In southern Iowa, Wright says, much of the land conversion is taking place on hillsides. The soil of those fields, without permanent grass to hold it in place, is now much more likely to wash into streams and ponds. And on the western edge of this region, farmers are taking a chance on corn and soybeans in places that sometimes don't get enough rainfall for these thirsty crops.

Why? There's one very simple reason: Corn and soybean prices are high, so farmers can earn a lot of money growing those crops. Meanwhile, funding has been declining for one important alternative — the government's Conservation Reserve Program, which pays farmers to protect wildlife and water quality by keeping land in grass.

Another reason, however, is getting increasing attention: crop insurance. The government subsidizes private insurance policies that cover the risks of poor harvests, or even that prices will fall. Because farmers don't pay for the full cost of this insurance, critics of crop insurance say that it encourages risky behavior: planting crops in areas that don't drain well, where rainfall is unreliable, or on hillsides where soil erosion is a problem.

Critics say that the government should drastically reduce its subsidies for such insurance. Not only is it fiscally irresponsible, they say. It's encouraging farmers to destroy the grasslands of the northern plains, a priceless and increasingly scarce natural treasure.

Copyright 2013 NPR.

grasslandsgrasslands

Pictures Don't Lie: Corn And Soybeans Are Conquering U.S. Grasslands