These days, U.S. corn and soybean farmers are beating the pants off past yields. In 2018, the average national 52.1-bushel-per-acre soybean yield shattered previous yield records. Ditto for 2018 corn yields, which USDA tallied at a record average of 178.9 bushels per acre.  For now, anyway. Some see some cracks in the system that threaten long-term productivity and sustainability. “Growing corn and soybeans is a simplified and convenient system,” says Jonathan Lundgren, an agroecologist and CEO of Blue Dasher Farms, Estelline, South Dakota. “But it’s backfiring.” • One half of all topsoil and soil organic matter has vanished since farmers broke the Midwestern prairie in the 1800s.  “If we lose the other half, we are in big trouble,” says Peter Scharf, a University of Missouri soil scientist. “Our biggest challenge now across the U.S. in keeping our soil is soybeans. They’re a great crop, but it’s hard to keep soil in place if tillage or rain removes residue on soybean ground.”  Soil degradation symptoms are often subtle, says Dwayne Beck, who manages the Dakota Lakes Research Farm near Pierre, South Dakota.  “Soils with half the organic matter they used to have do not hold as m...