So all I have to do to grow 450 bu corn is to stop using commercial fertilizer and use soybean meal. They did not say what rate they used, but the 300 extra bushels of corn and savings in fertilizer will pay for a few tons of soybean meal. 

With good growing conditions 2 to 3 times the yield would be about what I would expect from the use of fertilizer vs none. So I would say the control received no fertilizer and was likely low in something other than just N.