Considerations for Making N Management Decisions for Soybean.

Soybean yield exceeding 5.4 Mg ha^-1 has become increasingly common, and almost twice as much N is removed in the grain of 5.4 Mg soybean (315 kg N) as compared to 12.5 Mg corn (175 kg N). With such a high N demand by soybean, considering whether to apply supplemental N fertilizer becomes a realistic concern for soybean growers. A literature review illustrates that soybean response to N fertilizer has been inconsistent; but also indicates that N fixation represents only 65 to 70 % of total N uptake for soybean. For high soybean yields, e.g. up to 6.7 Mg ha^-1, N fixation by rhizobia appears to be limited to about 340 kg N ha^-1. If the difference between N demand and N fixation is not compensated by soil N mineralization, N will becoming a limiting factor; unless another source of N is provided – N fertilizer. In light of a N budget that appears insufficient to meet the demand of high-yielding soybean and the inconsistent response to N fertilizer for the many studies in the literature, making a successful N fertilizer application to soybean appears almost to be a random opportunity. This presentation will focus on identifying those conditions and / or characteristics for which a N application to soybean is appropriate, and as such, is successful (e.g. the value of additional yield exceeds the cost of the N application).