Effect of Nitrogen on Nodulation and Yield of Irrigated Soybeans 1

Wayne soybeans [Glycine max L. (Merrill)] were sown immediately after barley harvest in a 2 crop, 1-year rotation experiment, to determine the effects of additions of nitrogen (as ammonium sulfate) on nodulation and yield. Treatments consisted of all combinations of: (i) barley straw residue (burned or shredded); (ii) timing of nitrogen application (before planting, at flowering or a combination); and, (iii) rates of application (0, 56, 112, 168 kg/ha of nitrogen) in a split-split-split design. Nodule number per plant was linearly and inversely related to rate of nitrogen application. There were fewer nodules per plant if 56 kg/ha or more nitrogen was applied at planting time, but up to 112 kg/ha did not affect nodule numbers if applied at flowering. There appeared to be more nodules per plant on plots with the straw shredded, but the differences between shredding and burning the barley straw were not significant. Even though plants on the 0 nitrogen plots were yellow-green in early growth stages, there were no significant differences in yield due to straw treatment, timing, or rate of application of nitrogen.