While I was in Iowa recently, Chris Jones, an environmental scientist at the Iowa Soybean Association, showed me this fascinating graph (based on this study). It basically shows how much dirt was in one of the main rivers flowing through Iowa's farmland over the last century:
It doesn’t look like much at first, but becomes more and more interesting as you study it. Because the span of time here is so long (1916 to 2009) and because changes in agricultural policy have had a big effect on the erosion of topsoil into rivers, you can see historical events reflected in these numbers.
That big peak in 1973? That came just after Earl Butz, then the secretary of agriculture, urged farmers to plant fencerow to fencerow. Farmers cut into marginal land, and then heavy rains followed in Iowa. The newly disturbed soil washed off the fields and into the rivers, creating the spike on the graph.