NITROGEN-FERTILIZER MANAGEMENT EFFECTS ON CORN GRAIN-YIELD AND NITROGEN UPTAKE

Concern about the fate of N remaining in the soil after harvest has heightened interest in management techniques that will increase the ability of a growing crop to use N. There is considerable interest in sidedressing or split-applying N fertilizer as an inexpensive way to increase fertilization efficiency and thus to reduce the amount of N not used by the crop. The response of continuous corn (Zea mays L.) to (NH4)2SO4 incorporated to a depth of 4 in. was studied during 1986 and 1987 at two sites in Iowa. Preplant and sidedress (V6 growth stage) N applications were combined to create 21 treatments with six rates ranging from 0 to 175 lb N/acre. Nitrogen rate significantly increased grain yield, leaf-N content, grain-N content, and N uptake at both sites in both years. Grain yields in 1986 ranged from 70 bu/acre to 187 bu/acre at Site 1, and from 76 bu/acre to 223 bu/acre at Site 2. Grain yields in 1987 ranged from 77 bu/acre to 146 bu/acre at Site 1 and from 51 bu/acre to 175 bu/acre at Site 2. The difference in maximum yields attained was probably due to differences in growing season rainfall. Two types of regression equations describing the relative effects of preplant and sidedress application of N on the variables studied were fitted to the data. The models had similar R2s and exhibited similar trends. There was no effect of time of N application except for Site 1 in 1986, where sidedress application significantly increased grain yield, N uptake in the grain, and leaf N content compared with preplant application. The data show that response to time of N application was site specific and directly and positively related to growing season rainfall, and that the response was not due to an increase in efficiency of use.