Monitor: Kyle Slaby bounds up the slope behind his house, stopping at the sandstone outcrop he hopes will save his family's farm. The Slabys grow corn and soybeans on the ridgeline above. But these days there's more money a lot more in mining the sand below. "A lot of people look on it as an extension of farming," Mr. Slaby says. "It's another crop you're harvesting." Sand has become a valuable and deeply divisive commodity in the upper Midwest. Hydraulic fracturing, a method of extraction also...