BROADCAST NITROGEN-SOURCES FOR NO-TILL CONTINUOUS CORN AND CORN FOLLOWING SOYBEAN

Studies evaluating surface broadcast urea-based N sources in no-till c orn (Zea mays L.) have been primarily conducted with continuous corn. Most corn grown in Missouri follows soybean [Glycine mar (L.) Merr.]. Our objective was to evaluate no-till, continuous corn and corn follow ing soybean response to surface applied ammonium nitrate (AN) and urea -based N sources. Field studies were conducted at three Missouri sites during 1988-1990. Factorial combinations of rotation (continuous corn and corn-soybean) N source [AN, urea, urea ammonium nitrate (UAN), an d UAN + ammonium thiosulfate (UAN + ATS)], and N rate (67, 135, and 20 2 kg ha-1) were studied in a split-block design. Nitrogen rate and sou rce affected grain yields each site year. Ammonium nitrate was superio r to the urea-based N sources in 5 of 8 site-years. Averaged across si te-years, grain yields (Mg ha-1 were 8.22 for AN, 7.57 for urea, 7.09 for UAN + ATS, and 6.99 for UAN. Apparent fertilizer N losses were est imated at 11% for urea, 18% for UAN and 17% for UAN + ATS. Ammonium th iosulfate did not improve the performance of UAN. Grain yield response s to applied N were greater with continuous corn than corn following s oybean. Continuous corn also resulted in larger yield differences betw een N sources. In continuous corn, AN resulted in greater grain yield than urea by 0.81 Mg ha-1, than UAN by 1.35 and Mg ha-1, and than UAN + ATS by 1.32 Mg ha-1. For corn following soybean, the yield advantage for AN was 0.42 Mg ha-1 greater than urea, 0.98 than UAN, and 0.82 th an UAN + ATS. Therefore, reduced fertilizer efficiency of surface appl ied urea-based N sources appears to be a problem in both continuous no -till corn and no-till corn following soybean.