Irrigating soybeans is more art than science.  Like Dan mention, I have had years where my dry corners out-yielded the irrigated.  That year I'd just get finished with a circle and get a 1-2" thunderstorm.  My irrigated beans got too much water.

I'm more successful irrigating double crop beans than I am first crop.  I think it's because I start watering double crops early to get vegetative growth.  First crop beans you water them just to survive until they start setting pods

Beck's seed had an irrigator meeting this winter at Vincennes.  The speaker said we are watering beans too much at the wrong times.  Of course, he had the sensors and monitors that would help that ...