Even when the soil is quite alkaline, this still leaves sufficient available iron for crop growth. When we go looking for clear iron deficiencies, we have to go west, beyond Michigan, to the west half of Wisconsin. Incidentally, this also marks the limit of the last glaciation. The second factor is the history of the fields. Most of the fields showing yellow soybean leaves, also show stunting and yellowing when they are planted to cereals.
Cereals are quite sensitive to manganese deficiency, but are very insensitive to iron deficiency.
Also, manganese deficiency shows up on both eroded knolls and muck hollows, while a lime induced iron deficiency would only show up on the knolls. Finally, in the past a foliar spray of manganese has greened up the fields with deficiency symptoms. If the yellowing were caused by iron deficiency, the fields would have looked worse after spraying manganese, since manganese can induce an iron deficiency. In short, when we see yellow leaves with green veins, it is safe to assume it is manganese deficiency. Of course, if you don't believe me, you can always check for yourself with a tissue test.