What prediction does Jim’s grandfather make about the future of Nebraska’s corn fields?

From the text:

The cornfields were far apart in those times, with miles of wild grazing land between. It took a clear, meditative eye like my grandfather’s to foresee that they would enlarge and multiply until they would be, not the Shimerdas’ cornfields, or Mr. Bushy’s, but the world’s cornfields; that their yield would be one of the great economic facts, like the wheat crop of Russia, which underlie all the activities of men, in peace or war.