My guess would be that the number is around half of counties in Illinois today. Soybeans came to the Midwest from the Far East for use as a hay source, as well as for cooking material. Farmers and farm academics quickly saw the possibilities for soybeans as a cash grain crop. Here are some advantages of beans for grain. Whereas corn robs the soil of nitrogen, beans replenish it naturally.
Farmers can grow beans on land and in climates that are also good for corn.
They can use much of the same machinery for beans as for corn. On the rural-urban fringe, where land is awaiting construction activity, beans work well because they require fewer passes with machinery, allow surveying stakes to stand out, provide no place to hide construction debris, present less chance for fire, and are less likely to hide children who wander away from home. Finally, soybeans are an in-demand crop that brings cash to farmer. I do not intend to get into many of them, but here are some examples. Popcorn in southeastern counties along the Wabash.