RIDGE-TILL CORN AND UREA HYDROLYSIS RESPONSE TO NBPT

Ammonia volatilization from surface-applied, urea-based fertilizers ca n result in reduced fertilizer N availability to the crop, particularl y in minimum tillage systems with substantial surface residue, The obj ective of this study was to evaluate the potential for the urease inhi bitor N-(n-butyl)thiophosphoric triamide (NBPT) to minimize ammonia lo ss and preserve fertilizer N availability of urea and urea-ammonium ni trate (UAN) solution applied to ridge-till, irrigated corn (Zea mays L .). The study was conducted at three sites over 3 yr, 1990 to 1992, Th e soil for all three site-years was a Crete silt loam (fine, montmoril lonitic, mesic Pachic Argiustoll), Urea and UAN solution (28% N) were applied following planting via two methods, broadcast (BR) and surface band (SE), at two N rates, 100 and 200 lb/acre, with and without NBPT at the rate of 1.0 lb/acre, A nonvolatile check of UAN knifed into th e soil, with and without NBPT, was applied each year, Grain yield, sto ver yield, ear leaf N and chlorophyll, basal stalk nitrate, grain N, g rain moisture, and stover N were evaluated, A lab incubation study was also conducted to evaluate the effect of subsurface band application of UAN with and without NBPT on the rate of urea hydrolysis and fertil izer N availability to the crop, Nitrogen did not Limit yield in 1990, Precipitation soon after fertilization in 1990 and 1991 resulted in l ittle or no benefit from the use of NBPT. Limited precipitation and lo w humidity for an extended period following fertilization in 1992 resu lted in a 56.6 bu/acre increase in yield when NBPT was applied with ur ea (averaged over rates and application methods), but no yield increas e when NBPT was applied with UAN, No differences in yield were observe d between BR and SE application methods. Parameters which are indicati ve of N use efficiency, such as grain N, basal stalk nitrate, leaf N a nd chlorophyll, responded to source, rate, and application method vari ables even when grain or stover yield did not, The lab incubation foun d that urea hydrolysis was reduced when NBPT was added to UAN in a sub surface band, but not slowed to the extent that N availability to the crop would be reduced over the growing season, This study found the yi eld response of corn to urea or UAN solution containing NBPT to be hig hly variable, dependent mainly on the climatic conditions following fe rtilization, The use of NBPT can be an effective management tool to mi nimize the potential for yield loss due to ammonia volatilization from urea fertilizers applied to high residue conditions in years when cli matic conditions are conducive to N loss.