| Nitrogen (N) and phosphorus (P) fertilization of soybean was studied in two separate experiments in the Morris, St. Norbert and Homewood areas of the Red River valley in Manitoba from 2004 to 2006. All fields had a history of soybean production. In each experiment, six rates of fertilizer were applied using three application practices. For the nitrogen study: 0, 25, 50, 75, 100 and 125 kg N ha-1 were applied either as a single application at seeding with granular Bradyrhizobium japonicum inoculant, as a single application at seeding without B. japonicum, or as a split application with 25 kg N ha-1 applied at seeding with B. japonicum inoculant and the balance applied at R3 (early pod). For the phosphorus study: 0, 15, 30, 45, 60, and 75 kg P2O5 ha-1 was either seed-placed, banded 2.5 cm below the seed or seed-placed with Penicillium bilaiae -inoculated seed. Data was collected on emergence, nodulation, biomass, yield, seed weight, seed protein, seed oil, and seed phosphorus content. Nitrogen fertilization of soybean resulted in significant negative emergence, nodulation and seed oil content responses as well as a significant but small positive response of seed weight. Applying the N fertilizer as a split application resulted in a small positive response for seed protein content and a small negative response for seed oil content. Inoculation with B. japonicum was not significantly different from the non-inoculated practice. Overall, there was no significant effect found on soybean yield as a result of N fertilization regardless of the application practice used. For the phosphorus experiment, P fertilizer had almost no effect on soybean production. The P component of the mono-ammonium phosphate fertilizer did not negatively affect seedling emergence regardless of placement. Inoculation with P. bilaiae had almost no effect on soybean production. Only a mild response to P rate was observed for seed protein content (negative) and seed P content (positive). Overall, N fertilization of soybean in the Red River valley of Manitoba is not recommended and the use of expensive granular Rhizobium inoculants may not be necessary on land that has grown well-nodulated soybean crops in the past. Phosphorus fertilizer for soybean should only be applied to soils that are low in available phosphorus. |
