Yeah, Don's suggestion of soil testing does offer some possibilities.  If you, Grasseed, could use GPS to mark locations, and soil test on, say, 7.5' centers, or 10 ft, or 15 ft - whatever you think you give you the answer......  that might help to establish where the spreader was driving.  then, the real question becomes..... how do you spread to balance out the uneveness? 

I was thinking about spreading nitrogen overtop, perpendicular to the spreading of the manure, and that might help balance out the uneveness of the manure nitrogen; but it does nothing for the uneven P&K. 

My impression/experience is that Ammonium sulfate does not broadcast far.  Maybe that might be a nitrogen source that could be used to "fill the gaps" between the manure spreading passes - again using GPS to establish where the spreader was driving, and using AMS to split the middles....... just thinking out-loud here....