To an untrained eye, the fields of Iowa have a reassuring solidity. You cannot tell that the state has lost half its topsoil in the past century. According to a new report from the Environmental Working Group, Iowa’s soil is washing away at rates far higher than anyone realized.

For Iowa — and other Corn Belt states facing similar problems — this means an increasing loss of fertility that has to be replaced chemically. It marks a failure of stewardship, since these soils will have to feed future generations. And every particle that washes away causes problems downstream, including sedimentation — which can increase the risk of flooding — and the alarming dead zone in the Gulf of Mexico, the result of runoff of the chemical fertilizers farmers apply to make up for lost fertility.

The Agriculture Department says that a “sustainable” rate of topsoil loss for most of Iowa is 5 tons per acre per year, and the actual average soil erosion is 5.2 tons. But using Iowa State University statistics and an aerial survey, the Environmental Working Group concluded that average annual soil loss in much of Iowa is double the federal government’s estimates. This pace of erosion is caused partly by an increasing number of intense storms. As the report says, it has been exacerbated by a fundamental bias in federal farm policy and supports. In the dozen years before 2009, Iowa received nearly $17 billion in subsidies that fostered high-intensity farming and less than $3 billion to support conservation. In the recent budget battles, conservation programs were the hardest-hit farm programs.

Meanwhile, the race to profit from high crop prices — especially corn for ethanol — and the sobering jump in the cost of rented land in Iowa means that there is an intense push to create greater yield on more acreage and less incentive than ever to practice sound soil conservation.