A recent documentary noted how greed and foolish government policies encouraged farmers on the Plains to turn over millions of acres of native grassland. Ken Burns’ recent project reignited debate among historians about the extent to which human activity added to the massive dust storms in the 1930s. Sod that anchored the soil for thousands of years was stripped by plowing, allowing the dirt to blow freely. Historians contend Burns ignored numerous records of major dust storms before the sodbusting. Those interviewed by Burns were children in the 1930s. Riney Kehrberg, history department chairwoman at Iowa State University, interviewed adults at the time of the dust storms. Her book, “Rooted in Dust,” notes immense clouds of black soil, blotting the sky, were unusually severe but frustratingly commonplace. Fastcast: Rain returns.