OK, so I've never grown beans, much less in Iowa.

But alfalfa is a legume as well and supposedly "doesn't need any N" -- which I've now determined to be false.

Legumes fix N into the soil only when the soil temperature is high enough to get the natural action going. Cold ("frigid") soils that don't warm up into the 50's until late June will see a yield response with N applied early in the growing season.

We used to buy into the "no N on alfalfa" line, putting down only P, S, K, etc. This year, I put down 40 units of N in liquid (injected through the pivots) and the hay immediately showed an improvement in the crude protein levels in the alfafla hay -- as much as two percentage points in the second cutting (taken in early July). Protein depends on available N -- that I see such a response in the CP levels in the hay is a prime indication that the alfalfa was short of N and needed supplemental N to achieve what I want.

Here's a link to an abstract of a recent paper on early N applied to beans in the northern plains:

http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=165713