO(,3)(\u27-) leaching and/or denitrification from UAN. Ammonia volatilization was not considered a problem in the wheat experiments and urease inhibitors when added with urea fertilizers were not effective in increasing grain yield or N uptake. In 1983 experiments with corn (Zea mays L.) grown under no-till Raub silt loam (Aquic Argiudoll) and conventional till Chalmers silty clay loam (Typic Haplaquoll) practices, urea prills produced higher grain yield and tissue N content than UAN solution. No differences were observed between broadcasted and banded UAN or between fertilizers with and without urease inhibitors. In 1984, phenylphosphorodiamide and N-(n-butyl) thiophosphoric triamide added to urea prills significantly increased grain yield and N uptake of no-till corn grown on Chalmers silty clay loam and Avonburg silt loam. However, urease inhibitors were not effective when added to UAN. These studies show that urease inhibitors can be beneficial when urea is surface-applied on no-till corn, under conditions of adequate moisture to promote urea hydrolysis and ammonia volatilization, but without sufficient precipitation to move the urea into the soil