My whole point is that the potash has the same availability, whether it is corn or soybeans.  How well the corn root picks it up vs how well the soybean root picks it up is an entirely different question. 

I do agree that soybean and corn roots have completely different root structure, and those differences in root structure can affect how the plant uptakes nutrients.  Yes, mycorrhizal fungi can enhance nutrient uptake, and that will occur in both corn and soybeans.

In the big picture, I disagree with the statement that spring-applied can not be taken up by corn roots.  I have done it. I have seen the results.  To say that potassium applied in the spring is not available to corn plants is just non-sense.