With respect to the N........it makes no agronomical sense to add slurry for the purpose of increasing N to the plant for purposes of yield enhancement----------BUT, if the objective is to dispose of the N, then applying in front of soybeans will allow the consumption of N just like corn will so consume???????  In other words, since the soybean plant does not care as to N source, why not use it as a disposal system.

With respect to the addition of nutrients to enhance soybean yields--------it could easily make sense, economically-speaking, to add slurry ahead of beans------just for the P and K, and the bacterial action of live manure.  Correct?  (Obviously giving due regard to the tested components of the slurry)

And therefore.............if a person has the slurry to dispose of-------OR if the person is looking to add P&K, and is willing to allow the plant to consume soil N vs "atmosphere" N...............where's the downside?

Lastly.........given the potential for a change in rotation..........adding slurry ahead of projected soybeans would hedge the bet of possibly going to corn.

OR, changing from corn to soybeans at the last minute would cause the soybeans to chew the soil N.............

But in all cases, the N goes poof.

I think the bottom line is as you indicated-------- P will trump all anyway.

But Jim Doolittle is correct--------- much ado about nothing, and that is the real problem.......tieing up resources and putting the DNR in an enhanced position for  more controversy.