In my studies, I believe nitrogen has two main effects on the soybean plant. First, it increases internode length during vegetative stages. Second, it helps maximize conversion of dry matter during grain fill.
Commonly known but not widely published is the fact that soybeans require 5# of N per bushel. The plant can fix about 70% of its needs from the atmosphere and rhizo bacteria. The balance must be met from soil mineralization or supplemental fertilizer. For extremely high yield soybeans (70bpa+) they can run out of N in average soils.
Not surprisingly, high yield soybeans tend to follow high soil N mineralization springs and summers. With ample rain, additional N late can capitalize on available potential.
One interesting consideration for soybeans after drought stressed corn is the potential for residual Nitrogen. If it makes it to grainfill that could mean bushels. However, the more likely situation (not likely, but MORE likely) is that spring available N is higher than normal. (most of Indiana soils are still not saturated as I write on this 18th day of December.) Since soybean internode length is increased by N in early vegetative stages this may simply translate into taller beans and more shaded growing points, fewer nodes and flowers... and reduced yield potential. What should you do?
Plant early and reduce populations to compensate for higher N load. You're likely to get as much or more nodes and similar canopy dates. Ultimately this can translate into higher yield potential, along with reducing costs and lodging risk. The worst practice is probably abnormally high pops drilled in narrow rows on high OM soils following a drought and dry winter. You'll have tall, lodged soybeans that might yield in the 40's (but they'd be nice baling!)
Hopefully this provides you some food for thought. Happy planning!
(I drew this picture, mapping nodes, pods and distribution. Did you know 80% of yield is normally in bottom seven nodes?)
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