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When prices for corn and soybeans surged last fall, Bill Hammitt, a farmer in the fertile hill country of western Iowa, began to see the bulldozers come out, clearing steep hillsides of trees and pastureland to make way for more acres of the state’s staple crops. Now, as spring planting begins, with the chance of drenching rains, Mr. Hammitt worries that such steep ground is at high risk for soil erosion – a farmland scourge that feels as distant to most Americans as tales of the Dust Bowl and Woody Guthrie ballads.

Long in decline, erosion is once again rearing as a threat because of an aggressive push to plant on more land, changing weather patterns and inadequate enforcement of protections, scientists and environmentalists say.

“There’s a lot of land being converted into row crop in this area that never has been farmed before,” said Mr. Hammitt, 59, explaining that the bulldozed land was too steep and costly to farm to be profitable in years of ordinary prices. “It brings more highly erodible land into production because they’re out to make more money on every acre.”